Eyes on the east and west this week

May 8, 2017

The weekend’s calm conditions continue for a couple of days yet, but cloud and heavy rain are on the way.  A trough in the Tasman Sea brings a gradual increase in cloud and rain for those in the west but, for the most part, a broad high over New Zealand holds it at bay until mid-week.

On Thursday, the trough makes its way across the country, coinciding with ex-Tropical Cyclone Donna passing near to New Zealand.  There is still some uncertainty about the exact track of Donna, which is currently slow-moving in the tropics and causing gales and heavy rain in northern Vanuatu. The Fiji Meteorological Service – the organisation currently officially  responsible for Tropical Cyclone Donna – have categorised the system as a category 5, and expect it to weaken as it moves southeast out of the tropics.  The worst of the possible scenarios for New Zealand include the remains of the system passing close to the northern and eastern parts of the North Island.

The next few days will see showers in the west, heavier and persistent for Westland and Buller provinces. Elsewhere, the cloud-free skies of the weekend are a pleasant memory as sheets of cirrus cloud gather, and northerly winds bring down warmer, moister air from the tropics.

The approach of the trough from the Tasman will bring widespread rain to the North Island and upper South Island late Thursday into Friday.  If the remains of TC Donna draw near to New Zealand on Thursday, it will have weakened and transitioned into an ex-tropical cyclone by this point, but the added energy of the trough could lead to an intensification and the chance of some heavy falls of rain to the upper North Island.  There is also a smaller risk of some strong winds in eastern areas under this scenario, with the caveat that it depends on Donna’s track, the most likely of which keeps it away to the north.

“It is a question of probabilities,” explained meteorologist Tom Adams. “It is very certain that the trough in the Tasman will bring rain later this week. Additionally, there’s a chance that the remnants of Donna will pass close to New Zealand – but we must consider the whole range of possibilities. People should expect some rain on Thursday or Friday, and keep an eye on Severe Watches and Warnings in case there are areas where conditions could get severe.”

Once the trough has passed by New Zealand, including the Chatham Islands which could also be in for severe conditions, a strong southerly will sweep the country.  Although the odds are stacked for a wet Saturday, the weather looks like fining up again for Mother’s Day.

Satellite image showing the trough to the west of New Zealand, and Tropical Cyclone Donna near to Vanuatu

Official Severe Weather Watches and Warnings are reviewed and re-issued by MetService at least every twelve hours, and more often if necessary. To get the most up to date information on severe weather around the country, or any other forecasts, see metservice.com or on mobile devices at m.metservice.com. You can also follow our updates on MetService TV, at MetService New Zealand on Facebook@metservice  and @MetServiceWARN on Twitter and at blog.metservice.com

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