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Posted on Wednesday August 18th at 8:47 a.m.
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While Canterbury's winter has been wet and lacking in sunshine, polar blasts and snow have largely been absent.
Forecasters warn that may be about to change with stirrings in the polar vortex, the ring of westerly winds encircling the Antarctic.
The southern annular mode (Sam), which measures the strength of the vortex, has been at record highs in the past month, meaning the polar air has been locked around Antarctica.
However, computer models show Sam is expected to weaken over the next few weeks and return to negative values for the first time since April, allowing polar outbreaks to head north towards New Zealand, Australia, South Africa and South America.
MetService spokesman Bob McDavitt said it was too early to say which countries would get the freezing southerlies. However, the computer predictions meant the South Island was at risk.
"For most of the winter we haven't had the good old polar outbreaks. We've had a few goes at snow, but it hasn't been like previous years."
A southerly today and tomorrow would bring snow to Canterbury, but it was expected to be above 500 metres.
National Institute of Water and Atmospheric Research principal climate scientist Dr James Renwick said Sam's high values had been unusual.
"These positive episodes don't normally last for more than a month or two at the most, but it is very unusual this winter, it's lasted four months," he said.
Monthly values recorded since 1979 showed Sam was more positive than it had been for 30 years. "There is a trend towards the positive, which is being put down to ozone depletion, greenhouse gas emissions, and it will continue to be more positive as time goes on."