The first "winter blast" of the year is on its way, bringing rain, snow and gales - and a "significant nationwide temperature drop".
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"Over the next few days we wave goodbye to our mild May and open the door to winter!" NIWA says.
"Wet and windy weather will be abundant for the week ahead with chilly temperatures and even some snow moving in on Friday and into the weekend."
Weather Watch says the week kicks off warmer than average due to sub-tropical airflows crossing New Zealand, leading to heavy rain and windy westerlies for the West Coast.
But as the week progresses, the winds strengthen while the warmth falls away.
"By Wednesday the sub-tropical portion ends but the windy westerlies ramp up further with gale force nor'westers around the upper South Island and Cook Strait/Wellington area," Weather Watch says.
By the end of Thursday a southerly change starts to move into the lower South Island. By Friday those winds turn cold, with a "significant nationwide temperature drop" by the start of the long weekend.
"Single digit daytime highs are forecast for Southland and Otago by Friday and Saturday with snow possibly falling in Queenstown on Friday and Saturday," Weather Watch says.
"Daytime highs in Queenstown by then will be just 3 or 4 degrees above freezing at the warmest point of the day. State Highways are likely to be impacted by snow along with all alpine passes."
The North Island will be affected too, with windier sou'westers kicking in by Saturday.
The southerly peaks across New Zealand on Saturday and Sunday, with another surge of cold wintry southerlies due early next week.