WEATHER

NIWA National Climate Summary: A month of two halves; cold & dry to start, then wet & warm

Monday 6 August 2012, 2:55PM
By NIWA
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Rainfall
Extremely wet in Northland, Western Bay of Plenty, Waikato, Gisborne, southern Hawkes Bay, Wairarapa, Tasman, south Canterbury and parts of Otago.  In contrast, it was unusually dry in Southland.

Temperature
The first half of July was unusually cold and dry, with particularly severe frosts between 1 and 5 July.  In stark contrast, northwest winds produced unusual warmth mid-month, and the last half of July was extremely warm.  Temperatures for the month as a whole were near average for many regions.  The exceptions were the south and west South Island, and along the northeast coastal margin of both Islands (with above average temperatures).

Sunshine
An extremely sunny July for the western South Island and central-west North Island.  Very cloudy in Gisborne, Wellington, and Nelson.
Soil moisture   As at the end of July, below normal soil moisture levels were observed in south Canterbury for the time of year.  Near normal levels elsewhere.

Overview
July started unusually cold and dry, due to winter time anticyclones or ridges prevailing over the country during the first half of the month, bringing clear skies, light winds and a recipe for frost.  Frosts during the period 1 July to 5 July were particularly severe.  In stark contrast, northwest winds produced unusual warmth in eastern areas mid-month.  During the last two weeks of July, lows dominated over the north Tasman Sea, bringing unusually mild conditions, northeast winds and high rainfall to northern and eastern regions of the North Island, as well as Nelson/Marlborough.

For the month as a whole, higher than normal pressures were observed over New Zealand and to the southeast, with lower pressures than usual over the north Tasman Sea. This resulted in more northeast winds than usual over the North Island.

It was an extremely wet July (with more than 150 percent of July normal rainfall recorded) in parts of Northland, the Western Bay of Plenty, Waikato, Gisborne, southern Hawkes Bay, Wairarapa, Tasman, south Canterbury, and parts of Otago. Other regions which experienced above normal rainfall for July (between 120 and 149 percent of July normal) included  Taranaki and Westland.  In contrast, it was an unusually dry July for Southland, with rainfall totals less than 50 percent of July normal. It was the driest July on record for Invercargill and Tiwai Point.  Rainfall was also below normal for much of north Canterbury, Fiordland, and between Wanganui and Waiouru (with totals between 50 and 80 percent of July normal). Elsewhere, near normal rainfall (between 80 and 120 percent of July normal) were generally observed.  At the end of July, soils were much drier than normal in south Canterbury, but soil moisture levels were generally near normal elsewhere.

Because of the change mid-month from extremely cold and frosty conditions, to an unusually warm period, air temperatures for July as a whole were near average for many regions of the country (within 0.5°C of July normal).  The exceptions were the south and west of the South Island, and along the northeast coastal margin of both Islands (where above average temperatures were observed, between 0.5°C and 1.2°C above the July average).  Patches of below average temperatures (between 0.5°C and 1.2°C below the July average) were observed around Reefton.  The nation-wide average temperature in July 2012 was 8.4°C (0.5°C above the 1971-2000 July average), using NIWA’s seven-station temperature series which begins in 1909.

It was an extremely sunny July for western and alpine regions of the South Island, with sunshine totals exceeding 125 percent of July normal. It was the sunniest July on record for Queenstown and Mt Cook.  Above normal sunshine totals (between 110 and 124 percent of July normal) were observed in the central North Island, as well as Canterbury.  In contrast, it was a rather cloudy July for Gisborne, Wellington, and Nelson (with between 75 and 90 percent of July normal sunshine experienced).  Sunshine totals for July were near normal elsewhere (between 90 and 110 percent of July normal).

Further Highlights:
•       The highest temperature was 22.6°C, observed at Rangiora on 15 July.
•       The lowest temperature was -11.3°C, at Ranfurly on 2 July.
•       The highest 1-day rainfall experienced was 336 mm at North Egmont on 15 July.
•       The highest gust recorded was 132 km/hr at Cape Reinga on 29 July.
•       Of the six main centres in July 2012, Tauranga was the warmest, wettest, and sunniest, Christchurch the coolest, Dunedin the driest, and Wellington the cloudiest.