Introduction

There is a psychological barrier to predicting a rainfall expected only once every 150 years or more. It’s like backing the least fancied runner in a horse race. This is why meteorologists almost never forecast an extreme event. However, these are the ones that are the most important, and can have major effects on the community. Hence this publication.

A secondary purpose of this treatise is to have some sort of documentation of past events. Within it you will see some huge rainfalls being reported for events which to us occurred an enormously long time ago, such as the the record-breaking Hawkes Bay rainfall of 1924, and the cyclone of 1868. By 1870, just a few decades into the period of European settlement, New Zealand rivers had been responsible for 1115 recorded drownings. Drowning became known as “the New Zealand death”. But is the frequency of flooding increasing, and are the floods of today any worse? Our knowledge of river flow records is handicapped by lack of information on past events, as indeed it is for extreme rainfalls. A simple tabulation of high rainfall events will produce an increase over time, but one would expect that anyway as the rainfall network and population density increases. The consensus in New Zealand would appear to point to an increase in localised events, with little change in the frequency of widespread, major floods.

As global warming brings more extreme and more frequent floods, there is an increasing need for them to be accurately forecast. At present, MetService does an excellent job of forecasting heavy rainfalls (defined for verification purposes as exceeding 50 mm per six hours, or 100 mm per 24 hours). What we do not do so well with is the extreme events – the ones that inundate farmland and houses, cause massive landslips, drown stock, and take lives. So the main objective of this publication is to provide some insights into what makes an event extreme.

Although some of the content is technical, this publication is designed to appeal to a fairly broad range of meteorological knowledge. It is hoped that professional meteorologists, particularly heavy rain forecasters, will be assisted in doing their job. It is hoped some insights will be thrown up into what turns a mundane heavy rain event into something lethal.

This work gathers together 122 extreme events between 1858 and 2005. The events are divided into geographical regions, and are placed in the region that experienced the most severe weather. They have been selected based primarily (but not exclusively) on rainfall return periods, rather than severity of flooding. In no way can the list pretend to be exhaustive, and I am sure that a number of significant events, for various reasons, have not made it onto these pages.

Some caution needs to be exercised in interpreting return periods. An extreme rainfall that occurs over a small area is much less significant than one that occurs over a large area. A thunderstorm can deliver a high rainfall to an area as small as 1 square kilometre. The land area of New Zealand is 268,000 square kilometres. This means that thunderstorms are going to produce 268,000/100 = 2680 one-hundred-year rainfalls in various tiny parts of New Zealand in an average year. (Statisticians will tell me that the above is not entirely accurate, but the general idea is correct.)

Acknowledgements

MetService Expert Forecasters (Erick Brenstrum, John Crouch, Andy Downs, Allister Gorman, Bob Lake, Paul Mallinson, Ian Miller, Steve Ready): Severe Weather and Severe Convection Logs.
John Crouch: Much of the material in the Severe Convection Section.
Steve Ready: Data on Cyclone Bola.
Allan Penney, Errol Lewthwaite (NIWA ). Assistance with dredging the archives.
Marianne Watson of Horizons Regional Council, and Craig Thompson of NIWA : The rainfall recurrence interval map for the 2004 Manawatu Flood Event. (Fig. 112).

Bibliography

Cumberland, K., 1985: Rivers and Lakes. In: New Zealand. Whitcoulls Publishers, Wellington.
Mosley, M.P., and C.P. Pearson, 1997: Floods and Droughts: the New Zealand Experience. New Zealand Hydrological Society.

Northland/Auckland

Northland and Auckland are the first places in New Zealand in the firing line for ex-tropical cyclones. Lows of sub-tropical origin can also produce heavy rains. In heavy rain situations, the wind flow over the area is usually (but not always) easterly or northeasterly. Auckland, however, does receive some sheltering by the Coromandel Peninsula in flows from the east, especially from the City southwards. The exact direction of the flow is important in determining which parts of the Auckland Region,
if any, will receive heavy rain. In Northland, the Hikurangi area usually receives the most substantial falls.

Convective activity plays a major role in heavy rain events in Northland and Auckland, particularly in summer. Because these convective rainfalls tend to affect only limited areas, there are a large number of events which produce a 100-year rainfall somewhere or other.

A critical element in the forecasting of heavy rain events in this area, particularly Auckland, is the presence of a slow-moving low level convergence zone in the presence of suitable upper-level support.

1917
26th January–6th February
PROLONGED PERIOD
OF WET WEATHER

A 10-day period of wet weather included the large 24-hour rainfall of 231mm in 24 hours at Taheke on the 2nd February. Most bridges in Northland were
swept away.

A blocking high near the Chatham Islands maintained a long period of moist northeasterlies over the northern North Island, culminating in the passage of a front.

1935 15th February
AUCKLAND CBD
FLOODED

Considerable damage was caused to shops and warehouses in Auckland City, and to low-lying areas in the suburbs, after two hours of torrential rain. Workmen went swimming in Fort St to locate blocked stormwater gratings.

The flooding was caused by convective activity within a slack, low pressure system.

1936 1st-2nd February
THE ’36 CYCLONE

Rainfalls
Northland:
Whangarei 290mm/24hrs (return period over 150 years)
Russell 301mm/24hrs (return period over 150 years)
Warkworth 275mm/24hrs (return period 120 years)

Auckland:
Auckland City 162mm/24hrs (return period 45 years)

Stock losses: Heavy
Damage: £115,000
2012 Dollars: 12.7 million

Hawkes Bay:
Maraetotora 254mm/24hrs (return period 125 years)
Stock Losses: Serious
Road Damage: £13,400
2012 Dollars: 1.5 million

Total Deaths: Several
Total Nationwide Damage: £10 million
In 2012 Dollars: 1 billion.

An ex-tropical cyclone caused widespread devastation across a large part of the country. The centre passed just west of Northland and Auckland, then veered
southeast to cross the central North Island, emerging near Napier.

In parts of Auckland there were floods of record proportions, and Whangarei was flooded to depths of up to a metre. Napier was isolated by floodwaters. There was also extensive flooding in Whanganui, Manawatu and Wairarapa. The Ashley River in Canterbury rose to unprecedented heights. The wind blew in windows from Kaitaia to Picton, the worst hit being the Manawatu, where the grandstands of the A&P Association, the Awapuni Racecourse and the Sportsground were demolished.

The rainfall distribution for the event is shown in Fig 1.

Fig 2 shows the MSL chart for 9am February 2nd. The central pressure is drawn as below 970 hPa. Palmerston North is reporting a wind of 10 on the Beaufort Scale, implying winds of storm force (averaging between 48 and 55 knots). This figure also shows isallobars (rate of pressure change), in hectopascals per three hours. The biggest pressure falls are seen to be occurring off the southern Hawkes Bay coast.

1966 16th February
ONE-HOUR RECORD
GOES TO WHENUAPAI

A downpour over Whenuapai broke the official record for a one-hour fall, a record which stood for 35 years. Heavy rain was quite widespread through Northland and Auckland. The 24-hour rainfall at Whenuapai was 260mm, and about 400mm fell at Kokopu. The MSL analysis for 6pm 16th (Fig 3) did not suggest that anything extraordinary was about to occur.

The flow remained very anticyclonic-looking through to 6am on the 17th (Fig 4), when a north-south front suddenly appeared on the analysed charts.

A look at the upper air (Fig 5) reveals a trough in the Tasman Sea, exhibiting a slight negative tilt. The flow over the North Island at 200 hPa at midday on the 16th was anticyclonic.

1971 18-19th April
SEVERE ELECTRICALSTORMS AFFECT
TAIPUHA AND WHANGAREI HEADS

Major slipping occurred on hillsides, with roads damaged. Buildings were damaged and paddocks flooded.

The rainfall chart (not shown) showed a double centre. One centre with rainfalls over 6.5 inches (165mm) lay over the Settlement Road, Taipuha area. The maximum rainfall within this area was estimated at 250mm, and this fell in the two and a half hours to 10pm 18th. This is 2.59 times the 150- year return period rainfall, and would place this storm at number three in the hierarchy (see Appendix 1). The other centre lay over Whangarei Heads. The rainfall here was estimated at 230mm in the five and a half hours to 2.30am on the 19th. This is also a well over 150-year return period rainfall.

MSL charts (Fig 6) showed a frontal band lying slow-moving over Auckland, while Northland lay within a humid north to northeast airstream. The rain was accompanied by intense electrical activity. Upper level charts (not shown) revealed a negative-tilt trough moving onto Northland. The flow at this level appears quite difluent, suggesting upper-level divergence was a significant forcing mechanism.

1974 22-23rd February
KAEO – OKAIHAU STORM

The MSL chart (Fig 8) shows Northland lying within a moist and very humid northeast flow. Dewpoints were up around 21C. Satellite imagery (not shown) revealed a broad cloud band covering the whole of the North Island. Convection was a major contributor.

It’s a little hard to tell from the figure (Fig 9), but upper level divergence may also have been a significant factor. At 500 hPa, a ridge lay over Northland.

1975 30th May
LOCALISED STORM
SOUTHEAST OF
WHANGAREI

1981 19-20th March
KEMP HOUSE FLOODED

A massive flood stuck the Kerikeri area, and New Zealand’s oldest house, Kemp House, was inundated to ground floor window-sill level. The Waipapa Flats were completely covered with water, and in the Waipapa Landing area a house was completely washed away. Fifty boats were torn from their moorings. Most of the river valleys at the northern end of Kerikeri were scoured, and the
vegetation along their banks totally cleaned out. The isohyetal chart (Fig 12) shows a considerable area recording over 300mm in the seven-hour duration
of the event. 174mm in 2.5 hours was recorded, but intensities during shorter periods were not available.

The extreme rainfalls recorded put this event well up
in the hierarchy (see Appendix 1). The MSL chart (not shown) looked rather innocuous, with a large high centred
east of the South Island, and a weakly cyclonic easterly flow over Northland. The air was moist and unstable however, and the heavy rain was the result of an outbreak of severe convection.

Rainfalls

F. Hunt: 448mm/9.5hrs
(return period well over 150 years)
incl 174mm/2.5 hrs
(return period well over 150 years)
Deaths: One.

1985
SEVERE CONVECTIVE EVENT
AFFECTS PUKEKOHE

See Landslide Sweeps Through Te Aroha

1988
CYCLONE BOLA

See Bola Devastates Gisborne District

1997 30th June
HOUSES, SHOPS
FLOODED IN KERIKERI
AND KAEO

Rainfalls

Brynderwyn Hills: 310mm/24 hrs (return period over 150 years)
incl 96mm/1hr (return period over 150 years)
and 165mm/2hrs (return period well over 150 years)
Maungaturoto: 268mm/24hrs (return period over 150 years)
Purerua: 131mm/7hrs to 4pm (return period 80 years)

1999 21st January
HOKIANGA AREA
DEVASTATED

Homes were washed away in the small towns of Pawarenga, Panguru and Omapere after walls of water and logs swept down the valleys. In Pukekohe homes were flooded, and two rest homes had to be evacuated.
Intense convective activity was the main forcing mechanism in this event.

A broad slow-moving trough lay over northern New Zealand, with a weak easterly flow over the area (see Fig 15). Shears were low. Low-level convergence was also present, as the westerly sea breezes met the large-scale easterly flow. Diurnal afternoon heating, and warm, moist tropical air at low levels, all contributed to the development of large cumulonimbus clouds.
Fig 203, in Appendix 3, shows the sounding for Auckland for the time of the storm. It features tall ‘skinny’ CAPE, with a lifted index of –6.0C and a SLI of –2.8C.
The CHAMP model provided excellent guidance to forecasters, who were able to issue warnings for this event. The global models did not indicate heavy rain over the area, being unable to resolve the small-scale convective nature of the event.

Rainfalls

Panguru: 100mm/3hrs
(return period over 150 years)
Bayly’s Beach: 90mm/2hrs
(return period over 150 years)
Omapere: 120mm/5hrs
(return period over 150years)
Awakino: 160mm/4hrs
(return period well over150 years)
Pukekohe: 160mm/3hrs
(in fact just over 2 hrs)
(return period well over 150 years)

Deaths: Two

Evacuations: Hundreds.

Fig 15. MSL analysis for 6pm 21st January 1999.

2000 21st April
OFFICIAL ONE-HOUR
RECORD EXCEEDED

2000 28-29th June
EVACUATIONS IN
AWANUI

2001 30th May
ONE-HOUR RECORD
TAKEN BY LEIGH

Bibliography

Barnett, M.A.F., 1938: The Cyclonic Storms in Northern New Zealand on the 2nd February and the 26th March, 1936. New Zealand Meteorological Service Office Note 22.
Brenstrum, E.M., 2000: The Cyclone of 1936: The Most Destructive Storm of the 20th Century? Weather and Climate Vol. 20. p. 23-27.
Cowie, C.A., 1957: Floods in New Zealand, 1920-53: with notes on some earlier floods. Soil Conservation and Rivers Control Council.
McGavin, T. : Severe Weather Events Analysis. http://www.metservice.com/default/index.php?alias=notableweatherindex New Zealand Meteorological Service
Mosley, M.P., and C.P. Pearson, 1997: Floods and Droughts: the New Zealand Experience. New Zealand Hydrological Society.
New Zealand Herald 18th February, 1966.
________ 21st March, 1981.
NIWA, 2002: HIRDS v2.0 – High Intensity Rainfall Design System
New Zealand Gazette, 1936, Vol. I. New Zealand Government.
Pascoe, R.M., 2001: Forecasting Heavy Rain in Auckland. New Zealand Meteorological Service Internal Report 6.
Severe Weather Log. New Zealand Meteorological Service.

Waikato
(including Coromandel Peninsula)

Northwesterly airstreams are the main bringers of heavy rain to the Waikato area. The area is dominated by the Waikato, New Zealand’s longest river. Because of its length, and also its relative flatness, it takes several days of heavy rain beforethe river will flood. Unfortunately, it also takes a long timefor the river level to go down again. As with other parts of thecountry, stationary fronts can cause flooding, and convectionoften plays a part.
Moist northeasterlies ahead of a frontal band or associated with an ex-tropical cyclone are required to produce heavy rain in the Thames/Coromandel area. Orography plays a major part in these events, and in the most severe, convection is also a factor.

1907 10-23 January
MAIN TRUNK RAILWAY
BECOMES FERRY

A long spell of wet weather led to the biggest flood of the 20th century for the Waikato River. The main railway south from Auckland was flooded south of Pokeno to depths up to 4.5 metres. Train passengers were transferred to boats there and taken to Taupiri to catch another train. Refreshments were served on the way at Mercer station, although the station floor was under 30cm of water.

Vast areas of land were flooded, and crop and stock losses were heavy. The wettest period was from the 14th to the 16th, when a northerly flow between a high to the east and a slow-moving trough in the Tasman Sea brought a prolonged spell of rain. MSL analyses for the 14th and 15th (not shown) reveal a slow moving depression with central pressure about 990 hPa lying just west of the North Island.

1910
MINING TAILINGS DEPOSITED
ON FERTILE LANDS

1953 7 July
MOST EXTENSIVE
WAIKATO FLOODING
SINCE 1907

Although rainfall amounts were similar to the flood of April 1924, the flooding in 1953 was much more serious. About 10,000 hectares of developed and partly developed farmland along the banks of the Waikato River was inundated. About 2000 hectares of first-class dairying land was under water for two to three weeks, and considerable areas were affected for longer periods. The main highway was covered at many points, and in one place was impassable for several days.
However, river levels were well below those recorded in 1907.

Interestingly, it was not a northwest flow that brought the heavy rain. A rain band remained slow moving over the Waikato area for some time. Fig 21 shows the sequence of MSL charts for the period of heaviest rainfall. During this period, the low-level flow over most of the North Island was from the northeast.
Fig 22 shows that, as indeed might be expected, heavier falls occurred in the Bay of Plenty.
Fig 23 shows the system was cut off to a high level in the atmosphere, and thus can be expected to have been very slow moving.

1954 18-20th May
WORST FLOOD ON
COROMANDEL FOR
HALF A CENTURY

Parts of Paeroa were evacuated during this flood event and the Ohinemuri River rose seven metres. Low-lying areas of Thames were flooded to a depth of two metres, with the Waihou River rising nine metres. Although significant flood events occurred in June 1920 and April 1923, this was reported to be the worst flood in 60 years. In actuality, it was probably not as bad as that of 1910 (only 44 years before).
Fig 24 shows the classic set-up for heavy rain on the Coromandel Peninsula. The low remained virtually stationary, while its associated frontal band drifted slowly southwards over the affected area.

1958 15-25th February
MAJOR FLOODING
IN THE WAIKATO

1981 12-14 April
WORST COROMANDEL
FLOOD ON RECORD

1985 16-17 February
LANDSLIDE SWEEPS
THROUGH TE AROHA

Rainfalls
Coromandel/Thames:
Coromandel 331mm/24hrs (return period 85 years)
incl 266mm/8hrs (return period over 150 years)
Thames 251mm/24 hrs (return period 150 years)
Te Aroha 127mm/6hrs. (return period 100 years)

Auckland:
Pukekohe 166mm/24hrs (return period 50 years)

Deaths: 4
Damage: $7 million
2012 Dollars: 19 million
Evacuations: Hundreds

1996 29-31st December
CYCLONE FERGUS

1998 1-20 July
FLOODING LASTS
FOR WEEKS

Rainfalls
Waikato:

Matangitangi 203mm/72 hrs to 7.40pm 11th (return period 42 years)
Ngaroma 271mm/72 hrs to 4.55pm 11th. (return period 22 years)
incl 188mm/24 hrs (return period 7 years)

Bay of Plenty:

Ranger Stn 391mm/72hrs (return period 120 years)
incl 326mm/48hrs (return period 80 years)

Evacuations: 30 houses
Damage: $17.85 million
2010 Dollars: 26.2 million

Episode 1

A low deepened rapidly in the Tasman Sea. The low was triggered by strong CVA, also very strong upper level divergence in the poleward exit region to a 180-knot
southwest jet and the entrance to an anticyclonically-curved northwest jet (not shown).
Very moist air, with wet bulb potential temperature (WBPT) around 17C was brought out of the subtropics, and 850 hPa winds strengthened to 50-60 knots.
The MSL chart for midnight 1st July is shown in Fig 42.
The high slowed the movement of the cold front as it crossed the North Island.
Rainfall rates reached 60 mm per hour in the Bay of Plenty.
Forecast amounts were good for the area around Taupo, but underestimated elsewhere.

Episode 2

Strong north to northwest winds spread over the North Island ahead of an active cold front. A complex low lay in the Tasman Sea.
Fig 43 shows the mean sea level (MSL) analysis for midday on the 9th.
The development of a small low, seen west of Taranaki in Fig 43, occurred beneath strongly difluent flow (Fig 44), and the resulting increased gradient
enhanced the orographic contribution to the heavy rain.
The front subsequently weakened over the eastern Bay of Plenty.

Episode 3

A wave can be seen in the midday 10th July analysis over northern New Zealand (Fig 45).
The imagery revealed a large baroclinic leaf (not shown), which rapidly developed, then spread rain back over the whole of the northern half of the North Island. Although rain from this episode may not have been quite up to the criteria required for a warning, it was certainly of significance, coming as it did on the top of the previous heavy rain.
Computer guidance had led forecasters to expect a further burst of rain in the eastern Bay of Plenty only, and not for the wave to come back as far west as it did.
There was in fact a fourth burst of heavy rain on the 14/15th July. Another 50-60mm of rain fell over the affected areas in an easterly airstream.

2002 21 June
THE “WEATHER
BOMB”

Rainfalls
Coromandel:
125mm/25mins (return period well over 150 years)
Thorndon Bay: 150mm/2hrs (return period well over 150 years)
Tapu: 83mm/1 hour (return period over 150 years)
Waiomu: 31mm/15 mins (return period 140 years)
Te Aroha: 97mm/1 hour (return period over 150
years)
Putaruru: 120mm/2 hrs (return period well over 150 years)

Deaths: One.
Evacuations: Many
Damage: $13 million.
2012 dollars: 17 million.

The 250 hPa analysis (Fig 47) shows a strongly difluent flow over
the Coromandel Peninsula.

Fig 47. 250 hPa analysis for midnight 20th June 2002.

2003 25-28th February
COROMANDEL ROADS CLOSED

2003 26th November
FLASH FLOODING
IN HAMILTON

2004 28-29th February
EVACUATIONS
IN TURANGI

Rainfalls
Ngaroma: 218mm/36hrs (return period 80 years)
incl 183.5mm/24hr (return period 55 years)
Mangatoetoe: 289mm/36hr (return period over 150 years)

Evacuations: 90.
Damage: $4 million.
2012 Dollars: 5 million.

The Tongariro River burst its banks and 100 people in Turangi were evacuated. The return period for the flow in the Tongariro and Upper Waipa Rivers was about 100 years, although only 20 years for the Lower Waikato. Flooding occurred in many other parts of the Waikato Region, particularly Otorohanga, and low-lying areas adjacent to the lower Waipa and Waikato Rivers, with roads closed and farms inundated. Elsewhere in the country, rivers in Manawatu and Whanganui came close to bursting their banks. SH1 just south of Kaitaia was closed by flooding. There was surface flooding around Auckland (where there were also trees blown down), in Hastings, on the Kapiti Coast, and in the Hutt Valley. Flooding was exacerbated in many areas by the already wet catchments, after a stormy month.
The area of heaviest rain was in the upper reaches of the Tongariro and Waipa Rivers, where intensities reached 27mm per hour.

A deep low over the Tasman Sea moved slowly southeastwards, crossing the South Island on the night of the 28th. Its associated front was preceded by a very moist, strong northerly flow. The analysis for midday 28th February is shown in Fig 48. The front moved slowly across central and northern New Zealand, and linked up with the remains of Tropical Cyclone Ivy, which moved rapidly southsoutheast passing just east of East Cape early on the afternoon of the 29th. The presence of Ivy ensured that very humid tropical air was brought down onto the country – this is what turned the event into an extreme one in a small area. Orographics played a major role in the event, with upper level divergence also a factor.

2005 18-19th May
HOMES INUNDATED
IN WHANGAMATA

2005 16-17th July
SECOND 150-YEAR
FLOOD IN 3 MONTHS

within a short wave trough moving southeast over the area. Convection was a major contributor to the rainfall. No useful hourly falls are available for the Coromandel area, but parts of Auckland received up to 45mm in an hour, and several properties were flooded. There was also flooding and road closures in Northland.

The convection was the key to this event. The active portion of the front was quite small, but tracked right down over Northland, Auckland and Coromandel. Forecast soundings did not produce much CAPE, and actual soundings were unspectacular. The only clue to impending heavy rain was given by CHAMP12. This often
produces spurious heavy rain from low-level convergence zones, but can be useful as a “first alarm” level for convection forecasters. This needs to be done judiciously though (for example, “blobs” of high rainfall in inland areas on the CHAMP fields can usually be ignored in winter, where afternoon convection is not an issue and there is no warm sea close by to act as a moisture source).

Bibliography

Collen, B and J.W.D. Hessell, 1981: The Thames-Coromandel Floods of April 1981. New Zealand Meteorological Service Technical Information Circular 183.
Cowie, C.A., 1957: Floods in New Zealand, 1920-53: with notes on some earlier floods. Soil Conservation and Rivers Control Council.
Finkelstein, J., 1954: The Waikato Flood of July 1953. New Zealand Meteorological Service Tech. Note 108.
Holmes, V, and S.M. Burgess, 1985: The Thames-Coromandel Floods of February 1985. New Zealand Meteorological Service Scientific Report 17.
McGavin, T. : Severe Weather Events Analysis. http://www.metservice.com/default/index.php?alias=notableweatherindex New Zealand Meteorological Service
Munro, A.J., and M.G. Chapman, 1998: Waikato Regional Flood Event of 9-20 July 1998. Environment Waikato Technical Report 1998/15.
New Zealand Gazette, 1958, Vol. I. New Zealand Government.
NIWA, 2002: HIRDS v2.0 – High Intensity Rainfall Design System
Poole, A.L., 1983: Catchment Control in New Zealand. Water and Soil Misc. Pub. 48.
Severe Weather Log. New Zealand Meteorological Service.
White, Ron, 2003: Weather Bomb – The Coromandel Experience. Tephra June, 2003, Vol. 20.

Bay of Plenty

Floods are quite common in the Bay of Plenty, and can be very severe. This is because of the region’s exposure to the north – air arriving from the north tends to be warmer than air coming from other directions, and warm air can carry more moisture than cold air. Heavy rain events in this area generally occur
in strong pre-frontal northerly airstreams. If the front stalls over the Bay of Plenty, very large accumulations can ensue.
Convection is often a factor in heavy rain events. Such events do not need a northerly fetch to produce flooding, but in these cases the flooding is localised.

1944 22 February
“DISASTROUS” FLOOD

Rainfalls

Rotorua: 146mm/24 hrs (return period 10 years)
(mostly in three hours) (return period over 150 years)
East of Rotorua: about 380mm/3 days
Edgecumbe-Urewera strip: Up to 240mm/24 hrs

Stock Losses: Considerable.

1948 17-18th April
RECORD RAINFALL

Rainfalls
Tauranga: 230mm/24hrs (return period 95 years)
incl 221mm/12 hrs (return period over 150 years)
and 212mm/6 hours (return period over 150 years)
and 34mm/10 minutes (return period over 150 years)

Opotiki: 182mm/12hrs (return period over 150 years)
incl 145mm/6 hours (return period over 150 years)

1964 9-11th March
HOMES FILLED WITH
MUDDY WATER

This flood covered most of the Opotiki floodplain and filled nearly all Opotiki’s 650 homes with muddy water. The Whakatane River was also affected. Up to 300mm of rain fell in three days.
A depression which had formed in the north Tasman Sea moved onto Taranaki and combined with another centre near Southland. A frontal band associated with the
first low hesitated as it crossed the North Island. It was finally pushed through by the front associated with the second low. Fig 53 shows the MSL situation at midday 10th March.

The 500 hPa chart for this time (Fig 54) shows a sharp trough in the Tasman Sea. The rain finally eased after both fronts passed across the Bay of
Plenty during the afternoon of the 11th.

1970 11-14th August
WHAKATANE HOMES
EVACUATED

Rainfalls
Te Teko: 295mm/48hrs to 9am 15th (return period 120 years)
incl 262mm/24hrs to 9am 14th (return period over 150 years)
Lake Tarawera: 200mm/24hrs to 9am 15th (return period 95 years)

Evacuations: 100

Roads were closed, and the Rangitaiki River burst its banks at Te Teko and Edgecumbe.
An intense anticyclone lay east of the country, while a broad, complex trough covered the Tasman Sea (Fig 55).

The 500 hPa flow at this time showed a blocking signature (Fig 56).
The trough moved slowly onto New Zealand, and a strong, moist northerly airstream affected Bay of Plenty for a considerable time. The low seen near Northland at midnight on the 13th (Fig 57) subsequently moved southeastwards into the Bay of Plenty, bringing a burst of enhanced rainfall.

1979 21st March
FLOODS DERAIL
GOODS TRAIN

1998
WAIMANA ISOLATED

1999 30th April-2nd May
ROTORUA MARATHON
CANCELLED

2003 6th April
DREDGE WASHED
OUT TO SEA

Rainfalls
Ranger Stn: 175.5mm/6hrs (return period over 150 years)
Te Puke: 156mm/6hrs (return period over 150 years)

2004 17-19th July
100-YEAR-PLUS FLOOD
FOR WHAKATANE RIVER

The sub-tropical low moving southsoutheastwards gradually became the dominant feature, and the flow over the Bay of Plenty gradually turned to a light easterly
(Fig 67). During this time a narrow band of heavy rain persisted over the eastern Bay of Plenty. Even after the northerlies had gone, a zone of low-level convergence persisted in the eastern Bay of Plenty, between northeasterlies and southeasterlies.
Aloft, a major cut-off low lay slowmoving to the west of central New Zealand. A forward-tilting trough, with its associated CVA, moved around the low onto the North Island during the 17th (Fig 68), with the cut-off low then crossing the North Island on the 18th.

2005 3rd May
DELUGE HITS
TAURANGA

2005 18th May
MATATA AND
TAURANGA SWAMPED

Rainfalls
Bay of Plenty:
Tauranga AWS 347mm/24hrs (return period over 150 years)
incl 217mm/12hrs (return period about 150 years)
and 133mm/3hrs (return period over 150 years)
Awakaponga 95mm/1hr (return period over 150 years)
Coromandel:
Hikuai about 900mm/72 hours (return period over 150 years)

Evacuations: About 500.

Bibliography

Cowie, C.A., 1957: Floods in New Zealand, 1920-53: with notes on some earlier floods. Soil Conservation and Rivers Control Council.
NIWA, 2002: HIRDS v2.0 – High Intensity Rainfall Design System
Kerr, I.S., 1944: Synoptic Situations Associated with Flood Producing Rains in New Zealand. New Zealand Meteorological Service Circular Note 33.
McGavin, T. : Severe Weather Events Analysis. http://www.metservice.com/default/index.php?alias=notableweatherindex New Zealand Meteorological Service
Nabi, M., Using the MM5/3DVAR Scheme to Investigate the Impact of Model Forecasts: A Case Study. New Zealand Meteorological Service Student Projects Series.
New Zealand Gazette, 1964 Vol. I. New Zealand Government.
New Zealand Herald 20-22nd March, 1979
Severe Weather Log. New Zealand Meteorological Service.

Gisborne/Hawkes Bay

Easterly airstreams ahead of active fronts, or accompanied by slow-moving fronts, are the most common situations producing heavy rain in the east of the North Island. The most severe events include some convective element. Strong moist easterlies associated with ex-tropical cyclones can also produce heavy rain, the most notable example in recent years being Cyclone Bola (1988).
This area takes the top slot in the country in the extreme events hierarchy at the end of this publication (Appendix 1). The hierarchy is based on the multiplication factor by which the maximum rainfall for the event exceeds its 150-year return period. Using this criterion, the events of 1924, February 1938, 1953, April 1938, 1910, and 1941, all of which occurred in the Gisborne/Hawkes Bay area, take six of the slots in the top ten of heavy rain events in recorded history in New Zealand.
Also of interest is that not one of the above-mentioned events occurred in the last 50 years.

1867 25th May-4th June
BIGGEST FLOOD IN
LIVING MEMORY

Rainfalls
Napier: 381mm/4 days (return period over 150 years)

Stock Losses: Heavy

Several feet of floodwater flowed through Napier, and silt to a depth of 30-50cm was deposited. Land that had not been flooded within the memory of the oldest inhabitant was under water. After initial heavy rain fine weather followed, but on 3rd June rain recommenced, and continued on the following day.

1893 4th December
WAIPAWA RIVER REACHES
HIGHEST KNOWN LEVELS

1897 14-17th April
HOUSES WASHED
OUT TO SEA

Rainfalls
Hawkes Bay:
Tikowhai 507mm/96hrs
Manawatu-Whanganui:
Raetihi 160mm/96hrs
Ruanui 257mm/96hrs
incl 178mm/24hrs (return period over 150 years)

Deaths: 12
Stock losses: Hundreds of thousands
Damage: 150,000 pounds
2012 Dollars: 27 million

Hundreds of families homeless.

“A terrible calamity” was how The Hawkes Bay Herald described this event. Flooding extended from Wairoa to Woodville. It was estimated that three-fifths of the Heretaunga Plains were under water, with roads and railway washed away.
Clive, about 10km south of Napier, was worst affected. Only the tops of the houses were visible in some parts of the town, and in some instances houses were washed out to sea.
Of the people drowned, ten were in rescue boats that had also been swept out to sea.

It was reported that “Eleven and a half inches of rain (290mm) fell in 24 hours”, presumably in the Napier area. Another report gave the Napier rainfall as 355mm in four days. Inland, it was reported that 21 inches (533mm) fell in 30 hours in parts of the higher reaches of the Tukituki, Ngaruroro and Tutaekuri Rivers.
Heavy rain began in inland areas at first, soon spreading to the coast.
Extreme flooding also occurred on the Rangitikei River. In fact, this remains the largest flood in the Rangitikei in European history, with a return period for the flooding of maybe as much as 500 years. All six bridges across the river were destroyed, and settlers in the Lower Rangitikei had several feet of water through their houses. Near the river 100 acres of green forest, including a stand of three-hundred-year-old totara trees, was swept away.
The Whangaehu River also had an extremely high flood, as did the Manawatu River.
The Whanganui River also received a big flood, and southwesterly gales were reported by the Whanganui Herald.
The severe weather was caused by a depression with central pressure around 990 hPa, moving slowly eastwards across the North Island. There is evidence to suggest strong similarities between this event and that which caused the disastrous floods in Manawatu in February 2004. Heavy rain in Hawkes Bay and Wairarapa suggests a flow from the easterly quarter, yet Whanganui reported southwest gales. This is strongly suggestive of convergence between southeast and southwest flows, as occurred in the 2004 event.

1906 14-17th July
THREE METRES OF
WATER IN CAESAR’S PADDOCKS

1910 30th March-1st April
ORMOND VALLEY
A SEA OF WATER

1917 13th June
SEVERAL TOWNS
FLOODED

1924 11th March
MOST EXTREME EVENT
IN KNOWN HISTORY

Rainfalls
Riverbank, Rissington: 512mm/10hrs to 5.30pm 11th (return period well over 150 years)
incl 229mm/2.75hrs to 11.45am 11th (return period well over 150 years)
Tutira: 203mm/3hrs to 10am 11th. (return period well over 150 years)

Deaths: One.
Stock losses: Heavy.

This phenomenal downpour, in Hawkes Bay’s Esk Valley, resulted from a clash of airmasses, with warm air from a depression meeting a surge of cold air to the south. (See Fig 73.) Widespread thunderstorms were occurring within the warm airmass, suggesting that the air was already unusually unstable. This warm air became trapped in Hawkes Bay, then becoming undercut by a cold southerly, leading to violent convection.

Convective rainfalls can on occasion be confined to a single thunderstorm, but in this case the area of 500 mm-plus (20 inches approx) rainfall was quite large (Fig 74).

1936
CYCLONIC STORM

1938 19th February
THE KOPUAWHARA
DISASTER

Rainfalls
Tarewa: 200mm/24hrs (return period 10 years)
Puninga: 204mm/24 hrs (return period 10 years)
Tolaga Bay: 194mm/24hrs (return period 20 years)
Signal Station Gisborne: 263mm/3days (return period 65 years)

Deaths: 22
Road Damage: 43,800 pounds.
2012 Dollars: 4.5 million.

A camp for construction workers building the Wairoa-Gisborne railway had been located about four metres above normal water level on the banks of the Kopuawhara Stream, just north of Mahia Peninsula. It was made up of houses for married men up on higher ground, and a cookhouse and huts for 47 single men close to the river bank. In the early morning hours of 19th February, a five-metre wall of water hit the camp. Most of the huts and part of the cookhouse were swept away, and 21 lives
were lost.
Another life was lost at Boyd’s Camp. Many bridges were overtopped, and road damage was considerable.
The heaviest rain fell away from rain gauges. The Maungakotukutuku River recorded a freak discharge, which has been subjected to considerable review and scrutiny. Testimony to the extremity of the event was the considerable scour, and large boulders, logs, stumps and other debris deposited onto the downstream grass flats. The measured discharge was equivalent to a rainfall rate of 132mm per hour, over an area of 19 square km.

Pressures were high to the south of New Zealand and low to the north, with a northeast flow over the North Island. A mesoscale low developed off Gisborne within this flow, then moved southwards. Torrential rain occurred from East Cape to Hawkes Bay, and southwards to Cook Strait. Electrical activity was a major contributor to the heavy rain. The charts for 3pm 18th and 9am 19th February (Figs 75 and 76) show the dramatic difference from one day to the next, and highlight the extreme difficulty in predicting heavy rain at times.

Flood debris from Kopuawhara No. 4 public works camp,
Mahia Peninsula. 20 Feb 1938
PAColl-4431-01.

1938 23-25th April
ESK VALLEY FLOODS

Rainfalls
Puketitiri: 1001mm/3 days (return period well over 150 years)
Putorino: 815mm/3 days (return period well over 150 years)
Napier: 274mm/3days (return period 100 years)
incl 169mm/24hrs (24th) (return period 50 years)
Hastings: 271mm/3 days (return period over 150 years)
incl 194mm/24 hrs (24th) (return period 130 years)

Deaths: One.
Evacuations: Many
Damage to Roads and
Bridges: 200,000 pounds
2012 Dollars: 21 million

1941 May 4th
PORANGAHAU
INUNDATED

The Porangahau River rose 14 metres above normal and burst its banks, and the township was inundated to depths up to 1.2 metres. The Akitio River rose
15 metres in a few hours, with two bridges washed away. Manawatu was also severely affected by flooding.
A depression moved eastwards across the North Island. A cold front, followed by a strong southeast flow, stalled over southern parts of Hawkes Bay for a
considerable time, as can be seen from Figs 79 and 80.

1948 May 14th
“MOST DESTRUCTIVE”
FOR POVERTY BAY

Rainfalls
Gisborne:
Whatatutu 344mm/72hrs (return period over 150 years)
Kanakanaia 339mm/72hrs (return period 45 years)
Hawkes Bay:
Onepoto 307mm/72hrs (return period 20 years)
Tuai 260mm/72 hrs (return period 15 years)

Stock Losses: over 16,000.
Total Damage: 336,356 pounds
2012 Dollars: 23 million

This flood is still regarded as the most destructive in Poverty Bay history. The flood spread over practically all the flats along the main and tributary streams of the Waipaoa River, including the extensive Poverty Bay flats. 8500 hectares were inundated, and parts of Gisborne City were flooded. Further south, the Wairoa River rose to a record height and flooded buildings in Wairoa township. The flood flow of 11,450 cumecs has been exceeded only by floods on the Buller River.

A low developed on a cold front just east of Gisborne on the 12th. The low drifted slowly east, then northwest. A moist east to southeast flow was maintained over the area right through to the 15th, when the gradient over Gisborne started to weaken, and the low began to drift away northeastwards. Fig 81 shows the MSL analysis for 6pm on the 14th.

The rainfall distribution is shown in Fig 82.

1953 January 27-28th
EXCEPTIONAL
RAINFALLS

Rainfalls
Mangarouhi Valley:
349mm/12hrs (return period well over 150 years)
mostly in 6hrs!
Kauranaki: 224mm/9hrs
(return period well over 150 years)
Wanstead:
233mm/14.5 hrs (return period over 150 years)
Wilder Settlement:
208mm/9hrs (return period well over 150 years)

Stock Losses: Substantial.

1963 June 2-4th
SWIRLING TORRENT FILLS
TONGOIO VALLEY

Rainfalls
Tareha: 310mm/48 hrs (return period approx 100 years)
incl 195mm/24hrs (return period approx 100 years)

Stock Losses: Heavy

1988 March 6-8th
BOLA DEVASTATES
GISBORNE DISTRICT

Rainfalls
Gisborne/Hawkes Bay:
Te Puia Springs 419mm/24 hrs (return period over 150 years)
Aniwaniwa 771mm/3 days (return period over 150 years)
Glenross/Waimata 917mm/4days (return period well over 150 years)
Tutira 649mm/2 days (return period over 150 years)
Northland:
Whangarei 234mm/2 days (return period 30 years)

Deaths: Three
Evacuations: Hundreds.
Damage: $111 million.
2012 dollars: 205 million.

2001 9th December
DELUGE HITS NAPIER AND HASTINGS

Surface flooding caused millions of dollars worth of damage in Napier and Hastings when 50mm of thundery rain fell in the hour before noon – a return-period rainfall of over 150 years. Thundery downpours occurred in Hawkes Bay, Wairarapa, Wellington and Marlborough. Fig 91 shows the area lying in a convergence zone between warm moist northerly air and a cooler southerly.

Fig 92 suggests that overrunning was a significant factor.

2002
SECOND HUNDRED-YEAR FLOOD IN A MONTH

2003 25-28th February
HEAVIEST RAIN SINCE
BOLA IN SOME PLACES

Rainfalls
Gisborne:
Waikura 438mm/48hrs to end 27th (return period 130 years)
Hikuwai 423mm/60hrs (return period 120 years)
incl 381mm/48hrs (return period 100 years)

Coromandel Peninsula:
Pinnacles about 650mm/2days (return period 100 years(?))
Golden Cross about 400mm/2days (return period 35 years)

2004 15-16th February
MAJOR FLOOD HITS LOWER NORTH ISLAND

2004 18th October
LOCALISED STORMS

Thunderstorms caused localised flooding in Napier around 2am, and around 8am northwest of Porangahau.
The thunderstorms formed on and south of a convergence line between northwesterly and easterly flows on the southeastern side of a deepening low.

Napier reported a wind change from southeast to southwest during the thunderstorm episode. Thunderstorms were clustered along the Hawkes Bay coast.

Fig 94 shows several analysis fields for midnight 17th October. It can be seen that while WBPTs were very low, strong CVA and upper divergence were present over Hawkes Bay. Satellite imagery (Fig 95) attests to the fact that a very unstable atmosphere prevailed over the area. Indeed, the sounding from Paraparaumu, admittedly some way from the affected area, produced around 800j/kg of CAPE when edited with the actual Napier temperature and dew point (Fig 96).

2005 18th March
LOCALISED DELUGE IN
NORTHERN HAWKES BAY

2005 21st October
CROPS DEVASTATED
IN LABOUR WEEKEND
FLOODS

Rainfalls
Hikuwai: 371.5mm/36hrs to 11am 22nd (return period 120 years)
incl 273mm/15hrs to 10pm 21st (return period about 150 years)
Te Puia: 365.5mm/36hrs to 10am 22nd (return period 60 years)

Evacuations: 50
Crop Damage: $8.4 million.
2012 Dollars: 10 million.

Bibliography

Cowie, C.A., 1957: Floods in New Zealand, 1920-53: with notes on some earlier floods. Soil Conservation and Rivers Control Council.
Hill, H., 1897: On the Hawkes Bay Plain: Past and Present. Transactions of the Royal Society of New Zealand 1868-1961. National Library of New Zealand website.
Kidson, E., 1930: The Flood Rains of 11th March, 1924, in Hawkes Bay. New Zealand Journal of Science and Technology, Vol. XII, No. 1, pp 53-60.
McLintock, A.H., 1966: The Encyclopaedia of New Zealand. Published on the Internet.
New Zealand Gazette, 1910, January-June. New Zealand Government.
________, 1938, Vol. I. New Zealand Government.
________, 1941, Vol. II. New Zealand Government.
________, 1953, Vol I. New Zealand Government.
NIWA, 2002: HIRDS v2.0 – High Intensity Rainfall Design System
Poole, A.L., 1983: Catchment Control in New Zealand. Water and Soil Misc. Pub. 48.
Salinger, J. in Awesome Forces, 1998: Edited by Geoff Hicks and Hamish Campbell. Te Papa Press.
Sinclair, M.R., 1989: Preprints, 12th Conf. Wx. Analysis and Forecasting, Monterey, California, Oct. 2-6, 1989.
___________, 1993: A Diagnostic Study of the Extratropical Precipitation Resulting from Tropical Cyclone Bola. Mon. Wea. Rev., 121, 2690-2707.
The Hawkes Bay Herald, April 19, 1897.
The Manawatu Herald April 17, 1897, and April 22, 1897.
The Taranaki Herald, April 20, 1897.
The Whanganui Herald, April 17, 1897.

Taranaki/Whanganui/Manawatu

The predominant heavy-rain bearing flow in this area is the northwesterly. A straight northerly flow can bring heavy rain to Taranaki, while the Manawatu River, rising as it does east of the main divide, can receive flood rains in easterly flows. Also in easterly flows, significant spillover can occur on the western side of the Ruahines. Flows from between west and southwest generally do not contain enough moisture to bring heavy rain. Post-frontal northwesterly flows can occasionally cause problems. If the flow is strong and moist, heavy rain can continue, particularly at high altitudes.
That cyclonic southerly flows could bring heavy rain did not become obvious until a devastating flood in the Manawatu in February 2004. Research for this publication revealed that the 1897 flood, which still holds the record for the highest known flow in the Rangitikei River, also occurred in a cyclonic southerly flow. What happens is that broad-scale southeasterly winds converge over the Manawatu/Central Plateau area with south or southwesterly winds that have come through Cook Strait.

1880
MAJOR MANAWATU RIVER FLOOD

1897
BIGGEST FLOOD ON RECORD
FOR RANGITIKEI RIVER

1902 13-15th June
BIGGEST KNOWN
FLOOD FOR
MANAWATU RIVER

1904 22-25th May
WHANGANUI
“GREAT FLOOD”

Rainfalls
Whanganui:
102mm/3 days (return period 15 years)

Stock Losses:
Considerable.

Opinions differ as to which flood was bigger for Whanganui: 1904, or 1940. However, the 1904 event was the highest known at that time for the town, and was probably the bigger of the two. East Town in Whanganui township was extensively flooded.

A 992hPa low moved southeastwards onto the South Island, and another (985hPa) followed it. A prolonged period of northwesterlies affected the Whanganui River headwaters. Figs 101, 102 and 103 are the original charts for 9am on the 24th, 25th and 26th respectively. Fronts were not analysed in those days, but a possible frontal analysis is superimposed in red. This suggests that a front remained stationary over or near the area for three days.

1935
WORST FLOODING TO DATE
FOR NEW PLYMOUTH

1940 24-25th February
CENTENARY FLOOD

Rainfalls
Central Plateau:
Taumarunui 147mm/24hrs (return period 125 years)

Taranaki:
Whangamomona 229mm/24hrs (return period 130 years)
New Plymouth 150mm/24hrs (return period 30 years)
Tangarakau 218mm/24hrs (return period over 150 years)

This flood occurred during the year which celebrated 100 years since the signing of the Treaty of Waitangi. For Whanganui this was the third major flood in half
a century, equalling, in the opinion of many, the great flood of 25th May 1904. A period of southeasterly rain may have produced comparable flooding at the mouth, even if flooding in the upper and mid reaches was higher in 1904. At the height of the flood, one of the Whanganui River steamers sailed over the top of a bridge and into a garden, where peaches were picked from the tree tops.

In Taranaki, parts of New Plymouth were under water, and roads everywhere were blocked by slips and washouts. The RNZAF dropped supplies to remote settlements in north Taranaki which were isolated by floodwaters.

On the 20th a high centre moved to the northeast of the country, where it remained for several days. On the 23rd rain commenced in Taranaki. A disturbance approached from the Tasman Sea, and one of its centres developed into a small but active depression.
The southeastward passage of this low brought the heavy rain to the central North Island (Fig 104).
The rain occurred firstly within a long fetch of northerlies, and later in a southeast flow as the low moved away.

Fig 104. MSL analysis for 3pm, 24th February, 1940

(central pressure in the low is about 985 hPa).

1941
EXTENSIVE FLOODING IN MANAWATU

1953
MAJOR FLOOD FOR MANAWATU RIVER

1971 23-24th February
WORST FLOODING
IN NEW PLYMOUTH’S
HISTORY

New Plymouth’s city centre was flooded, with shops ruined and the main routes in and out of the city closed. Wide areas of Waitara were covered with metres of water, and in rural Taranaki roads were closed by slips and flooded rivers, and schools were closed.

A low developed just south of Norfolk Island on the 21st, to the south of a weak tropical depression. The low then moved southwards while deepening, with a northeast flow developing over the North Island. Frontogenesis then occurred west of the North Island, downstream from a strong upper trough in the Tasman Sea. The MSL chart for 6am on the 24th (Fig 105) might suggest that the flow over Taranaki was anticyclonic. However, the tropical depression had played a role in bringing very
warm and moist air down into New Zealand latitudes, and strong ascent was occurring within a warm conveyor belt over Taranaki.
Fig 106 shows that the rain was associated with a sharp upper-level trough.

1990 8-11th March
CYCLONE HILDA

Rainfalls

Pukeiti: 751mm/6days incl 445mm/24hrs to 9am 9th (return period 140 years)
Lake Mangamahoe: 498mm/6 days incl 294mm/24hrs to 9am 9th (return period over 150 years)
North Egmont Visitor Centre: 970mm/6 days
incl 419mm/24hrs to 9am 9th (return period 45 years)

Stock losses: Major.

Although it did not come anywhere near New Zealand, ex-tropical cyclone Hilda was blamed for this event, with moisture from the system finding its way onto
Taranaki.
Waitara was evacuated owing to fears of a breach which did not eventuate. The Whanganui River broke its banks though, leading to the evacuation of several houses. Farmland was inundated, and there was extensive damage to roads, with many remote parts isolated.
Hilda moved into the north Tasman Sea, then transitioned on the 8th into a large low-pressure area.
The heaviest rainfalls occurred in the 24 hours to 9am 9th March.

The MSL analysis for midnight 8th March (Fig 107) shows a very anticyclonic-looking flow over Taranaki, and the low-level flow over the area is quite northeasterly.
However, the 500 hPa analysis (Fig 108) shows air at this altitude being brought from very low latitudes.
This air was rising as it moved through the eastern Tasman Sea and onto Taranaki – this is the warm conveyor belt (WCB). Satellite imagery (not shown) revealed the frontal band extending right up into tropical latitudes. Very humid air was being advected onto Taranaki – dew points reached as high as 21C.

The similarity to the 1971 event was quite marked, and the main causes of the heavy rain were the same, viz.:

1. a tropical cyclone bringing tropical air into the north Tasman Sea, which was subsequently dragged further south by troughing,

2. a strong WCB, and

3. a blocking high to the east.

1995 20-21st April
THE BIG WET

Rainfalls

New Plymouth AWS: 134mm/5hrs (return period over 150 years)

New Plymouth was cut off by floodwaters, with flooding in some central city streets. There were landslides in the city, drain covers popped, and raw sewage spilled across the streets. However, the heavy rain was quite localised, with no major rivers or streams breaking their banks.

On the 19th and 20th a high to the east extended a ridge across the North Island, while a front approached from the west. The front moved onto Taranaki and then stalled.

Fig 111 shows the MSL chart for midday on the 21st. It was just before this time that the New Plymouth AWS received its downpour of 134mm in 5 hours. The flow over Taranaki looks anticyclonic, as it did for much of the period of the 1971 and 1990 Taranaki floods. The rain eased off shortly after this time, but it did continue to fall, with some heavy bursts, right through to the 23rd. Flooding in Taranaki was not as severe as in the events of 1971 or 1990. Blocking was again a major factor, and the WCB affected the area for a considerable time. The difference was probably the absence of a long fetch from the tropics.

2004 15-16th February
PARTS OF MANAWATU
DEVASTATED

Rainfalls

Manawatu-Whanganui:
Apiti, Table Flat 200mm/24 hours (return period 130 years)
Kopua 176mm/24 hours (return period 100 years)
Dannevirke, Pine Gr. 225mm/24 hours (return period over 150 years)

Wellington:
Birch Lane (Lower Hutt) 234mm/48hrs (return period 50 years)
incl 199.5mm/24hrs (return period 50 years)
and 161.5mm/12hrs (return period 50 years)

Hawkes Bay:
Shagrock 228mm/24hrs (return period 150 years)
incl 48mm/1hr (return period 150 years)
Wallingford 197mm/24 hrs (return period 125 years)

Evacuations: Many
Stock Losses: High.
Damage: $112 million.
2010 dollars: 140 million.

This was the most devastating flood in New Zealand for many a year. Insurance payouts of 112 million 2004 dollars made it the fourth most expensive natural disaster since 1968 – after the 1987 Bay of Plenty earthquakes, the loss of the Wahine in 1968, and the Otago/ Southland floods of 1984.

Farmland was inundated, and stock losses were high. The region lost 100 million cubic metres of soil (one-sixth of Mount Ngauruhoe), which was either deposited on farmland or washed out to sea. It was the third largest flood on record for the Rangitikei River, after 1897 and 1926, while for the Whangaehu River it was the biggest, much bigger than the 1897 event.

In the Wellington Region slips and flooding were widespread, with 50-year flows in the Ruamahanga River in the Wairarapa, also the Waiwhetu Stream in the Hutt Valley.

In Hawkes Bay there were evacuations in Porangahau Village. Fences and gates were demolished, and there were numerous slips, with roads blocked.

Rainfalls exceeded the 150-year return period over a wide area, coloured purple in Fig 112.

Fig 112. 24-hour rainfall return periods for the storm of 15-16th February, 2004.
Legend:
Over 150 years – Purple.
Over 100 years – Dark blue.
Over 75 years – Light blue.
Over 50 years –Teal.
Over 25 years – Dark green.
Over 10 years – Light green.
Over 5 years – Yellow.
Over 2 years – Orange.
Less than 2 years - White.

See Acknowledgements

This figure is taken from a case study of a winter storm that dumped up to 40cm of snow on the central plateau. Low-level air blowing through Cook Strait is being overrun by air of higher potential temperature coming from the southeast (the warm conveyor belt), with warm advection adding to the upward motion.

In the February 2004 event, an intrusion from the north of dry air with high Isentropic Potential Vorticity added deep instability to the mix. The 2004 storm in fact resulted from a polar outbreak more typical of winter – but with temperatures much higher, precipitation amounts were correspondingly much greater.

Another interesting aspect of the storm was the presence of a striated cloud head over the southern North Island as well as a striated delta further east. The striations over the southern North Island can be seen clearly in Fig 115.

On the right of the picture can be seen part of the cirrus shield of a larger striated delta – this pattern has been associated with major cyclogenesis. The striations over the North Island were moving westwards, with both wavelength and amplitude increasing with time and distance downstream. This suggests that energy was being fed into the system. The striations showed both in radar returns, and as peaks in rainfall intensity (not shown).

Bibliography

Dixon, R.S., K.A. Browning, and G.J. Shutts, 2000: The mystery of striated cloud heads in satellite imagery. Atmospheric Sciences Letters.
Feren, G., 1995: The “striated delta” cloud system – a satellite image precursor to major cyclogenesis in the eastern Australian – western Tasman Sea region. Wea. and Forecasting, 10, 286-306.
Horizons (Manawatu-Whanganui Regional Council) website: www.horizons.govt. nz
Miller I.D., 2005: Some 3D aspects of a striated cloud head over New Zealand including radar and raingauge data. www.ams.confex.com/ams/pdfpapers/97036. pdf
New Zealand Gazette, 1940, Vol. I. New Zealand Government.
______, 1926, Vol. III. New Zealand Government.
New Zealand Herald 8th November 1965
______, 12th March 1990
______, 21st April 1995
NIWA, 2002: HIRDS v2.0 – High Intensity Rainfall Design System
Pascoe, R.M., and P.E. Bruce, 2003: Cyclogenesis East of the North Island and the Central North Island Snowstorm of 15th August, 2001. Unpublished. (Available
from New Zealand Meteorological Service.)

Wairarapa/Wellington

The weather in this area is greatly influenced by Cook Strait. In Wellington there can be only two rain-bearing wind directions because there are essentially only two wind directions – north and south. In Wairarapa, the main flood-bearing wind is the northwesterly. The Tararua Ranges can receive huge rainfalls, which often spill over well to the east of the divide, and because the Tararuas are so steep the Wairarapa rivers rise very quickly. Wairarapa can also receive heavy falls in easterly flows, the brunt of which is borne by the eastern hills.

Wellington City and the Hutt Valley get their heaviest rainfall in southerlies. Forecasters particularly look out for where a shallow southerly is overlain by moist northerlies – the so-called “overrunning situation”. However, for overrunning to produce extreme rainfalls there needs to be other forcings at work, such as upper-level divergence.

1858 17th January
WATER FROM HILL TO HILL

An early settler wrote: In the year 1858 there was a tremendous flood in the Hutt River; the water reached from hill to hill; great trees came floating down carrying away houses with them; thirteen people were drowned.

As a result, many settlers left the Hutt Valley.

1880 24th March
BIGGEST FLOOD ON RECORD
FOR WAIRARAPA

1897
WIDESPREAD FLOODING

1898 17th June
EQUAL BIGGEST IN THE HUTT RIVER IN PAKEHA TIMES

This flood was of a similar magnitude to that of 1858. These two floods have been estimated to have reached 2000 cumecs in the Taita Gorge, and as such are thought to have been the biggest floods in the Hutt River in Pakeha times. Water was knee deep in Lower Hutt township.

Surprisingly, this was a northerly rainfall. A period of northerly gales brought minor wind damage, and the railway line was washed out near Paraparaumu. Snow melt was thought to have been a factor.

1924 17-19th December
MASTERTON FLOODED

1932 28-30th August
WORST KNOWN
FLOODING FOR
SOUTHERN WAIRARAPA

Rainfalls
Putara: 370mm/3 days (return period 70 years)
including 203mm/24 hrs (return period 15 years)

Stock losses: Heavy.

Widespread heavy rain affected the lower North Island over a three-day period. Several Wairarapa rivers broke their banks, and the town of Masterton was flooded. Particularly badly affected was southern Wairarapa, where the flooding reached record proportions.

Individual rainfalls were not unusually high, but heavy rainfalls were sustained over three days, and affected all parts of the Wairarapa catchments. In this case a period of northwesterly rain was followed by a period of southeasterly rain. Snow melt in the northwesterly stages of the event added to the flooding.

Fig 116 shows a strong northwest flow onto the area.

In Wellington there were several gusts exceeding 100km/hr, while at Eketahuna the worst gale for 20 years uprooted trees and damaged buildings.
Fig 116 shows a wave developing west of central New Zealand. This wave had the effect of slowing the front.

Subsequently a cut-off low developed from this wave (Fig 117).

A weak southerly reached the southern North Island, and there was a considerable period where this weak southerly was overlain by the moist humid northerly
airstream. The heaviest rainfalls in Wairarapa occurred in the 24 hours to 9am on the 29th, the flooding reaching its climax on
the night of the 28th.

In winter, this is a classic snow-producing situation, and in fact snow fell to amazingly low levels, with 20cm on the ground near Masterton and 15cm at Featherston.

1939 11th December
BIG HUTT RIVER FLOOD

Rainfalls
Kelburn: 127mm/13 hrs (return period 130 years)

Stock Losses: Large

This flood, described at the time (and still today) as the greatest in living memory, covered hundreds of hectares, isolated houses, flooded roads, destroyed and damaged bridges, and drowned large numbers of stock. There were also slips and flooding in Wellington City. At one point at Silverstream the entire flat between the hills forming the valley was one sheet of turbulent water, fences and roads being invisible.

The storm was brought by a deep low moving eastwards over northern New Zealand. Northeast gales blew over the Auckland Peninsula, while strong to gale southerlies were experienced around Wellington.

1947 27-29th June
WORST SINCE 1897

Rainfalls
Masterton:
129mm/24hrs. (return period 50 years)
Greytown: 106mm/24hrs (return period 20 years)
Martinborough: 182mm/24hrs (return period over 150 years)

Deaths: One.

Stock Losses: Large.

A week of southerly rain brought the worst floods to the Wairarapa since 1897. 9000 sheep and 300 cattle were lost, and 60,000 acres flooded. Masterton and many other Wairarapa towns were flooded, and southern Wairarapa was cut off. It was a hundred year event for the Ruamahanga River.

The main episode occurred between the 26th and the 28th, with a long period of strong, cyclonic southerlies affecting central New Zealand.

1976 20th December
THE “HUTT VALLEY
FLOODS”

Rainfalls
Taita: 230mm/8hrs (return period well over 150 years)

Damage: $30 million
In 2012 dollars: 218 million.
Evacuations: Many.
Deaths: One.

When one mentions “The Hutt Valley Floods” this is the event which comes to the mind of many people, although February 2004 has doubtless supplanted that in the minds of some. The photograph of the Petone overpass (below) shows that this was an event of unusual severity. Several houses in the Hutt Valley were demolished, and Lambton Quay in Wellington lay under 30cm of water.

The heavy rain was produced by mesoscale processes, the synoptic situation, shown in Fig 118, being nothing out of the ordinary.

The cause was a convergence line,extending from the hills west of Wellington City and up the Hutt Valley, between weak southeasterlies and weak northwesterlies. Fig 119 shows the position of this convergence line at 9am 20th December. Contributing factors were high instability and considerable upper-level moisture.

During the period of the intense rainfall, the winds through a deep layer of the atmosphere (2km to 10km) above the convergence line appear to have been northeasterly in direction, and quite light in strength. The dashed arrow in Fig 119 indicates the direction of this upper-level flow. This northeasterly, containing a high
level of moisture, overran the cooler and denser southerly air below.

Fig 120. 24 hour rainfall during 19 and 20 December 1976. All points within the dashed line received 24 hour rainfalls of return period greater than 50 years.

1998 26th June
FLASH FLOODING
IN WELLINGTON

Rainfalls
Kelburn: 69.5mm/1hr 40mins to 9.10pm (return period over 150 years)

Evacuations: Several.

Vehicles were submerged and several properties damaged when a short period of intense thunderstorm activity struck parts of Wellington. People were evacuated from homes in Thorndon and a hotel in Oriental Bay which were struck by slips. Lightning also caused power cuts. In Karori, 80mm was reported to have fallen in an hour
and a quarter.

Fig 121 shows the MSL situation for midnight on the 26th.

A cold front had moved onto the southern North Island, followed by a southerly change. Nothing appeared to be particularly unusual, but cooling aloft, relatively humid air at low levels ahead of the front, and strong CVA ahead of an upper trough caused high instability in a strongly convergent low-level flow. Multi-cellular
cumulonimbus clouds developed over the hills west of Wellington.
Heavy rain had been forecast for the Tararuas only. However, the heavy rain around Wellington affected only a small area, from Karori to the CBD.

2002 10th January
CENTRAL WELLINGTON
FLOODED

Rainfalls
Wellington:
Kelburn over 50mm in less than 1 hour (return period over 150 years)

Hawkes Bay:
Hastings 77mm in 90 minutes. (return period well over 150 years)
Napier 70mm in 90 minutes. (return period over 150 years)

Water was knee-deep in Wakefield and other city streets after a violent thunderstorm struck Wellington City. Slips caused the evacuation of homes. The trigger was the sea-breeze convergence zone.

Thunderstorm activity also occurred in Wairarapa, within a shallow trough of low pressure containing humid, unstable air and light winds. Further north, streets and properties were flooded in Napier and Hastings, in what was described as the second hundred-year event in a month.

Fig. 122 shows the likely sounding over Wellington that day, adjusted from the actual Paraparaumu sounding. It shows the deep instability and high, ‘skinny’ CAPE characteristic of soundings within high rainfall storms. The low storm motion also contributed to the high rainfall. The “straight-up-and down” structure of the storm, as seen in the radar cross-section (Fig. 123) points to the low shears that prevailed over the area, leading to intense rainfall over a small area.

2003 3-4th October
LANDSLIDES AT PAEKAKARIKI

A deluge struck the Kapiti Coast, caused by a slow-moving convergence zone between northeast winds to the north and northwest winds to the south. A Convair freight aircraft became caught in severe icing and crashed into the sea, killing both pilots. Paekakariki was worst hit. A huge landslide crashed into a motel, and the Paekakariki Hill Road was rendered impassable by slips. State Highway 1 was flooded, with motorists trapped, and the Kapiti Coast was declared a disaster area.

The severe flooding affected only a small area. A private weather station 3 to 4km north of Paekakariki recorded 92mm in 24 hours, including about 60mm in 6 hours.
However, rainfalls were likely to have been considerably higher in the hills immediately above Paekakariki. Fig 124 shows the radar rain accumulation for the period 7am to 10pm 3rd October 2003. Based on this, the return period of the rainfall was estimated at greater than or equal to 120 years.

Fig 124. Radar rain accumulation for 7am-10pm 3rd October 2003.

A maximum of 93mm lies just south of Kapiti Island, and another of 124mm in the Marlborough Sounds.

2004
WAIWHETU STREAM FLOODS HOMES

2004
MIRAMAR FLOODED

2005 30th March
COASTAL WAIRARAPA
SETTLEMENTS
ISOLATED

Residents of the coastal settlements of Mangatoetoe and Mataikona were isolated for several days. Bridges, fences and roads were washed away, and dozens
of houses were extensively damaged. Thousands of tonnes of logs – debris from forestry harvesting – were catapulted over farmland and hurled through the sides of farm buildings, wiping out equipment and killing farm stock. There was intense electrical activity out to sea, which encroached onto the coast in some places. Southern parts of the Wainuiomata Coast Road were hit by the worst flooding in living memory, with the road washed out in several places. One place (Homeburn) recorded 444mm in 36 hours.

Waipukurau and Waipawa, in central Hawkes Bay, experienced 25 to 30mm of rain in 15 minutes (return period about 150 years), and water poured
into four businesses. There was also flooding in Picton, in the Marlborough Sounds.

The extreme rainfalls were the combined result of a frontal band, with strong gradients and high dew-point air, which remained slow moving over the area for a considerable period, and convective activity embedded within the front (see Fig. 127).

Bibliography

Kidson, E., The Wairarapa Floods of August 1932. New Zealand Journal of Science mand Technology. Vol. 14, No. 4. p. 220-227.
Kilmister Reminiscences, Alexander Turnbull Library, MS 1117, dated 12 May 1932.
McGavin, T. : Severe Weather Events Analysis. http://www.metservice.com/default/index.php?alias=notableweatherindex New Zealand Meteorological Service
Mosley, M.P., and C.P. Pearson, 1997: Floods and Droughts: the New Zealand Experience. New Zealand Hydrological Society.
New Zealand Gazette, 1940, Vol. I. New Zealand Government.
NIWA, 2002: HIRDS v2.0 – High Intensity Rainfall Design System
Poole, A.L., 1983: Catchment Control in New Zealand. Water and Soil Misc. Pub. 48.
Severe Weather Log. New Zealand Meteorological Service.
Tomlinson, A.I., 1977: The Wellington and Hutt Valley flood of 20 December 1976. New Zealand Meteorological Service Technical Information Circular 154.
Wellington Regional Council Website. www.gw.govt.nz

Marlborough/Nelson

A strong northerly ahead of an active front is the main bearer of heavy rain for the top of the South Island.

Marlborough is most vulnerable in flows from 330 to 360 degrees. As the flow tends more northerly, spillover onto the south side of the Wairau River tends to increase. The event of July 1983, one of the biggest known floods, occurred with isobars from almost due north. As the flow tends northeasterly, the proportion of total rainfall spilling south of the Wairau continues to increase. However, total rainfall tends to decrease, with the area gaining increasing shelter from the North Island. As the flow tends more easterly, Marlborough again becomes exposed, and the catchment of the Taylor River, which runs through Blenheim, receives its heaviest falls in flows from between northeast and southeast.

If a dominant direction for an impending event can be identified for Nelson, this is very useful information to hydrologists in determining which river catchments will be affected. The wettest part of the Nelson area is the Tasman Mountains, which receive rain in the more westerly flows. The northeasterly can be extremely wet in Golden Bay. Heavy rain can occur in this area even if there is a southeasterly blowing out of Cook Strait, as convergence develops between this relatively localised
southeasterly and the broadscale northeast flow.

Purely convective major flooding events are rare, but the summer months can see big thunderstorms developing in the high dry mountains south and southeast of Blenheim.

1866 1-3rd February
PROLONGED RAINFALL
LEADS TO SEVERE FLOODING

1867 27-28th January
UNUSUALLY HIGH
RAINFALL

1868
VAST SHEET OF WATER

1877
7th February
BIGGEST ON RECORD FOR MOTUEKA RIVER

1923 5-8th May
BLENHEIM
UNDER WATER

Rainfalls
Marlborough:
Spring Creek, 165mm/2 days (return period 50 years)
incl 102mm/24hrs (return period 10 years)
Duntroon, 234mm/2 days (return period over 150 years)
incl 161mm/24hrs (return period 65 years)

Canterbury:
Emscote, Stag and Spey 934mm/4 days (return period well over 150 years)
incl 775mm/2 days (return period well over 150 years)
and 424mm/24hrs to 9am 7th (return period over 150 years)
Keinton Combe, 366mm/24hrs (return period well over 150 years)
incl 330mm/12hrs (return period well over 150 years)
Christchurch: 86mm/24hrs (return period 15 years)

Deaths: Two.
Stock Losses: Huge
Road and Public Works Damage: 41,800 pounds.
2012 dollars: 3.9 million.

This was the worst of several events to have flooded Blenheim in its history. The town was inundated to a depth of 1.2 metres, and rowing boats plied Market Place. There were huge stock and property losses. The town is now protected by a detention dam. It is estimated that 125-150mm fell in Blenheim overnight (return period over 150 years) but the gauge overflowed. This event and that of 1868 are the only two known occasions when the weather pattern changed from eastsoutheast to
northnorthwest (or vice versa) and brought major floods to both Wairau floodplain river systems.

There was also severe flooding along the Kaikoura coast, and in Canterbury. Phenomenal rainfall was recorded at Keinton-Combe, and at Emscote, Stag and Spey, putting this event into the top five in the hierarchy (see Appendix 1). Three spans of the Waiau River bridge were washed away, while the Conway and Hurunui River bridges were completely destroyed. There was also extensive flooding in Christchurch.

On the 3rd, there was evidence of the development of a low to the west of the country, and by the 5th the low was centred west of Waikato. At the same time pressures were rising to the south, leading to a strong easterly gradient over Canterbury and Marlborough. The low was very slow-moving, and did not start to move away to the northeast of the North Island until the 9th.

Flooding, Market Street North, Blenheim ca 1930s. 1/4-016885-G.

Permission of the Alexander Turnbull Library, Wellington, must be obtained before any re-use of this image.

1924 1-2nd November
HIGH RAINFALL AT PICTON

1926
THIRD BIGGEST KNOWN FOR WAIRAU RIVER

1931 3-4th April
BIG RAI VALLEY FLOOD

Rainfalls
Rai Valley: 563mm/40 hrs (return period well over 150 years)
incl 290mm/14hrs (return period over 150 years)
Yncya Bay: 343mm/48hrs (return period well over 150 years)
Bainham: 611mm/48hrs (return period well over 150 years)

West Coast:
Karamea 428mm/24hrs (return period well over 150 years)

Stock Losses: Large.
Road Damage: 5400 pounds
2012 Dollars: 540,000.

The Rai River rose by over six metres. Considerable areas in the Rai Valley were flooded, and the township was cut off. Much of the countryside was scarred by slips, and roads were blocked.

A low developed off the Queensland coast. It deepened as it moved southeast, and crossed the central South Island. Northerly gales affected a wide area, with Cook
Strait experiencing its most severe northerly gale for many years. There were large floods in the Hutt and Otaki Rivers in the lower North Island.

1933 31st January
RAI FLOODED AGAIN

Rainfalls
Rai Valley: 512mm/36 hours (return period well over 150 years)

Deaths: One.

1939 25th November-2nd December
BIGGEST FLOOD ON RECORD
FOR WAIMEA RIVER

Takaka was isolated, with parts of the town under one and a half metres of water. Old settlers stated that the flooding was the heaviest in the history of the district.

Closer to Nelson City, this is the biggest flood on record for the Waimea River.

A blocking high near the Chathams held up a front. In the early stages of the event, the geostrophic flow over the area was northeasterly, and winds in Cook Strait were southeasterly. The low level flow over the Nelson Province was from the eastnortheast. With time, the flow gradually shifted around to the north, as rain continued to fall. The rain had started on the 25th November, and did not cease until the front cleared the area on the 2nd December.

1954 16-17th June
TREMENDOUS
RAINFALL AT COBB

1967 10th August
WORST SINCE 1920’S

Flooding caused in Takaka by this event was variously described as “the worst and most damaging floods in (the region’s) history”, and “the worst since the 1920s”. Many homes in Takaka were invaded by floodwaters.

Blocking was a major factor. An intense anticyclone lay well east of New Zealand. An approaching cold front was slowed in its eastwards progress, before finally pushing through the Golden Bay area on the morning of the 11th. Fig 129 shows the analysis for noon on the 10th.

The 500 hPa flow (Fig 130) shows the typical blocked situation that leads to so many extreme rainfalls. It features a medium wave trough over eastern Australia, with short wave troughs moving through it; and a medium wave ridge east of New Zealand.

1970 30-31st August
THE BROOK STREAM FLOOD

Rainfalls
Nelson Airport:
137mm/24hr to 9am 30th (return period 50 years)
incl 102mm/6hr to midnt 29th (return period 100 years)
Roding: 233mm/48hrs (return period 65 years)

Deaths: Two.

A long spell of wet weather culminated in two days of incessant rain. All the streams flowing through Nelson City overflowed. One of them, Brook Stream,
increasing its usual flow by more than 100 times, took off in new directions. Property, bridges and servicing pipes were extensively damaged, and one person was drowned. Another died when her house was hit by a slip. A blocking high to the east led to a frontal band parking itself over theNelson area (Fig 131).

The 500 hPa pattern (Fig 132) shows the typical blocked pattern.

1975 April 1st
LARGEST PEAK FLOW
INTO COBB RESERVOIR

Rainfalls
Nelson:
Cobb Dam 121mm/17hrs (return period 3 years)
Uruwhenua 198mm/24hrs (return period 8 years)

Marlborough:
Onamalutu 157mm/24hrs (return period 25 years)
Waihopai 75mm/24hrs (return period 3 years)
The Branch 127mm/24hrs (return period 20 years)

Damage: Minor

Effects from this flood were not severe, but it did produce the largest ever peak flow into the Cobb reservoir. In fact, the weather situation that produced the high
rainfall was very similar to that of the 1954 flood – also a major event for the Cobb. In the Wairau River, the flow was similar to that of 1954. The cause was a“bomb” that developed from a wave near Lord Howe Island.

Development occurred in the equatorwards entrance to a 150 kt jet, and in an area of strong vorticity advection. (Fig 133).

Figs 134 and 135 show a deepening of the low from 998 hPa to 968 hPa in an 18-hour period.

This is almost two bergerons. (A bergeron is considered the threshold to be called a “bomb”). Deepening rates of this magnitude are rarely seen in the New Zealand area.

A period of very strong northwest winds affected the northern South Island ahead of the front, with the wind strength supplemented by the rapid movement of the low, and by outflow from thunderstorms accompanying the front.

No heavy rain warning was issued for this event.

1976 9th April
MAJOR FLOOD FOR
MOTUEKA AND RIWAKA
RIVERS

Rainfalls
Moss Bush: 151mm/6hrs (return period 111 years)
incl 139mm/4hrs (return period 171 years)
and 92mm/2hrs (return period about 110 years)

Deaths: One.

A small but fairly intense low moved southeastwards over the northern South Island. The rain included a short period of localised very high-intensity rain in the Riwaka area, generated within a convergence zone between northeasterlies and easterlies. A lady standing on
a bridge in the Graham Valley died when it was swept away.

The MSL chart for 9am 9th April is shown in Fig 136. At this time, the wind at Farewell Spit was 070/25kt, while a 20kt easterly prevailed at Nelson City, and there were southerly gales in Cook Strait.

Upward motion was assisted by strong upper divergence, as illustrated in the strongly difluent flow that can be seen over the northern South Island in Fig 137.

There was also CVA over the area.

1983 15th April
COBB DOWNPOUR

Successive 30-minute rainfalls at Cobb were 31mm, 38mm, 35mm and 61mm, with a maximum hourly fall of 106mm.

A low moved southeastwards from the northwest Tasman Sea, and an associated front became stationary over the northern South Island for a time. The low
weakened away as it crossed the northern South Island.

However, the extreme rainfall occurred while the low and cold front were still well out in the Tasman Sea.

Fig. 138 shows the mean sea level analysis at the time the heavy rain started. This was obviously a localised convective event.

Fig. 139 shows the area lying in the equatorwards entrance zone to a jet, but apart from that there is little clue to the cause of the extreme rainfall.

1983 July 8-9th
RIVER WORKS
OVERWHELMED

Rainfalls
Marlborough/Nelson:
Uruwhenua 462mm/48hr (return period 140 years)
Patriarch 338mm/48hr (return period over 150 years)
Wairau Valley 225mm/48hrs (return period 130 years)
The Leatham 256.5mm/48hrs (return period over 150 years)

West Coast:
Reefton 214mm/48hrs (return period 110 years)
Upper Mai Mai 216mm/24hrs (return period 100 years)

Number evacuated: 500 plus.
Stock losses: About 26,000.
Damage: $6-7 million
2012 dollars: 18-21 million.

The Wairau flood control scheme failed to cope with the July 1983 event. Floodwaters spilled out over much of the Wairau Plain, causing hundreds of evacuations. The flow had a return period of about 150 years, and was probably comparable to that of the 1868 event.

Residents of Takaka were also evacuated. Here floodwaters were reported to be 30cm deeper than in the 1967 event.

On the West Coast, extreme rainfalls were recorded in the Reefton area and in the Grey Valley. However, floodwaters did not enter Greymouth.

The synoptic situation over the period of this storm featured a complex, slow-moving depression covering the whole of the Tasman Sea, with an anticyclone east of the
country also slow-moving. A narrow band of moist tropical air flowed onto the country for a period of 48 hours during the 8th, 9th and 10th of July.

The mean sea level analysis for midday 9th July, 1983 is shown in Fig 140. A broad frontal cloud sheet covered the whole country (Fig 141).

A large upper level low covered much of the Tasman Sea (Fig 142).

The warmth of the air at low levels made the air unstable, thus assisting the upward motion process.

The direction of the flow was also a critical factor in the magnitude of the flooding. The flow was very northerly, which meant that spillover to the south side of the Wairau River was very high, making it a “whole catchment” event. Strong winds also contributed to high spillover.

The rainfall pattern (Fig 143) shows the typical distribution of rainfall in this area in northerly flows.

A heavy rain forecast was issued for this event, but the amount forecast was only “120mm or more in the next 24 to 36 hours”. The acquisition of radar coverage has been a great help in forecasting for this area.

1985 10th January
ELECTRICAL STORM
WREAKS HAVOC

Rainfalls
Roding: 198mm/24hrs (return period 65 years)
incl 141mm/4hrs (return period well over 150 years)
Takaka Hill: 144mm/6hrs (return period 70 years)
incl 133mm/4hrs (return period 130 years)

Homes Evacuated: Two

Nelson residents had to resort to boiling their drinking water after flooding washed away the inlet to the Roding River reservoir. At Pelorus Bridge fifteen campers were evacuated by front-end loader. In Golden Bay Takaka was isolated, two homes at Rockville were evacuated, and hundreds of cattle were washed down the Aorere River.

This was a major event for the Roding area (south of Richmond). The main front, as seen in Fig 144, consisted of an active line of convection. (The second front behind it was weaker.)

A difluent 300 hPa flow (not shown) affected the northern South Island, with a trough lying just west of the South Island.

1990 12th August
GOLDEN BAY ISOLATED

Rainfalls
Moss Bush: 349mm/72hrs (return period 90 years)
incl 314m/48hrs (return period 116 years)
and 224mm/24hrs (return period about 50 years)

Deaths: One.
Evacuations: 45.

Flooding cut roads and damaged homes. A civil emergency was declared for Motueka, and twelve people were rescued from Riwaka by coastguard boat. Access to Golden Bay was cut.

A large low moved from the central Tasman Sea southeastwards to cross the central South Island. Central pressures within the low were down to about 980hPa. A moist northeast flow affected the Nelson area for a considerable period. A warm front crossed the area at 6am 11th. The MSL chart for noon 11th August is shown in Fig 145.

At this time the wind at Stephens Island was 100/35kt, while that at Farewell Spit was 060/30kt. This indicates low level convergence within the Nelsonregion. Nelson City was reporting continuous heavy rain at this time. The small low seen off the West Coast was moving southsouthwestwards, and its presence helped prolong the
period of heavy rain. During the latter stages of the event, several convective bands crossed the area, as the flow turned slowly around to the northwest.

The 500 hPa chart for midnight on the 11th (Fig 146) shows the characteristic blocked pattern. A negative-tilt trough can be seen extending onto northern and central New Zealand from the Tasman Sea.

2004 17th February
MASSIVE RAINFALL
FLOODS WAIKAWA

Rainfalls
Marlborough:
Waikawa 160mm/24hrs (return period 25 years)
incl 85mm/1hr to 8.15am (return period well over 150 years)
and 55mm/30 mins to 7.35am (return period well over 150 years)

Wellington:
Miramar 30mm/1 hour (return period 35 years)
incl 14mm/10mins (return period 60 years)

A convergence zone spread into the Cook Strait area between a cool moist southerly flow and a milder northeast flow. The northeasterly overran the denser southerlies, leading to rapid ascent of moist air. The slow-moving nature of the convergence zone led to some extremely heavy rainfalls. The synoptic situation is seen in Fig 147.

In the Marlborough Sounds, 1000 people were evacuated due to fears that a dam above Picton would burst. Floodwaters swept away camper vans and flooded houses.

In the Wellington Region severe surface flooding occurred in Miramar, and a rock slide smashed into a house in Karaka Bay.

Bibliography

Challands, N., Pointer, M.W., and Quayle, A.M., 1983: The Marlborough- Nelson Bays floods of 8-10 July 1983. New Zealand Meteorological Service Technical
Information Circular 194.
Cowie, C.A., 1957: Floods in New Zealand, 1920-53: with notes on some earlier floods. Soil Conservation and Rivers Control Council.
Fenemor, A.D., 1989: Motueka and Riwaka Catchments Water Management Plan. Richmond: Nelson Catchment Board/Tasman District Council.
Nelson Evening Mail, 1867, January 28, National Library of New Zealand, Digital Collections.
New Zealand Gazette, 1923, Vol. II. New Zealand Government.
_________, 1925, Vol. I. New Zealand Government.
_________, 1931, Vol. II. New Zealand Government.
NIWA, 2002: HIRDS v2.0 – High Intensity Rainfall Design System
Pascoe, R.M., 1982: The flood of 1 April 1975 in the Wairau River, Marlborough, New Zealand. New Zealand Meteorological Service Office Note 105.
______A scheme for forecasting spillover rainfall in Marlborough and Wairarapa. Unpublished. Available from New Zealand Meteorological Service.
Poole, A.L., 1983: Catchment Control in New Zealand. Water and Soil Misc. Pub. 48.
Severe Weather Log. New Zealand Meteorological Service.
The Dominion, 11th August 1967.
______, 18th February, 2004.
Thomson P. A. In: One-Eyed and Blinkered, 2001, by Cynthia Brooks. Marlborough Civil Defence.
Williman, E.B., 1993: Wairau River Flood Frequency Analysis. Marlborough District Council

West Coast

The West Coast is the wettest part of the country, lying well within the roaring forties and prone to heavy rain from fronts that pass through at regular intervals. The rainfall is greatly augmented by orographic uplift from the Southern Alps, which rise to 3754 metres. Heaviest rainfall occurs in the northwest flows ahead of these fronts, and in the largest falls the maximum rainfall occurs 10-20km upwind of the highest ground. Extreme rainfalls can occur when the front becomes slow-moving, or when it contains active thunderstorms. Sometimes flooding can result from a number of active fronts passing across the area in the space of several days. The West
Coast holds the official rainfall records for all durations from 12 hours upwards. However, rain is distributed relatively evenly throughout the year, and extreme events are less common than elsewhere in the country.

1872 8-9th February
FIRST “GREAT FLOOD” FOR GREYMOUTH

1926 3-5th November
NATIONAL RIVER
FLOW RECORD

This flood holds the record for being the worst ever for Westport, the peak being higher than the 1950 flood by some feet. Practically the whole of the town was flooded. This event is likely to have seen a flow in excess of 12,000 cumecs in the Buller River – with the possible exception of the 1872 event, the highest anywhere in New Zealand in the last 150 years. The river broke its banks and flooded practically the whole of Westport.

This was also a major event for Marlborough. Large parts of Spring Creek and Tuamarina were inundated, and the Ferry Hotel at Tuamarina was inundated to a depth of over a metre. In Canterbury, the Waimakariri rose to an abnormal height and inundated much of the surrounding countryside.

The flood followed a wet ten days, culminating in the passage of an active front. Snow melt was also a factor.

1935 20-22nd February
WORST EVER
FOR HOKITIKA

Rainfalls
West Coast:
Hokitika 233mm/24 hrs (return period 80 years)

Taranaki:
New Plymouth 184mm/24hrs (return period 65 years)
Stratford 243mm/24hrs (return period 70 years)

Damage (West Coast): 3,380 pounds
2012 dollars: 370,000.
Road Damage (Taranaki): 25,517 pounds
2012 dollars: 2.75 million.

It takes a lot of rain to flood Hokitika, the river being usually well contained in its riverbed. However, they got a flood in February 1935. The lower portions of the town were flooded to a depth of 1.2 metres, and many houses and businesses were invaded. The southern span of the Kaniere Bridge over the Hokitika River was swept out to sea. The Ahaura Valley was devastated, with Humphreys estimated to have received 432mm/24 hours (2.22 times the 150-year return period).

This event also affected Taranaki, where flooding was serious and damage widespread. Water swept through business premises in the central part of New Plymouth; 20,000 pounds worth of damage (2.1 million 2012 dollars) was done to stock alone.

Unstable air seems to have prevailed over the country throughout the month, with frequent thundery outbeaks. These continued until the last week, but the later stages of the event were associated with more general rain. A north to northwest flow prevailed over the country during the last two days, and a blocking high to the east caused a succession of fronts to slow considerably as they crossed New Zealand. Fig 148 shows the MSL analysis for the latter stages of the event.

1953 26th March
TORRENTIAL RAIN IN SOUTH WESTLAND

Described by the inhabitants of Whataroa as the heaviest rain they had ever experienced, 17 inches (432mm) was estimated to have fallen in ten hours (1.57 times the 150-year return period). The Waitangi-taona River changed its course.

Fig 149 shows that a front stalled in its northeastwards progress over the South Island. The low near the Kermadecs (off the far northeast of the map in Fig 149) is
suggestive of blocking, as is the 500 hPa chart for 4pm 26th (Fig 150).

1978 26-29th March
STATE OF EMERGENCY
DECLARED AT HAAST

After a whopping 610mm fell in the 24 hours to 9am on the 27th, a state of emergency was declared in Haast. The town was isolated by slips, and SH 73 closed. There were a lot of trampers in the area for Easter, and many people were evacuated by helicopter.

Two active frontal systems passed over the West Coast. The first can be seen over Fiordland in Fig 151. The second front lies attached to a wave depression in the mid Tasman Sea. Close to this second front a ship was reporting thunderstorms. The first front stalled briefly over northern Fiordland, and was caught up by the second. Fig 152 shows the wave depression close to Haast at 6am on the 27th..

In the latitudes of Northland the same ship, moving eastwards with the front, was still reporting thunderstorms – active convection was a feature of this event.

Upper air charts showed a northwest jet extending from the central Tasman Sea to the area south of the South Island (not shown), and a very weak shortwave trough in the eastern Tasman Sea. Upper level divergence assisted the frontal uplift. The 500 hPa flow (Fig 153) looks like the classic blocked set-up – however, systems
were mobile. CVA associated with the short wave trough seen in the eastern Tasman in Fig 153 was also a factor. The 500 hPa trough over eastern Australia at midnight on the 26th weakened as it crossed the Tasman Sea, and moved over Westland early on the 29th.

1982 10-12th March
HUNDRED-YEAR
FLOODS

The three-day rainfall at Alex Knob up the Waiho Valley is a New Zealand record.

There were hundred-year floods in the Waiho, Whataroa, Paringa and Whanganui Rivers, and a 50-year flood in the Waitangi-taona River. Damage throughout South Westland was extreme and widespread. Franz Josef reported 350mm/13 hours, and 650mm for three days, but with the rain gauge having overflowed. Another gauge, 4km from Alex Knob, overflowed after 1310mm rain.

The MSL charts for midday on the 10th, 11th and 12th are shown in Fig 154. These charts illustrate a rather atypical West Coast heavy rain situation, which usually involves a strong northwest flow. A clue to the causes of the heavy rain can be seen from a closer examination, particularly of the midday 12th analysis. Firstly, the front became slow moving over the area for a time. Blocking was therefore a significant factor. A more significant clue though is the connection of the front to the low between Queensland and New Caledonia. A nephanalysis done at this time (not shown) depicted a broad frontal zone extending from this low and down over the South Island. Humid air from the subtropics was rising through the atmosphere as it moved towards New Zealand, the so-called “warm conveyor belt” – thus creating this broad and active band of cloud. With a trajectory from such low latitudes, this activity was unusually intense.

The 500 hPa chart, seen in Fig 155, confirms that air was being dragged from very low latitudes.

A similar pattern – this connection of the front to the tropics or subtropics – is also seen in several Taranaki extreme rainfall events, particularly that of
8-11th March, 1990.

1988 19-20th May
STATE OF EMERGENCY
DECLARED FOR
GREYMOUTH

Rainfalls
Westport Aero: 272mm/13 days
Hokitika Aero: 221mm/13 days
Greymouth Harbour: 251mm/13 days

Evacuations: 400
Damage: $4 million.
2012 Dollars: 7.5 million.

Most of Greymouth was flooded, with the water over 1 metre deep in places. Both the Grey and Inangahua Rivers had record floods. There was extensive
damage in the Grey Valley.

The flooding followed 12 days of solid rain. Towards the end of the period, northwesterly rain was produced by a low moving southeastwards in the Tasman Sea. A front crossed the area at about noon on the 19th, but with a low remaining in the central Tasman Sea, the flow remained northwesterly and rain continued, before easing around midnight.

1988 13-14th September
WORST EVER
FOR GREYMOUTH

Rainfalls:
Inchbonnie: 462mm/4 days (return period 15 years)
Hokitika Aero: 204mm/4 days (return period 3 years)

Deaths: One.
Damage: $8-10 million.
In 2012 dollars: 14-18 million.

The Greymouth CBD has been flooded several times in its history, often to depths of up to 1.5 metres, including three times in the 1980s. However, this was the worst flooding in Greymouth’s history, with floodwaters reaching 2.5 metres deep. 200 houses were flooded, and in the Grey Valley 500-600 sheep were lost. The flow in the Grey River was estimated to have a return period of greater than 120 years.

Snow melt was a major factor in this event, and rainfalls were not out of the ordinary. A series of troughs affected the area, followed by a particularly active one on the 12th and 13th.

1994
NEAR RECORD RAINFALLS

1995
RECORDS FALL

1997 14-16th December
GREYMOUTH SAVED BY FLOOD PROTECTION

The lower Grey River reached to near the top of its banks, but recent flood protection was equal to the task, and Greymouth was (just) saved from flooding. 300-350mm fell in the ranges of north Westland and Buller.

A strong moist northwesterly flow ahead of a front dragged a lane of subtropical air onto the northern South Island (Fig 156).

1999
TORRENTIAL RAIN IN FIORDLAND
AND SOUTH WESTLAND

2004 8th January
HIGHEST ONE-HOUR
RAINFALL RECORDED
IN NEW ZEALAND

A major weather system crossed the country, bringing flooding and slips to the West Coast and closing roads. Further afield, a woman was killed in Queen Charlotte Sound when the tent she was sleeping in was blown away, and three Cook Strait power pylons near Molesworth were destroyed. There was also wind damage on the West Coast, with trees brought down. Northwesterly wind damage on the West Coast is generally caused by strong gusts (or tornados) associated with active
convective cells.

The extreme rainfall, which was probably quite isolated, occurred within an instability line on the leading edge of an active cold front. Unfortunately, the satellite image for the time of onset of the extreme rainfall was not available. However, Fig. 158 clearly shows the instability line at 10pm.

The two Cropp stations are located very close together, inland from Hokitika. The return period calculated by HIRDS2 is probably too high.

Fig 159 shows the MSL analysis at the time of the extreme rainfall. The weather system was accompanied by a strong upper trough, with significant upper divergence, and 850 hPa wet bulb potential temperature up to 20C (not shown). No sounding near the affected area is available.

Bibliography

Benn, J.L., 1990: A Chronology of Flooding on The West Coast, South Island, New Zealand, 1846-1990. The West Coast Regional Council.
Henderson, R.D., 1993: Extreme storm rainfalls in the Southern Alps, New Zealand. In: Extreme hydrological events – precipitation, floods and droughts
(Proceedings of Yokokama Symposium, July, 1993) I.A.H.S. Publ. No. 213:113-120.
New Zealand Herald 14th and 15th September, 1988.
NIWA, 2002: HIRDS v2.0 – High Intensity Rainfall Design System
Poole, A.L., 1983: Catchment Control in New Zealand. Water and Soil Misc. Pub. 48.
Severe Weather Log. New Zealand Meteorological Service.

Canterbury

Flood events in Canterbury occur in two categories. The more common are the northwesterly events, where heavy rain occurs on the West Coast and in the Southern Alps. Significant rainfall can spill over into the headwaters of the large Canterbury rivers, particularly the Rangitata, Rakaia and Waimakariri. The main factors favouring high spillover are unstable air and strong winds normal to the Alps. In these cases, flooding is confined to the immediate environs of the main rivers.

More significant but less common are the easterly events, often associated with a large, slow-moving depression over or near the North Island. These situations can produce widespread rain throughout the region. Surface flooding occurs, as well as flooding in the smaller rivers and water courses. In these cases, a slow-moving front is usually found over the area, and convection is a factor in the most severe.

1868 2-4th February
EX-TROPICAL CYCLONE SWEEPS OVER NEW
ZEALAND

Rainfalls
Marlborough/Nelson:
Nelson City 292mm/48hrs (return period over 150 years)
incl 179mm/24hrs (return period 150 years)

Canterbury:
Mt Peel 205mm/24 hrs (return period 150 years)

Deaths: At least 15
Number Evacuated: Hundreds

The storm of 1868 brought widespread flooding from Marlborough to Otago, also to Nelson, and sank several ships. It appears to have been an ex-tropical cyclone. Reports from the Marlborough area were of a period of easterly gales followed by strong northerlies, and atmospheric pressure reaching a minimum of 968 hPa. At New Plymouth, the glass bottomed out at a similar pressure, with southeast gales giving way to a dead calm, then southwest gales, soon afterwards turning to northwest gales. Heavy rain and devastating flooding extended as far south as Otago, where a number of lives were lost, including nine at Totara Station, near the
Waioreka River. The Taieri Plain, from a point about 11 miles from Dunedin, was covered by an almost unbroken sheet of water. Canterbury was the worst affected, with the flooding there still being the highest on record. Christchurch was flooded, with 60cm of water flowing through the Magistrates’ Court, and one and a half metres in Fendalton Road. In Kaiapoi there was nearly two metres of water in some streets. At Kaikoura houses and farm land were washed out to sea. The whole of the Wairau Plain from above Blenheim to the sea was reported to have become “one vast sheet of water”. The Blenheim rainfall recorder had an accident occur to his gauge on one of the days involved, but estimated about 90mm to have fallen there over 48 hours. In Nelson, this was the third year in a row in which major flooding had occurred at the end of January or beginning of February.

Easterly gales led to the sinking of four ships off the Otago coast, with further loss of life. Another ship was lost off the coast of Hawkes Bay in a northeasterly gale. Gales were also reported from the West Coast, with the barometer falling to about 980 hPa. An easterly gale also caused problems for shipping around Auckland.

The approximate track of the low, as inferred from newspaper reports around the country, is shown in Fig 160.

1923
NEARLY 1000MM IN FOUR DAYS

1926
WAIMAKARIRI FLOOD

1945 20-21st February
TEMUKA INUNDATED

Rainfalls
Strathmore: 354mm/48 hrs (return period over 150 years)
incl. 234mm/24 hrs (return period 150 years)

Damage: 372,000 pounds
2012 Dollars: 30 million.

One and a half metres of flood water flowed through Temuka, and 23 bridges were either destroyed or rendered impassable after two days of heavy rain.

The charts for noon 18th and noon 19th (Figs 161 and 162) show the complexity of the build-up to the heavy rain event. In particular, note the situation on 19th February. Two depressions lie in the Tasman Sea, while warm air from the subtropics is about to collide with cold sub-antarctic air.

By 20th February (Fig 163) the two lows have combined into one deeper one, while one can imagine a very strong baroclinic zone lying over South Canterbury.

The analyses for noon 20th February (Fig 163) and noon 21st February (Fig 164) show the classic isobaric pattern for heavy rain in the eastern South Island.

The intensification of an anticyclone to the south of New Zealand, bringing the cold air northwards and halting any southeastward movement of the low, was an important factor in producing the extreme rainfalls.The chart of rain distribution for this (Fig 165) shows how little orography influenced rainfall intensities. Convection
must have been a major contributor.

Flooding was widespread, with many towns isolated. The flooding was made worse by the fact that the ground was already well saturated.

1986 12-13th March
HAKATARAMEA VALLEY
EVACUATED

Rainfalls
Kakahu Bush: 145mm/12 hrs (return period over 150 years)
incl 106mm/6hrs (return period over 150 years)
Rangiora: 101mm/6hrs (return period over 150 years)
Heatherleigh: 167mm/24hrs (return period well over 100 years)

Deaths: One.
Damage: $66 million.
2012 Dollars: 157 million.
Stock losses: Nearly $15 million.
2012 Dollars: 35 million.
Evacuations: 2000.

After surviving several years of drought, all the farmers in the Hakataramea Valley were forced to evacuate their farms when a very intense rainstorm brought flooding to large parts of South Canterbury and North Otago. Many homes in Temuka were evacuated, as was the entire town of Pleasant Point.

Return periods of the 24-hour rainfalls were of the order of 75 years. However, since most of the rain fell in only 12 hours, this was an event with a return period of well over 100 years. In some places total rainfalls were up to 250mm, including a burst of up to 50mm in two hours at the end.

Three features collaborated to make this an extreme event. Firstly, there was a tropical cyclone near the Kermadecs. Warm air from this system spread down onto the South Island in a weak anticyclonic flow. There the air became caught up in the second system – a large, slow-moving depression centred in the Tasman Sea. The third system, a high-latitude anticyclone, then brought cool southeasterly air up the South Island east coast. Convergence between this cool air and the warm tropical air then led to intense convection. The thermal gradient in the lowest 3-500 metres across the convergence zone was 6-7C.

The sequence of MSL charts from 10th to 15th March can be seen in Fig 168.

The 300 hPa analysis for 6am 13th March (Fig 169) shows the South Island lying within a strongly difluent flow, in the poleward exit region to a jet stream. Strong upward motion is therefore indicated.

The satellite photograph shown in Fig 170 shows intense convection occurring over the Banks Peninsula/North Canterbury area. Cloud top temperatures are -60 to -70C, indicating cloud top heights of about 13,000 metres. At around this time, Rangiora in North Canterbury was raining at up to 37mm per hour. Timaru can be seen to be lying beneath warmer tops, yet even there, it was raining at up to 19mm per hour. The clouds look less convective there, but synoptic-scale forcing was
obviously very strong. Further south, in South Canterbury and North Otago, the rain was enhanced by the ranges near the coast. The fact that the Hakataramea
Valley (in the lee of the Hunter Hills) was badly affected, was another indication of intense convection. Compare this with the 1985 Coromandel Peninsula event, where high rainfalls also occurred well downstream of the ranges in the South Auckland area.

Rainfalls for the 24-hour period to 9am 13th March are shown in Fig 171, while Fig 172 shows that most of this rain fell in less than 12 hours.

Also of interest in Fig. 172 is how abruptly the rain intensified, and later ceased. As the high to the south moved northeastwards, the flow over Canterbury turned northeast, and the convergence zone broke up.

Insights

Temporal and Spatial Distribution

Fig. 195 shows the distribution of events throughout the year. Of the 122 events, 34 were considered predominantly convective. February is usually thought of as a dry month in New Zealand, and for most places is in fact the driest month of the year. However, as can be seen from the figure, it is also the month for extreme rainfalls. This is due to a combination of the high sea and land temperatures in the late summer. September, with its combination of low sea and land temperatures, has the lowest frequency of extreme events. Also of note in Fig. 195 is that convective events occur well into winter, due to the lag between land and sea temperatures. Then
in November and December, although sea temperatures are still cold, convection starts again due to warming land temperatures.

Only seven ex-tropical cyclones have produced extreme rainfalls in the last 150 or so years. A notable absentee from this list is Giselle, which, while producing hurricane force winds in Cook Strait and sinking the inter-island ferry “Wahine”, did not produce extreme rainfalls.

Events were also plotted on a map of New Zealand. This is not shown, since for most events it was not known precisely where the maximum rainfall occurred. However, the plot did highlight the following:
There were no extreme rainfall events in Fiordland. Rainfall there is the most reliable of anywhere in the country, but variability is low.
There were not many events for Manawatu, although there were a few memorable ones.
Gisborne, Hawkes Bay and Wairarapa showed up strongly, also Marlborough and Nelson.

Something also needs to be said about the decadal distribution. This is shown in Fig. 196, but this tells us more about the New Zealand population over
the years and the growth of the rainfall network than the actual incidence of extreme rainfalls. However, there is an interesting dip in the number of events from the 1940s to the 1960s.

Event Classification

Following Hand et al (2004), the events were classified according to the main forcing mechanism, and plotted on a chart of rainfall depth versus duration. This is shown in Fig 198.

The classifications were as follows:

(a) Convective Events [abbreviated to Conv. in Appendix 3 and marked with a diamond in Fig. 198]: Forcing was either from insolation or a meso-scale feature such as a convergence line or sea breeze.

(b) Convective with Frontal Forcing [abbreviated to C*** in Appendix 3 and marked with a cross in Fig. 198]: Forcing was from a synoptic scale feature such as a front. Of the ten most extreme events (see hierarchy Appendix 1), six were of this category.

(c ) Frontal with Convective Forcing [abbreviated to F*** in Appendix 3 and marked with a triangle in Fig. 198]: Rainfall was widespread and continuous over a large area and clearly associated with a synoptic scale frontal system. There was significant embedded convection characterised by pulses of very heavy rain or
thunderstorms.

(d) Frontal [marked with a square in Fig. 198]: Rainfall was associated with a synoptic scale frontal system.

(e) Orographic [abbreviated to Orog. in Appendix 3 and marked with an asterick in Fig. 198]: Rainfall was not directly associated with a frontal system or deep convection. The main rainfall mechanism was the prolonged ascent of moist cloudy air over high ground. Such a situation would characteristically give continuous
drizzle near sea level, but orographic enhancement would lead to many hours or even days of steady rain in the hills.

(f) Ex-tropical cyclone [abbreviated to Ex-TC in Appendix 3 and marked with a star in Fig 201].

A comparison of Fig. 198 with Fig. 16 in Hand et al shows that in the New Zealand cases also, each storm type tends to occupy a unique portion of the graph. However, this does not occur in quite such a neat fashion as for the British events. This is not surprising, since there are very few parts of New Zealand for which orography does not play a role, and in some places it plays a very significant role. For example, the 610mm in 24 hours on the 29th March 1987 was a West Coast event, so that while convection on the front played a major role, so did orography. Another notable example of a storm type being out of place was the Kerikeri storm of 19-20th March 1981, where a huge amount of rain fell in a brief convective event. Other anomalies were the 512mm in 10 hours in Hawkes Bay in 1924 (the most
extreme rainfall in the data-base) and the 349mm in 12 hours in 1953. The latter three anomalies all fall in the top five in the Extreme Rainfalls
Hierarchy (Appendix 1).

These four exceptions aside, event totals of over 300mm were all associated with frontal (non-convective) or orographic forcing (plus two ex-tropical cyclones).

Blocking

Many heavy rainfall events occur when a front becomes slow-moving. Often the eastward progress of a front will be stalled by a ‘blocking high’. On many occasions, the front will just weaken away as it moves into the high. However, a certain 500 hPa pattern became a familiar sight to the author during this study. This pattern featured a broad trough over Australia and the western Tasman Sea, and a broad ridge over New Zealand and the Chathams. Examples can be seen in Figs. 56, 106, 108, 130, 132, 146, 150, 153 and 155. Such a large-scale pattern tends to be slow-moving, and is typical of ‘blocking’.

Late January 2005, the 500 hPa pattern shown in Fig. 199 was noted.

This kind of pattern brought two weeks of northeasterlies, with fine, warm weather in most places, although in the northern North Island it was very humid with outbreaks of rain. However, early in February, the trough in the Australian Bight amplified as it approached Victoria, leading to the development of a deep, slow-moving low. In the 24 hours to 9am 3rd February, 120mm was recorded in Melbourne, Australia, the highest ever in its 149-year record. The lesson from this situation is that the synoptic-scale systems within the block need to be in the “right” place for extreme rainfalls to occur

Another well-documented blocking pattern is the so-called “omega block.” This pattern is associated with easterly or northeasterly rainfalls and is seen in Figs 31, 60, 84, 181 and 191.

The Southern Oscillation

An attempt was made to find a relationship between extreme rainfall and ENSO (El Nino Southern Oscillation) based on the following
preconceptions:
(a). A suggestion in some studies that neutral values of the Southern Oscillation Index (SOI) favour heavy rainfalls.

(b). A study by Environment Bay of Plenty, (Ellery and Blackwood, 2005) that showed a strong correlation between Bay of Plenty floods and negative values of the Interdecadal Pacific Oscillation (IPO). The IPO is a climatic fluctuation with a period of one to three decades–during its positive (negative) phase, El Nino (La Nina) predominates. La Nina years tend to bring an increased frequency of northeasterlies to the North Island, so intuitively, areas such as Bay of Plenty, Coromandel Peninsula, Auckland, and Northland should have more heavy rain events during this phase of the SOI. On the other hand, El Nino events bring an increased west to southwest flow to the South Island, so that during El Ninos, the West Coast and the Southern Alps should have a greater number of heavy rain events.

A comparision of the historical record of extreme rainfalls with values of the SOI and IPO is handicapped by the following facts:

(a) The steady but unrealistic increase (apparent from the data) in the incidence of events over the historical record.
(b) The fact that the incidence of extreme rainfall events may not bear much relationship to the incidence of mere “heavy” rain events. It certainly bears no relationship to average rainfalls. This problem is highlighted by the fact, as already mentioned, that the driest month of the year (February) is also the month with the greatest number of extreme events.

It was found that 75% of extreme rainfall events occurred with neutral values of the SOI (ie between -10 and +10), 14% with positive values, and 11% with negative values. Bearing in mind that neutral values of the SOI occur 69% of the time anyway, and also that the data shows an apparent but probably unrealistic maximum since the year 2000, when the SOI has been neutral, this would suggest that at least on a nationwide basis, the state of the SOI has little bearing on the occurrence of extreme events.

The regional distribution of extreme rainfall events among the three phases of the SOI is shown in Fig. 200. One would expect, from the average flows known to occur with the various Southern Oscillation regimes, that in the first group of four regions, more than 15% of events should occur in association with positive (greater than 10) values of the SOI. In fact, this was true only for Northland/Auckland.

In the second to last group, one would expect that more than 16% of events would occur in association with negative (less than -10) values of the SOI. The 50% in Canterbury represents only one event, and in fact only 10% of events in this area occurred in association with negative values, while 20% occurred in association with positive values.

However, the last group, mainly easterly and southerly rainfalls, show a strong preference for neutral values of the SOI (92%). This appears consistent with the nature of El Ninos and La Ninas, since El Ninos bring increased westerlies to these areas (ie dry conditions) while La Ninas are associated with anticyclonic regimes (also dry conditions).

A relationship with the IPO (also known as the Pacific Decadal Oscillation) was not much easier to find. Fig 201 (from Rodionov) shows monthly values of the IPO since 1900.

Using a method described in Rodionov 2004, regime shifts are calculated to have occurred in 1948, 1976, and 1999.

The two areas that are the most interesting to compare with extreme rainfall incidence are the Bay of Plenty and the Southern Alps. The former (latter) should receive a greater number of extreme rainfalls during the negative (positive) phase of the IPO ie (more La Ninas (El Ninos)).

The table shows the number of extreme events for the two areas in each phase of the IPO.

Before about 1950, data on rainfall events is probably deficient. That aside, the table bears out the theory reasonably well, particularly since the latest negative phase has only just started, and the Bay of Plenty could accumulate many more events before it ends. However, Northland/Auckland and the Coromandel Peninsula might be expected to show similar trends to Bay of Plenty, and this did not occur (not shown).

Also, according to Rodionov (2006), it is still not completely certain whether or not there really was a regime change in 1999.

Summary of Insights

(A) February is the month with the highest incidence of extreme rain events, while September has the least.

(B) Convection can play a role in events well into winter.

(C) No extreme events show in the record for Fiordland.

(D) Events can be classified according to the main forcing mechanism, and similar types tend to cluster together when plotted on a graph of rainfall amount versus duration.

(E) Blocking is a recurrent theme in extreme rainfall situations.

(F) On a nationwide basis, there is no predisposion of extreme rainfalls towards positive, negative or neutral values of the SOI, apart from in Canterbury and Otago, where easterly and southerly events strongly favoured neutral values.

(G) Bay of Plenty and the Southern Alps show some correlation with phases of the IPO, with the negative (positive) phase favouring Bay of Plenty (Southern Alps) events, while the positive (negative) phases correspond to a paucity of events in the Bay of Plenty (Southern Alps). This correlation with the IPO did not show up anywhere else.

(H) The most extreme events were the convective ones where forcing was from a synoptic-scale feature such as a front.

(I) Of the non-convective extreme events, a strong warm conveyor belt, with air at all levels coming from low latitudes, was noticed on a number of occasions. This tended to occur in combination with blocking.

Bibliography

Ellery, G., and P. Blackwood, 2005: Impacts of the Bay of Plenty Floods of May, 2005. Presented to “Enhanced Weather Services for Decision Making” Seminar,
MetService, Wellington, 3rd Nov. 2005.
NIWA Science, 2005: Past Climate Variations over New Zealand. http://www.niwascience.co.nz/ncc/cliver/pastclimate
Rodionov, S.N., 2004: A sequential algorithm for testing climate regime shifts. Geophys. Res. Lett., 31, doi:10.1029/2004GL019448.
Rodionov, S., J. Overland, and N. Bond, 2006: Pacific Climate Overview – 2005. http://www.beringclimate.noaa.gov/reports/np_05.pdf

Appendix 1

A Hierarchy of Extreme Rainfalls in New Zealand

Included in the following table are all rainfalls which reached or exceeded 1.50 times the 150-year return period value.

Most return periods have been calculated using NIWA’s HIRDS2 program. This does appear to have a few deficiencies, particularly in the very wet areas such as the Coromandel Peninsula and the Southern Alps. In these areas, return periods calculated using HIRDS2 appear to be far too high. In fact HIRDS1 gives much more realistic values for the Southern Alps.

Kopuawhara Disaster (est. from river level)

Appendix 2

Chronology of Extreme Events

* well-founded estimate
**actual max intensity for event probably considerably more

Appendix 3

Severe Convection

Severe convection plays a major role in many heavy rain events. Thunderstorms can occur in single cells, multi-cells, or so-called super cells. They can also occur in large clusters, and in fronts. Thunderstorms are generally responsible for the short-period extreme rainfalls, and play a major part in many of the longer-period extreme events. Up until recently, scattered thunderstorm activity was considered too difficult to predict, and heavy rain forecasts were attempted only when an area in excess of 1000 square kilometres was expected to be affected. However, in 2005, a dedicated severe convection shift was instituted at MetService. As well as heavy rain, the possibility of hail, severe straight line winds from convection, and tornados are now considered. Those parts of the country likely to be affected are assigned a probability (low, moderate or high) with the forecasts posted on the MetService website.

The fundamental tool for forecasting convection is the sounding of temperature, dew-point, and wind, as determined from weather balloons released at least twice per day from various parts of the country. There are only three sites in New Zealand where temperature soundings are made (Whenuapai, Paraparaumu and Invercargill), so we need to interpolate between these to get an idea of the atmospheric regimes at other places. Also, we need to know what the soundings are likely to look like at some future time. Computer models have become better and better at this. Although there is still a lot of room for improvement, forecast soundings are now good enough for useful convection forecasts to be made.

Beginners Guide to Using Tephigrams for Convection Forecasting

A parcel of air is lifted from the surface, possibly by flowing over a hill, or within a convergence zone. As it rises, its temperature falls off at the “dry adiabatic” lapse rate (sloping brown “potential temperature” lines on the tephigram – see Fig 202).

At the same time, its dew point falls, but not as rapidly as temperature, travelling up the mixing ratio (dashed green) lines. When the parcel’s temperature reaches its dewpoint, it becomes “saturated”. Whether it continues to rise will probably now be dependent on whether or not it is warmer than the surrounding air (or it may be forced up over a hill). If it continues to rise, it does so at the moist adiabatic lapse rate (solid green lines). The air will continue to rise until it is no longer warmer than the surrounding air. Because it will have some velocity at this point, it will “overshoot”.

Tephigram Parameters

The positive area between the parcel trajectory and the environmental temperature is the Convective Available Potential Energy (CAPE) (coloured red in Fig 202).

The green area in Fig 202 is equal to the red area, and represents the loss in kinetic energy of the air parcel as it overshoots.

The temperature difference between the lifted parcel and the environmental temperature at the 500 hPa level is the Lifted Index (LI).

The temperature difference between the lifted parcel and the environmental temperature at 700 hPa is the SLI700.

The altitude at which the environmental temperature reaches 0C is the freezing level.

The wet bulb temperature is the temperature a parcel of air would have if its moisture level was increased to saturation. This is roughly mid-way between its temperature and dewpoint. The altitude at which the wet bulb temperature reaches 0C is called the WBZ (wet Bulb Zero).

Heavy Rain Thunderstorm Sounding

A typical sounding for a thunderstorm producing heavy rain is shown in Fig 203.

This example is from an Auckland thunderstorm on 21st January, 1999. It features:

(1) “Tall skinny” CAPE. For heavy rain to occur, CAPE should be moderate to high, in the 1100-2000 j/kg range, indicating a low warm cloud base and a high equilibrium level.

(2) The sounding should show a deep moist environment. This is not so obvious in this example – however, locations close to water have a source of moisture which can readily be entrained into the atmosphere.

(3) A large lifted index (LI) in the range –4.5C to –6.0C.

(4) A high freezing level of greater than 3000 metres.

(5) A high WBZ of greater than 2500 metres.

(6) Slow storm motion of less than 10 knots (or a succession of cells moving over the same area – the so-called train effect).

(7) Weak environmental wind shear.

(8) High relative humidity near the ground.

A couple of personal observations are made at this point, based on the editor’s experience in forecasting convective rainfalls.

(A) A sounding or forecast sounding is not the last word when it comes to forecasting rainfall intensities. The role of any forecast dynamic processes must also be considered and added.

(B) Mesoscale models can be very good at picking up zones of moisture convergence (although not always in the right place). High intensity rainfall appearing in a mesoscale model can be treated as a “first alarm” level for severe convection. If the 100km or 60km models are indicating dynamic support in the form of strong upward motion, CVA, or upper divergence, then high intensity rainfall can be forecast with a fair amount of confidence. With the CHAMP12 model, for example, a forecast of 30-50mm per three hours, along with dynamic support from the coarser models, has been seen more than once to lead to actual rainfall intensities of 30-50mm per one hour. Sometimes these rainfall rates are accompanied by little or no thunderstorm activity.

(C) Conversely, if CHAMP12 is forecasting high intensity rainfalls without dynamic support, there is a good chance that it will be wrong. Many of the events described above were the result of severe thunderstorm activity. For the historically earlier cases, soundings are, of course, not available. Even for some of the later cases, (in particular the record Leigh rainfall of May 2001) the nearest soundings were not close enough to be representative of the airmass within the storm.