Climate Summary for Summer 2006-07

• Rainfall: Below normal in most regions, especially Waikato, Marlborough, and Otago; above normal in parts of Northland and Canterbury
• Soil moisture: Severe deficits developed during February in the east from Gisborne to Otago, Auckland, Waikato, eastern Bay of Plenty, Wanganui, Manawatu, Wellington, and Nelson
• Temperature: Cold, especially in the North Island and the east of the South Island
• Sunshine: Above normal in Northland and Westland; below normal in South Taranaki and North Canterbury

Mean summer temperatures were well below average. The national average temperature of 15.7°C was 0.9°C below normal and the lowest for summer since 1992/93. December was particularly cold. Summer temperatures were as much as 1.5 °C below average in quite a few eastern areas from coastal Wairarapa to North Canterbury, as well as parts of King Country and Wellington. However, summer temperatures were near average in Nelson. Rainfall was below normal in the west of the North Island from Auckland to Taranaki, as well as Eastern Bay of Plenty, and much of the north, west, and south of the South Island, with less than 50 percent (half) of normal in parts of Waikato, Marlborough, and Otago. By the end of summer severe soil moisture deficits had developed in most eastern areas of both islands from Gisborne to Otago, and spread to Auckland, Waikato, Eastern Bay of Plenty, Wanganui, Manawatu, Wellington, and Nelson. Above average summer rainfall occurred in parts of Northland and Canterbury. Sunshine hours were above normal in Northland and Westland, and below normal in South Taranaki and North Canterbury. The overall summer climate pattern was dominated by more anticyclones in the south Tasman Sea producing more frequent cool southerly winds over the whole of New Zealand. At the same time, seas around New Zealand were about 1°C below normal.

Major Highlights

• The highest temperature recorded during the summer was 33.4 °C recorded in Murchison on 7 February. The lowest temperature for the summer was -2.5 °C at The Chateau, Mt. Ruapehu on 10 December.
• The highest wind gust was 167 km/h from the northwest recorded at Rock and Pillar, Otago, on 22 January
• There were at several high rainfall/flood-producing events – one occurred in Christchurch (74 mm) and over Banks Peninsula on 20 December and in South Canterbury (with flooding in Ashburton) on the 29 December; another occurred in parts of Coromandel and Bay of Plenty on the 9 January, and over 200 mm occurred in parts of Northland over 5-7 February.
• A hailstorm affected Riwaka on 1 December, with slight damage occurring to apples in some orchards.
• Of the five main centres, Auckland was the warmest, driest, and sunniest, Dunedin the coolest, and Wellington the wettest. Summer temperatures were below average in all five main centres. Auckland was sunnier, cooler and much drier than normal; Hamilton much cooler and drier than normal; Wellington much cooler than normal; Christchurch much cooler, wetter and cloudier than normal; and Dunedin cooler and drier than normal. 

Rainfall

Summer rainfall was less than 50 percent (half) of normal in parts of Waikato, Marlborough, and Otago, and also below normal in the west of the North Island from Auckland to Taranaki, as well as Eastern Bay of Plenty, and much of the north, west, and south of the South Island. Rainfall was above normal in parts of Northland and Canterbury.

Temperature

Seasonal mean temperatures were well below average throughout much of New Zealand. These were at least 1.0°C below average over the North Island and the east of the South Island, and 1.5 °C below average in coastal Wairarapa, parts of King Country, Wellington, North Canterbury, and the Kaikoura Coast. Summer temperatures were near average in Nelson.

Sunshine

Summer sunshine hours and/or solar radiation were above normal in parts of Northland and Westland, and below normal in South Taranaki, and North Canterbury. Sunshine hours were near normal elsewhere.

Full report

http://www.niwascience.co.nz/ncc/cs/sclimsum_07_1_summer.pdf/view_pdf

For further information, please contact:

Dr Jim Salinger – Principal Scientist– Auckland  j.salinger@...

Stuart Burgess – Climatologist – Wellington  s.burgess@...

Geoff Baird – Communications Manager  g.baird@...

Acknowledgement of NIWA as the source is required.

NOTABLE WEATHER IN NEW ZEALAND - SUMMER 2006-07

(Thanks to Ben Tichborne and Trevor McGavin)

What we normally expect as summer was slow in arriving this season. Recalling 2004, December was unseasonably cold and (in areas exposed to the south) wet, with several southerly outbreaks. From them on it became more settled, with high pressure systems becoming predominant by February. However, in many areas (especially in the east) persistent onshore flows meant it stayed generally cloudier and cooler than normal.

DECEMBER 2006

* 1st - Cold southerlies spreading over North Island, but easing over South Island. Snow on the mountains. Only 10C max in Kaikoura.

* 2nd - Cold southerlies continuing in east of North Island; fresh snow visible on Tararuas in morning. Morning frosts in many South Island areas.

* 5th - A few afternoon thunderstorms about the eastern Waikato and Coromandel Peninsula.

* 8th - Colder southerly flow develops over South Island, with light snow on the mountains.

* 9th - Cold southerlies over most of NZ. Some hail and thunderstorms in east and south of both islands, also north of North Island. Snow on ranges of both islands; down as low as 600 metres in parts of North Canterbury.

* 12th - Warmest day of the month, with a 32C max in Blenheim and 31C in Ashburton, due to northwesterly flow.

* 16th - Some heavy rain on South Island West Coast.

* 18th-23rd - Trough and unsettled weather over North Island, followed by wintry outbreak. (see details below)

* 27th - Rain damages cherry crops near Alexandra.

* 29th December-3 January - Cold unsettled period spoils the holiday period (see details below)

JANUARY 2007

* 4th - Chilly overnight in many areas. Light frost at Martinborough, Wairarapa.

* 6th - Cooler southerly change brings rapid drop in temperatures (e.g. from 26C to 14C) in Canterbury and Otago in afternoon.

* 9th - Some heavy rain in north of North Island.

* 10th - Heavy rain in Bay of Plenty (106mm in 16 hours at Haparapara) and Northwest Nelson (103mm in 24 hours at Little Devil)

* 12th - Foggy in Tauranga. Warm 30C max in Tawa.

* 13th - Sea fog causes disruption to Wellington Airport. Elsewhere in the southwest of the North Island, a foehn northeasterly flow results in warm 27-29C maximums. Even warmer in Alexandra, with a 32C maximum there.  Heavy rain in Westland, e.g. 223mm at Cropp; 206mm at Waiho.

* 14th - Warm 32C max in Hastings. Heavy rain in Bay of Plenty, e.g. 94mm in 24 hours at Kaimai Summit. Slip in Rotorua as result of rain falling at about 10mm per hour.

* 22nd - Maximums into the low 30s in many eastern places, eg 34C max in Napier.

* 23rd - Continuing very warm in the east from Marlborough northwards. (cooler southerly change further south) 23C max in Blenheim; 33C in Napier and Hastings.

* 24th - 31C max in Alexandra; 30C in Hanmer. Foggy day in New Plymouth. Warm and humid in north of North Island.

* 25th - Warm and humid over most of the North Island, with 19-20C overnight minimums. 32C maximums in Napier and Hastings. Some afternoon thunderstorms in Gisborne area. (29C max in Gisborne)

* 29th - Colder west to southwest airflow spreads onto west and south of South Island, with fresh snow on the mountains. Only 8C in Wanaka at 4pm.

FEBRUARY 2007

* 1st - Heavy rain in Tararuas and Horowhenua, e.g. 125mm at Angle Knob.

* 2nd - Two waterspouts sighted on Lake Ellesmere (south of Christchurch), during a southerly flow. (thunderstorm reported in South Canterbury)

* 4th - Heavy showers in Bay of Plenty.

* 5th - 31C maximum in Alexandra.

* 6th/7th - Heavy rain causes disruption of northern areas. (see details below)

* 8th - Hot 32C maximum in Upper Hutt in a foehn northeasterly flow.

* 9th - Some afternoon thunderstorms about the Canterbury foothills; also in the central North Island and Hawke’s Bay.

* 10th - Thunderstorms developing during the day in parts of Auckland, Coromandel Peninsula, and Waikato, some with hail. (e.g. in Matamata)

* 17th-19th - Very warm days in Central Otago, e.g. 31C maximums in Alexandra on 17th and 18th.

* 23rd - Only 13C maximum in Ashburton in a damp southerly flow.

* 25th - Heavy rain in Fiordland, e.g. 61mm at Milford Sound.

* 26th - 30-32C maximums in Canterbury due to a very warm northwesterly flow. (reported maximums up to 35C at some suburban Christchurch stations)  Cool southerly change later in afternoon, sends temperatures plummeting by about 12C. 30C maximums in Napier and Hastings.  A few afternoon thunderstorms in eastern parts of Northland.

MAJOR EVENTS

18th-23rd December - Trough and unsettled weather over North Island, followed by wintry outbreak.

A trough and low pressure system moved in from the north to give a wet spell to the North Island, followed by another one which resulted in a spell of wintry southerlies from the 20th-23rd.

A cold front (which had moved onto the island the previous day) became very slow moving over the North Island on the 18th, with warm, humid air pushing onto northern areas. This delivered some heavy falls of rain, including a mid afternoon downpour in Rotorua (28mm in 2 hours; 5mm in 10 minutes) which caused surface flooding . A girl was swept down a drain, but fortunately rescued.

On the 19th, a low moved from subtropical latitudes moved southwards over the northeast Tasman Sea, pushing more warm, humid air over the North Island. This time, it brought some overnight heavy rain to the Coromandel Peninsula overnight 19th/20th, including 29mm in Whitianga between 3 and 4am.

At the same time, a trough and cold front moved onto the South Island. During the 20th, these two features combined to form a complex low pressure system to the east of the South Island. This resulted in cold, wintry southerlies spreading over South Island, with snow on the high country to about 700 metres in Central Otago and inland Canterbury. (The Hermitage, Mt Cook was blanketed) Heavy rain also fell in parts of Canterbury, with some flooding reported in Christchurch. (56mm was recorded there up to 5am the next morning)

From the 21st-23rd the low remained slow moving to east of South Island, maintaining the cold, showery southerlies over South Island, while southwesterlies covered the North Island. Further snow showers affected the South Island high country (flurries being reported as low as 500 metres in Central Otago during the early morning of 21st), and also the central North Island mountains. Thunderstorms and hail were reported in many parts of the North Island on 21st, especially in the Bay of Plenty.

The weather improved over the country on the 23rd, as the low started to move away and the flow weakened.

Mean sea level analyses for midday NZST (1pm NZDT) 18th December to midnight NZST 23rd December in 12 hour steps are shown here.

29th December-3 January - Cold, unsettled period spoils the holiday period.

The last spell of wintry weather in an unusually cold December unfortunately coincided with the New Years holiday period.

A cold front which moved onto the South Island overnight on the 29th morphed into a complex low pressure system during the day. Cold unstable air moved onto the South Island, resulting in afternoon thunderstorms in Otago and later in parts of Canterbury.

On the 30th and 31st this low pressure system, in a manner similar to the cold outbreak of the 20th-23rd December, became slow-moving to the east of the South Island, maintaining cold southerlies over the South Island and Wellington/Wairarapa. Heavy rain in Mid and South Canterbury overnight and morning of the 30th caused serious flooding in places, especially around Ashburton (six houses flooded) and Geraldine (where a camping ground was evacuated). In the high country, snow fell above about 900 metres in Canterbury, and 700 metres further south. Daytime maximums were predictably well below average, e.g. 10C in Christchurch and 11C in Ashburton and Masterton on the 31st. Over most of the North Island, a lighter southwesterly flow prevailed, though showers (and a few thunderstorms) peppered many parts of the island. Except on the South Island West Coast, any New Years celebrations held outdoors weren't the most comfortable events, due to the cold and (in many places) wet weather.

The low pressure system gradually moved further to the east during the first two days of 2007, allowing the cold southerlies to spread over the North Island. Afternoon thunderstorms affected many parts of the North Island, especially central and northern areas. The southerly flow eased on the 3rd, with the weather improving in most areas.

Mean sea level analyses for midday NZST (1pm NXDT) 29th December to midnight NZST 3rd January in 12 hour steps are shown here.

6th/7th February - Heavy rain causes disruption to northern areas.

Very moist, warm and unstable air resulted in heavy downpours in some northern North island areas during this period, with major disruption caused by flooding in parts of Northland and the western Waikato. A subtropical low already in the northeastern Tasman Sea on the 5th, led to a warm, humid east to northeast flow over northern areas becoming even more so on the 6th, with widespread rain.  Disturbances in this flow brought thunderstorms with heavy downpours to several places, especially the far north. Flooding there damaged a bridge on SH1 near Te Kao, cutting off people further for several days until it was repaired on the 10th. Waitangi Day ceremonies at the Treaty House were disrupted by rain and strong winds. Further south, heavy overnight downpours on the 7th caused flooding and slips to parts of the western Waikato, such as Raglan.

Elsewhere, Murchison recorded a hot 33C maximum on the 7th, as the subtropical air mass moved into higher latitudes and shed its cloud cover. Meanwhile, conditions eased in the north during the day, as the low pressure system weakened.

Mean sea level analyses for midnight NZST 5th January (1am NZDT on 6^th)to midday NZST 7th January in 12 hour steps are shown here.

CHRISTCHURCH WEATHER - SUMMER 2006/07

By Ben Tichborne

DECEMBER 2006

December 2004 was unusually cold and wet, December 2005 was unusually warm, and December 2006 - well it was unusually cold and wet, just like 2004.  Four cold southerly outbreaks were the dominant features of this month's weather.  Fresh snow coated the Alps on the 1st, following the wintry last day of November.  When the southerly died out at night, clear skies combined with little wind resulted in one of the city's coldest December nights on record, with a light frost being recorded.  The second cold snap arrived on the 8th, with only light southerlies and cloud in Christchurch, but the winds had freshened by early the next morning with some hail showers. More snow fell on the Alps, coming down as low as 600 metres on the North Canterbury mountains behind Oxford.

The other two cold southerly outbreaks were much more prolonged and wetter with snow on the inland high country, as lows became slow moving to the east of the South Island.  Heavy rain caused some flooding around parts of the city on the 20th, and showers persisted through to the 23rd, including some hail on the 21st. Southerlies reached gale on Banks Peninsula.  The last cold southerly of the month began with scattered thunderstorms in parts of mid and South Canterbury on the afternoon of the 29th.  The next day saw more heavy rain in many parts of the area, with flooding in Ashburton and Geraldine.  Christchurch escaped with lighter rain, but southerlies again rose to gale force for a time. Some further rain fell the next day, but fortunately the evening was dry for New Years celebrations, even though temperatures were far from summery.

JANUARY 2007

A predominance of onshore airflows meant that summer continued to be much cooler than normal, even though this month was more settled than December.  The cold southerly that ended December persisted through the first two days of the New Year, with some drizzly showers, clearing on the 3rd. From the 9th-16th and again from the 25th-28th, complex low pressure systems brought humid and cloudy weather with some rain or drizzle at times.  By contrast, there were also a few warm days with west or northwest flows.

FEBRUARY 2007

High pressure systems were dominant over NZ this month, with generally settled weather over most of the country.  However, the positioning of these systems resulted in onshore airflows still being predominant over Canterbury.  So while Christchurch had below normal rainfall (with only a few days recording rain or drizzle), it remained cloudier and cooler than average. There were some warm days, especially the 26th, which saw some city weather stations recording 34-35C maximums.

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