Autumn 2007: Warm; record dry east of the North Island

• Rainfall: Well below normal in the east, and Manawatu and Wellington
• Soil moisture: Significant deficits in several eastern regions
• Temperature: Above average, especially in the east and inland areas of the South Island
• Sunshine: Well above normal in Gisborne, Marlborough and parts of Canterbury

Low rainfall records tumbled in the east of the North Island, with the driest autumn on record. It was very dry and sunny in the east, from Gisborne to Otago, and Manawatu and Wellington. Mean autumn temperatures were above average, especially in inland and eastern areas of the South Island. Queenstown experienced its warmest autumn in over 135 years of record. The national average temperature of 14.0 °C was 0.7 °C above normal, the highest for autumn since 1999.

Rainfall was less than 50 percent (half) of normal in many eastern regions, from Gisborne to Otago, and also below normal in the west of the North Island from Waikato to Wellington, as well as eastern Bay of Plenty. Unusually, significant soil moisture deficits (at least 110 mm) still existed at the end of autumn in many eastern regions, especially Hawke’s Bay and Central Otago. Autumn temperatures were above average throughout New Zealand. Frequent north westerlies produced mean temperatures 1.5 °C above average in parts of Canterbury and Central Otago. March was warmer than normal, April cooler than normal, and May was the warmest on record. Sunshine hours were above normal in eastern regions of both islands from Gisborne to Otago, with Gisborne having its sunniest autumn in over 100 years. The overall autumn climate pattern was dominated by more anticyclones over the North Island and to the east with stronger westerly flow over the South Island.

• The highest temperature recorded during the autumn was 32 °C recorded at Darfield on 5 March, and Culverden on 24 March. On 1 May, many Northland locations recorded new extreme May maximum temperatures ranging from 24 to 25 °C. The lowest temperature for the autumn was –4.1 °C recorded at Motu on 31 May
• There were at several high rainfall/flood-producing events – the worst being that of 28–29 March when extremely high rainfall totalling over 400 mm in eastern parts of Northland resulting in severe flooding. Two-day rainfall totals were amongst the highest on record in the area. Significant soil moisture deficits still existed at the end of autumn in the east of the North Island and eastern Otago. This resulted in Hawke’s Bay farmers selling stock due to insufficient feed for winter.
• Unseasonable snowfall occurred at Lake Rotoiti on 14 March.
• Damaging winds, with gusts, exceeding 130 km/h, were measured in Auckland with westerly gales and winds blew over grape vines in part of Marlborough over 13–15 March. Damaging winds attributed to a tornado occurred in Stratford on 21 March. High gusts, exceeding 130 km/h from the southwest, were measured in parts of Auckland over 12–13 April, and a wind gust as high as 156 km/h was recorded from the west at Taiaroa Head on 27 May.
• Lightning strikes resulted in power outages to many homes in the Wellington region over the night of 13/14 March.
• Of the five main centres, Auckland was easily the warmest, and wettest, Dunedin the driest and coolest, and Wellington the sunniest. Autumn temperatures were above average in all five main centres. Rainfall was near normal in Auckland, and below normal in the four other centres. Autumn sunshine was near normal in Hamilton, and above normal in the four other main centres.

Rainfall

Autumn rainfall was less than 50 percent (half) of normal in many eastern regions, from Gisborne to Otago, and below normal in the west of the North Island from Waikato to Wellington, as well as eastern Bay of Plenty. Autumn rainfall was near normal elsewhere.

Temperature

Seasonal mean temperatures were at least 0.5 °C above average throughout much of the North Island and at least 1.0 °C above average throughout much of the South Island. Temperatures were about 1.5°C above average in parts of Canterbury and Central Otago.

Sunshine

Autumn sunshine hours and/or solar radiation were at least 110 percent of normal in eastern regions of both islands from Gisborne to Otago, and near normal elsewhere.

·         Full details are at http://www.niwascience.co.nz/ncc/cs/sclimsum_07_2_autumn.pdf/view_pdf

Dr Jim Salinger – Principal Scientist, Climate

NIWA National Climate Centre – Auckland

Phone +64 9 375 2053  j.salinger at niwa dot co dot nz

Stuart Burgess – Climatologist

NIWA National Climate Centre – Wellington

Phone +64 4 386 0569  s.burgess at niwa dot co dot nz

Geoff Baird – Communications Manager

Phone +64 4 386 0543 g.baird at niwa dot co dot nz

Acknowledgement of NIWA as the source is required.

NOTABLE WEATHER IN NZ DURING AUTUMN 2007

(compiled by Ben Tichborne with help from Bob McDavitt and Trevor McGavin)

Weather patterns varied greatly during this season, with warm settled periods (May was the warmest on record for NZ as a whole), but also some unsettled and stormy outbreaks, most notably the devastating Northland floods in late March.

MARCH

# 3rd - Warm in many inland areas, eg 30C maximum in Alexandra; 28C in Tokoroa.

# 4th - 30C maximum again in Alexandra.

# 5th - 31C maximum in Alexandra and 26C-30C maximums in many eastern places in a northerly flow.

# 6th - Very warm again in eastern areas. 35C maximum in Hanmer, and 31C-32C maximums in many other Canterbury places. 30C maximum in Gisborne.

# 7th - Heavy rain about and west of the Southern Alps. Cool 12C maximum in Invercargill in a southeasterly flow.

# 10th - Warm day in north of North Island, eg 29C maximum in Kawerau.

# 11th - 31C maximum in Culverden; 30C in Alexandra.

# 12th-15th - Cold, stormy period. (see details below)

# 17th/18th - Unsettled weekend, including major flooding in Buller region. (see details below)

# 21st Locally damaging wind gusts throw roofing iron at school children near Stratford. Reported in the media as a tornado, but no thunderstorm activity (necessary for tornado development) observed in Taranaki at the time.

# 23rd - Warm 32C maximum in Culverden; 29C in Blenheim.

# 28th-30th - Very heavy rain causes severe flooding in parts of Northland (see details below)

APRIL

# 4th - Some afternoon thunderstorms in Hawkes Bay and Rangitikei areas.

# 5th - Thunderstorms in several northern North Island areas, eg Northland and Waikato.

# 6th - Some thunderstorms in Northland.

# 9th - Thunderstorms in Northland with small hail reported.

# 10th - Convergence zone over Auckland results in heavy rain, along with thunder and hail. Flooding disrupts motorway traffic for a time around 3pm.

# 11th - Heavy rain in Westland results in trampers being caught out by flooding rivers. Cold southerly spreads over South Island behind cold front, with some snow on the high country.

# 12th - Cold south to southwest flow develops over North Island; trough and low forming to the east. Thunder and hail in many central and northern areas, plus fresh snow on the mountains. Snow also overnight above about 600m in inland Canterbury (15cm reported on hills above Hanmer) and 800m on Kaikoura ranges.

# 13th - Strong cold southwesterly flow spreads over whole country by afternoon behind another cold front, with gales in exposed areas. Gusts reach 81 knots at Mid Dome, Southland, 66 knots at Taiaroa Head, and 60 knots in Auckland. Avocado trees damaged in western Bay of Plenty.  Snow showers to low levels in parts of Otago and Southland, settling on higher areas, eg 6cm in Naseby. Snow flurries in Dunedin's hill suburbs. Snow on ranges further north and hail in some areas.

# 18th - Rather cold southerly flow, with maximums reaching only 12C in Wellington and 13C in Christchurch.

# 19th - Frosts in many inland areas, eg -1C minimums in Taupo and Te Anau.

# 22nd - Chilly 11C maximum in Oamaru, due to cloud cover and an onshore breeze.

# 27th - Some heavy rain in northwest of South Island, eg 120mm at Aorere. Chilly 9C maximum in Dunedin, under a cloudy onshore flow.

# 28th - Some heavy showers and thunderstorms in north of North Island, eg midday downpour in Whakatane. Only 10C maximum in Christchurch, under a drizzly southerly.

# 29th - Frosty morning in many inland South Island areas, eg 0C minimum in Queenstown.

MAY

# 1st - Warm in north and east of  North Island, eg 25C maximums in Dargaville, Whangarei, Gisborne, and Hastings. Afternoon thunderstorms developing in north of North Island.

# 2nd - Overnight downpour (about 50mm) causes flooding in Wellington, disrupting several roads and leaving 10cm of water in a Lambton Quay basement. Steady rain also results in some surface flooding around Christchurch. Afternoon thunderstorms in Bay of Plenty/Rotorua and Coromandel Peninsula.

# 4th - Morning fog around Auckland and Waikato, causing disruption at Auckland Airport.

# 5th - Warm 20-22C maximums in eastern areas. 23C maximum in Ashburton. Fog closes Rotorua Airport for a time.

# 6th - 23C maximums in Blenheim, Masterton, Napier, and Hastings. 20-22C maximums in Northland and Auckland.

# 7th - Warm spell continues in northern areas, eg 23C maximum in Kaitaia, and 21-22C maximums elsewhere from Waikato northwards.

# 8th - A summery 25C maximum in Kaitaia.

# 9th - Heavy rain in Fiordland, eg 112mm at Homer Tunnel. Northwesterly gales in some exposed parts of Canterbury, eg 40 knot gust at Le Bons Bay.

# 11th - Some heavy rain on South Island West coast, eg 50mm at Haast. Severe wind squalls or tornadoes cause some damage around Greymouth. 22C maximum in Napier, but colder southwesterly flow spreading over South Island with fresh snow on the high country, including southern passes.. 20cm recorded at Remarakables Ski-field.

# 13th - Frosty morning in some areas, eg 0C minimums in Hamilton and Masterton. However, rising to 21-22C maximums in Gisborne, Hawkes Bay, and the far north.

# 14th - Westerly gales in far south (100kph recorded in Invercargill); northwesterly gales in southern Hawkes Bay. Warm in some eastern areas, eg 24C maximum in Kaikoura and 23C in Oamaru.

# 15th - 21C maximums in Gisborne and Whitianga. More westerly gales in Southland

# 16th - Northwesterly flow results in warm temperatures in eastern areas, eg 24C maximums in Kaikoura and Timaru. Squally and severe west to southwest gales in the far south; gusts over 59 knots in Invercargill and 86 knots at Mid Dome, and damage to trees, power-lines etc.

# 17th - Warm again in many eastern areas and Northland/Auckland, with 20C+ maximums. (22C in Gisborne and Kaikoura)

# 18th - Warm spell continues in east, with 20-21C maximums in many places. 20C maximums also in far north and in Alexandra.

# 19th - An unseasonably warm 20C maximum in Tokoroa. (also in Kaitaia)

# 20th - Warm spell continuing in east. 23C maximum in Kaikoura. 20C maximums in the far north and also (unusually) in Invercargill. Northerly gales in some areas. Heavy rain in west of South Island, eg 160mm at Homer Tunnel; 201mm at Cropp.

# 21st - Yet another summery day in the east. 24C maximum in Oamaru; 23C in Ashburton.

# 22nd - 19-22C maximums in many North Island place in a northerly flow. 23C maximum in Kaitaia.

# 23rd (see details below) - Heavy rain causes flooding around Nelson and Taranaki..

An active cold front crossed NZ on this day, delivering a period of heavy rain to many areas exposed to the north and west of both islands. Worst affected were the Nelson region and North Taranaki, with widespread flooding. Stoke (near Nelson) recorded 98mm in 24 hours, with 53mm falling between 8-9am. North Taranaki was next to be hit, with several buildings flooded, including a pet motel. (where animals were rescued) A man just managed to get off a swing bridge before raging waters washed it away.

Because this front moved over the country in northwesterly flow, little or no rain fell in eastern areas, where unseasonably, dry and warm weather continued (Kaikoura recorded a balmy 25C maximum), contributing to increasingly bad drought conditions in the east of the North Island.

# 24th - Thunderstorms in west of South Island. Still warm in many eastern places, eg 23C maximum in Napier.

# 25th - Heavy rain in Fiordland. 20-21C maximums in Gisborne and Hawkes Bay.

# 26th - Northwesterly gales developing in the far south.

# 27th - Southwesterly gales in many exposed areas, severe in coastal Otago and Southland. Gusts up to 79 knots at Mid Dome, 60 knots in Invercargill, and 53 knots at Port Chalmers. (where containers were blown over)  Wildly fluctuating temperatures in Kaikoura: westerly breeze springs up about 2am, sending thermometer quickly shooting up from 4-18C; it rises up to 23C in morning, but drops to 15C by about midday, with a cooler southerly change. Napier also basks in a 23C maximum.

# 29th - Frosty morning in many areas, eg -3C minimum in Culverden and -2C minimums in Taupo, Waiouru, and Masterton. However, maximums still rise to be well above normal in many places, eg 22C in Gisborne.

# 31st - 22C maximum in Kaitaia; 21C in Kaikoura. Heavy rain in Fiordland; 59mm falling at Homer Tunnel in seven hours.

MAJOR EVENTS

# 12th-15th March - A cold, stormy period

A long spell of warm, settled weather over most of NZ came to a brutal end during this period, as cold fronts swept over the country, delivering gale force winds to many areas, while cold southwesterlies brought snow to unseasonably low levels on the South Island high country.

A trough of low pressure moved onto NZ from the Tasman Sea on the 12th. There were two intense areas of convection. One crossed the north of the North Island overnight 12th/13th, with a period of thunderstorms, heavy rain, and gale force northerly winds, resulting in damage in several areas. In the South Island, warm northwesterlies preceded a second active part of the trough. (which also brought heavy rain to Westland and Fiordland, eg 202mm at Milford Sound and 104mm at Haast) After a warm day in the east (Ashburton reached 30C), the night of 12th/13th was still unusually balmy with Christchurch not falling below 20C.

This was to change by dawn, as a cold southerly replaced the northwesterly; daytime temperatures being much colder than those of  the previous night in eastern South Island areas. A strong southwesterly flow covered NZ by the end of the 13th, with fresh snow on alpine areas of both islands. Winds reached severe gale-force in some northern areas, including Auckland.

Another trough and cold front crossed the country on the 14th, bringing even colder air, with more snow on the mountains of both islands. The air during the rain accompanying the trough was particularly wintry about and west of the Southern Alps, with snow to unseasonably low levels. Milford Sound shivered on an icy 3C during the morning, with snow reported as low as 300 metres. The mountains around Queenstown received a white coating, while low cloud and rain disrupted operations at Queenstown Airport. Snow also fell on the Lindis, Arthurs, and Lewis Passes, and even at St Arnaud, Nelson Lakes. (where long-time locals claimed it was the earliest they could ever recall snow falling and settling at lake level )

As the cold front crossed the North Island, during the afternoon, squally westerly gales (northwesterly about Wellington) rose to severe gale force in many areas, with thunderstorms and hail. Gusts reached 83 knots at Manukau Heads and 77 knots on Mt Kaukau. Reported damage included downed trees and power lines in Auckland and Waikato, and damage to buildings in Auckland, Kapiti Coast, Porirua, and Wellington. Surface flooding affected Kapiti, where there was a three hour power cut.

On the 15th, a moderating southwesterly flow covered NZ. Further showers affected western and southern areas exposed to the flow, but sheltered inland areas were fine but with the morning being particularly cold. Mt Cook Village recorded a -1C minimum.

Mean sea-level analyses for midday NZST 12th March to midday NZST 15th March in 12 hour steps are shown here.

# 17th/18th March - Unsettled weekend, including major flooding in Buller region

Several fronts crossed NZ during this weekend, resulting in unsettled weather in some areas, most notably in the Buller region, where heavy rain caused the worst flooding in many years.

On the 17th, a warm northwesterly flow initially covered the South Island ahead of  a cold front. Ashburton rose to a summery 31C maximum, but a cooler southerly change sent temperatures dropping in the afternoon. However, it was over the Alps that the real action was taking place. The combination of moist air coming off the Tasman and the approaching cold front led to a prolonged period of heavy rain in Westland and Buller, with widespread flooding. More than 80mm was recorded in both Hokitika in only seven hours, but Westport received the same amount in only two hours and recorded its worst flooding in 25 years.

On the 18th, the first front crossed the North Island, delivering heavy rain to the Tararua and Taranaki. Angle Knob recorded 238mm and North Egnmont 120mm, while flooding was reported at Mt Messenger and the WOMAD Festival in New Plymouth was disrupted.

As the front departed, it left a cool disturbed southwesterly flow over NZ. Further disturbances in this flow moved over the South Island during the day. Thunderstorms were reported in parts of Otago (including Dunedin) in the morning and Canterbury in the afternoon. Some particularly heavy thunder and hail storms developed in mid-Canterbury and then spread into parts of Christchurch, with surface flooding affecting the Eastgate Shopping Mall. Southerly winds rose to gale for a time about Banks Peninsula, disrupting yachting events on Lyttleton and Akaroa Harbours.

Mean sea-level analyses for midday NZST 17th March to midnight NZST 18th March in 12 hour steps are shown here.

# 28th-30th March - Very heavy rain causes severe flooding in parts of Northland

This event was very severe in eastern parts of Northland, with widespread flooding causing much damage.

On the 28th, an intense anticyclone lay slow moving just south of the Chatham Islands, while a complex area of low pressure lying from the Central Tasman Sea to the tropics (where Cyclone   had formed) tried moving east, but was prevented by the anticyclone. Consequently, a warm moist northeasterly flow (which had been around for the previous few days) increased over northern areas of the North Island.

Accompanied by fronts coming in from the northwest, rain became increasingly heavy over eastern parts of Northland and persisted through the next day, with some record high totals recorded. Totals included 419mm at Maungaparerua (Kerikeri), 328mm in Kaeo, 280mm in Whangarei, and 175mm in Kaikohe.

Consequently, much damage resulted from floods and slips. Houses were washed away at haruru Falls, while Kerikeri's stone house was flooded. Roads were closed and houses flooded. The town of Opua was cut off for a while.

In western parts of Northland, the rain, though still heavy, was much lighter. This rain was actually more of benefit to these areas, as they had been previously experiencing dry conditions.

Northland may have felt the worst of this system, but some other areas exposed to the northeast got a dousing as well, most notably the Hauraki Gulf and Coromandel Peninsula and the Bay of Plenty, but also Northwest Nelson.

Heavy rain had cleared from Northland on the 30th, but it took a long time to repair the damage done by this storm.

Mean sea-level analyses for midday NZST 28th March to midnight NZST 30th March in 12 hour steps are shown here.

# 23rd May - Heavy rain causes flooding around Nelson and Taranaki

An active cold front crossed NZ on this day, delivering a period of heavy rain to many areas of both islands exposed to the north and west. Worst affected were the Nelson region and North Taranaki, with widespread flooding. Stoke (near Nelson) recorded 98mm in 24 hours, with 53mm falling between 8-9am. North Taranaki was next to be hit with several buildings flooded, including a pet motel. (where the animals were rescued) A man just managed to get off a swing bridge before raging waters washed it away.

Because this front crossed the country in a northwesterly flow, little or no rain fell in eastern areas, where unseasonably, dry and warm weather continued (Kaikoura recorded a balmy 25C maximum), contributing to increasingly bad drought conditions in the east of the North Island.

Mean sea-level analyses for midnight NZST 22nd May and midday NZST 23rd May are shown here.

CHRISTCHURCH WEATHER - AUTUMN 2007

MARCH

This month saw long periods of settled weather, but also two stormy outbreaks. After an unusually warm night overnight on the 13th, cold southerlies arrived early in the morning and persisted through the day with rain. Another front followed the next day, bringing even colder air. Temperatures dropped as low as 7C later in the afternoon; hail being reported in parts of the city. Fresh snow fell on the inland high country, including the Arthurs and Lewis Passes.

A cool southwesterly flow covered Canterbury on the 18th, with a disturbance in this flow crossing the area in the afternoon. This brought thunderstorms to some areas, including parts of Christchurch. Heavy rain and some large hail led to flash flooding in parts of the city. Southerly winds rose to gale for a time about Banks Peninsula, disrupting yachting events on Lyttleton and Akaroa Harbours.

APRIL

Anticyclones and ridges were predominant to the west and southwest of NZ this month, which meant more cooler airstreams from the south than usual. However, this didn't mean much weather to Christchurch. Apart from the wet Easter Saturday on the 7th, the only interesting period was from the 11th-13th with two cold fronts. The first arrived late afternoon of the 11th, bringing a period of rain through to the early hours of the next day, plus some snow down to about 600m about the inland high country. The second front brought cold showery weather overnight on the 13th, plus a brief hailstorm early afternoon of that day.

MAY

The month began with two days of drizzle and rain thanks to a complex low pressure system crossing NZ, with onshore southerlies over Canterbury. But this weather was completely out of character with the rest of the month. Apart from a brief rain squall on the afternoon of the 11th (as the result of a cold front and cool southerly change), there was no measurable rainfall in Christchurch. Instead, persistent west or northwest flows gave an Indian Summer of unseasonably warm (several days above 20C) and sunny weather. Any snow on the mountains from beginning of the month and the 11th soon melted and the slopes were still bare at the end of the month.

Ben Tichborne