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National Climate Summary – July 2011

National Climate Summary – July 2011: Lows anchored south of country; freezing polar blast

• Wind: Westerly winds south of Christchurch extremely strong and frequent.
• Temperature: Monthly mean temperatures near average in many regions, but temperatures on some days during the month anything but typical.
• Precipitation: Well above normal for the west and south of the South Island, as well as for Wanganui to Waiouru. Very dry in the eastern South Island. Heavy snowfall event on 25 July.
• Sunshine: An extremely sunny July for the east of the South Island, as well as coastal Wairarapa. Cloudy for the west and south of the South Island.

Low pressures were anchored south of New Zealand and the Chatham Islands during July, producing an extremely windy and stormy month overall. Mean sea level pressures over the southern half of the South Island were unusually low for the month as a whole, and the monthly “westerly wind” index for Christchurch southwards was the second-strongest for July, since records began in 1941.

The month of July started out unusually warm in eastern areas of both islands, but a polar blast during 24-26 July delivered a bitterly cold air mass over the country – so that mean temperatures for July were near average, overall, for many regions of the country. Extremely cold air affected Canterbury, the Kaikoura coast, Nelson, Wellington, Wairarapa, Manawatu, Hawkes Bay and Taranaki during 25-26 July, and snowfall was heavy and to low levels over Canterbury, the Kaikoura Ranges (both Inland and Seaward), the Richmond Ranges, Tararua and Rimutaka Ranges, the Central Plateau, and around Mt Egmont. Brief dustings of snow were also reported in the ranges of Motueka and Northland on the 25th. The average temperature in July 2011 was 8.0°C (0.1°C above the 1971–2000 June average) using NIWA’s seven-station temperature series which begins in 1909.

The frequent westerly winds during July resulted in a very wet month for western areas of both islands. July rainfall totals were well above normal (exceeding 150 percent of July normal) across the south and west of the South Island, as well as for Wanganui to Waiouru, reflecting the stormy, southwesterly nature of the month. It was the wettest July on record around Invercargill, with more than 200 percent (double normal) July rainfall there. Two localised areas which were also very wet were the Firth of Thames, and Tolaga Bay. In sharp contrast, the entire eastern South Island was extremely dry (with less than 50 percent of July normal rainfall). Below normal rainfall (between 50 and 79 percent of July normal) was also experienced around Wellington, the Bay of Plenty, and eastern parts of Northland. Elsewhere, July rainfall was close to normal.

Sunshine totals were well above normal (exceeding 125 percent of July normal) for the entire east coast of the South Island, as well as coastal Wairarapa. It was the sunniest July on record at Cheviot. In contrast, it was very cloudy for the southwest of the South Island (Westland, Fiordland, Southland and central Otago). This contrast in sunshine hours across the Southern Alps reflects the enhanced westerly winds experienced during the month. It was also rather sunny for Nelson, Hawkes Bay, and much of Northland and Auckland (with totals between 110 and 125 percent of July normal). Elsewhere, July sunshine hours were close to normal.

Further Highlights:
• The highest temperature recorded was 19.1°C at Hastings on 11 July.
• The lowest temperature recorded was -10.2°C at Manapouri on 26 July (a new July and all-time record there).
• The highest 1-day rainfall experienced was 128 mm recorded at Arthurs Pass on 10 July.
• The highest gust was 189 km/hr observed at Cape Turnagain on 12 July.
• Of the six main centres in July 2011, Auckland was the warmest, Christchurch the coldest, Tauranga and Auckland jointly the sunniest, Hamilton the wettest and Dunedin the driest.

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