Humid air heralds the school holidays

MetService News Release
25 September 2017

Humid air heralds the school holidays

Hot, humid air is flowing across the Tasman Sea from Australia, bringing with it a raft of severe weather. In classic spring fashion, it will move quickly with changeable weather for the rest of the week, but the next twenty-four hours will be wild, wet and windy.

This image shows the air mass and wind flows, where greens and yellow show a tongue of warmer, more humid air stretching across the Tasman from Australia.

As the air moved across the Tasman it picked up a lot of moisture, which is leading to heavy rain in western parts, with warnings issued from Mount Taranaki to Westland. In the ten hours between midnight and 10am today, over 100mm of rain had fallen on some stations on the West Coast, with more to come.

The western areas experiencing heavy rain have also echoed to the sound of thousands of lightning strikes, with 8000 strikes over Westland alone in the last twelve hours. The thunder and lightning will mostly be confined to western South Island today, but tomorrow a moderate risk spreads up into Taranaki and the Kapiti Coast. Winds are also reaching severe gale in places, and gusts of around 130km/h are expected in central parts from Hawke’s Bay to Canterbury today and tomorrow.

“The strong northwesterly winds and humid air lead to high temperatures in the east, due to an effect known as foehn heating,” explained meteorologist Tom Adams. “The northwesterlies can be a fickle phenomenon, however, as sometimes they remain aloft and don’t bring the warmer air down to the surface.”

Dunedin oscillated between warm northwesterlies and cooler northeasterlies, reaching 22C yesterday evening, then turning northeast and cooling to 17C at 1am, before turning northwest and reaching 22C in the city at 3am. Fairlie maintained the northwesterlies overnight and hit 24C at 5am, nearly 3 degrees warmer than Niue airport at the same time.

This severe weather will ease as it moves over the North Island tomorrow, although heavy falls of rain are expected for all but the eastern-most parts. The weather eases on Wednesday as a ridge of high pressure moves across the country, leaving Wednesday and Thursday as the best days to visit the Wellington Spring Festival, but by the end of the week another band of wind and rain is set to provide an active end to the school term.

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