Wintry weather to set in for Canterbury
Significantly colder weather is looming, with Canterbury set for a return to wintry conditions at times over the next week.
Labour Weekend looks likely to be its traditional changeable self, with a cold blast of polar southerlies possible on Labour Day.
MetService says the mild nor'westers that have been buffeting the region are about to swing to much colder, squally southwesterlies this afternoon, bringing a risk of heavy showers, hail and thunder inland, and even snow to about 500 metres further south.
The cold winds are expected to ease tomorrow, with mostly fine weather developing, and Saturday looks likely to be milder, with another spell of northwesterlies before a return to southwesterlies later on Sunday.
At this stage, computer models suggest Monday could become very cold and showery across the region, followed by a couple of days' respite before more southwesterlies later next week.
MetService forecaster Larissa Maringchamko said the general atmospheric pattern across the South Island was tending more southwesterly over the next week. That would allow cold fronts to move up the east coast more frequently.
This afternoon's change was likely to bring thunderstorms and hail, but mostly inland about the Canterbury foothills, she said.
''After that it should be clearing to a nice day on Saturday,'' she said. ''On Sunday, it looks like fresh northerlies with some rain, and then Monday winds changing southerly and bringing showers."
The Antarctic Oscillation - a measure of the strength of the polar vortex that keeps the coldest air trapped near Antarctica - is in a negative phase at the moment, meaning the vortex is weaker and cold air can break out more easily and move north towards New Zealand.
The last time the oscillation was strongly negative was in June, when Christchurch had heavy southerly snow and weeks of frost.