Autumn 2018

A warm start to autumn, then cooler and unsettled at times.

Temperature

Nearly all of the North Island observed above average (0.51 to 1.20°C of average) temperatures during autumn, along with isolated pockets of near average (-0.50 to 0.50°C of average) and well above average (>1.20°C of average) temperatures. In the South Island, most locations recorded near average (-0.50 to 0.50°C of average) temperatures during autumn, with a handful of stations observing above average (0.51 to 1.20°C of average) temperatures.

Rainfall

Autumn rainfall in the North Island was generally near normal (80-119% of normal) or above normal (120-149% of normal), with a handful of locations also recording well above normal (>149% of normal) rainfall. In the South Island, above normal (120-149% of normal) or well above normal (>149% of normal) rainfall was observed in most locations, with a few spots also recording near normal (80-119% of normal) rainfall.

Soil moisture

As of 1 June, soil moisture was near normal in most of the North Island, along with the western and southern South Island. However, soils were wetter or even much wetter than normal in small portions of the lower North Island and nearly all of the eastern South Island.

Overview

Autumn 2018 began on a warm note, as March was characterised by significantly higher pressure than normal to the east of New Zealand. This pressure pattern, in concert with the decaying La Niña in the tropical Pacific, caused more northeasterly winds than usual over the country. Warm, humid air masses, combined with the remnants of the marine heatwave in the Tasman Sea, influenced higher than usual temperatures over New Zealand as well as some heavy rainfall events.

However, the pressure pattern for April resulted in more southwesterly winds than normal for much of the country. Several low pressure systems and cold fronts passed over New Zealand, bringing adverse weather to many locations. Storms occurring on the 10th-11th and 28th-29th days of the month were particularly damaging with the former bringing destructive winds to Auckland, while the latter brought heavy rain to parts of the country, resulting in flooding and the declaration of a state of emergency in Rotorua.

In May, mean sea level air pressures were much lower than normal over and to the south of New Zealand. The first half of the month was relatively warm and dry throughout the country. However, during the second half of May, a blocking anticyclone became established over the southeast of Australia, which delivered a prolonged period of disturbed westerly and southwesterly winds over the country. A ridge of high pressure covered the lower South Island during the final days of May, bringing heavy frosts and record cold temperatures to parts of Central Otago, Southland and the Mackenzie Basin.

Further highlights

  • The highest temperature was 31.8°C, observed at Kawerau on 5 March.
  • The lowest temperature was -8.8°C, observed at Mt Cook Airport on 31 May.
  • The highest 1-day rainfall was 205 mm, recorded at Secretary Island on 19 March.
  • The highest wind gust was 187 km/hr, observed at Akitio on 21 May.
  • Of the six main centres in autumn 2018, Auckland was the warmest, Dunedin was the coolest and least sunny, Auckland and Wellington were the equal-wettest, Christchurch was the driest, and Tauranga was the sunniest.

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Contact

For further information, please contact:

Mr Chris Brandolino, Principal Scientist – Forecasting, NIWA National Climate Centre
Tel. 09 375 6335, Mobile 027 866 0014

Mr Seth Carrier, Meteorologist/Forecaster, NIWA National Climate Centre
Tel. 09 375 4508

Research subject: Climate