From anticyclones to active westerlies

By Daniel Corbett, Metservice weather ambassador

The winter so far has had a bit of variety with polar outbreaks, large anticyclones, moisture-laden Tasman Sea fronts and broad troughs.

The anticyclones were the dominant feature during the late part of April, much of May and into the first few days of June.

July's weather eventsThe persistent southwest flow and polar air brought significant snow to the western coasts of Fiordland, Westland and Buller in July. Polar chilled air brought severe frosts to parts of New Zealand. Lake Pukaki dropped down to around -10°C at night for several nights and low cloud and fog was common in places too.

A moist northeast flow helped fuel an active front that brought heavy downpours and thunderstorms to Auckland in early July. Radar rainfall estimates were about 30-40mm an hour in the heaviest rain, resulting in flash floods to the city and several days later the already moist air combined with light winds brought dense fog to Auckland and Waikato.

During the second week of July a slow-moving front with moisture from the sub-tropics brought several days of heavy and persistent rain. By mid-month another large anticyclone brought settled weather again to much of New Zealand.

Winter's outlook Equatorial sea surface temperatures have continued to warm over the past month and the area of above-average sea surface temperatures over the eastern Pacific has now begun to slowly extend westwards. The subsurface waters of the eastern equatorial Pacific have also shown signs of warming in the past few weeks. The warmer zone of water that did extend from the Coral Sea down to the Kermadec group has relaxed to normal. The waters around New Zealand are also about average. Global models are picking the current neutral conditions to begin to develop into El Nino conditions over the next several months.

Conditions in the central Pacific are currently in-between a La Nina and an El Nino, indicating more variability in our weather.

In the shorter term the active westerlies will keep the weather systems rolling in from the west across New Zealand every few days interspersed with anticyclones bringing some fine and dry spells. These anticyclones have recently been tracking a bit further north, partly due to the seasonal shift, but also maybe hinting at other atmospheric changes, such as the building El Nino.

The risk for further spells of polar-chilled air arriving behind departing weather systems will still be possible over the next few weeks.

Any low-pressure system developing in the central Tasman Sea may draw moist winds from the tropics southwards, clashing with cold southern air. This may lead to thunderstorms with possible downpours, lightning, hail and gusty winds. The active fronts from the Southern Ocean will also tend to have more bite as we progress through the meat of the winter season.

During late winter, a change of weather pattern is expected, with more frequent fronts moving on to the country from the south Tasman Sea followed by episodes with cool southwest winds lasting several days.

This may bring windy showery conditions to all western districts and dry and mild conditions to the east and is currently the most likely pattern for spring. A few polar outbreaks may be mixed in, caused by a low deepening over the Chatham Islands, bringing a brief period of chilling southerly winds, typical of winter.

- Hamilton News

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