The West Coast is in for days of prolonged rain from midweek as a "significant winter storm" stalls over the South Island on Wednesday, with heavy rain expected to swell rivers before heavy snow closes in on inland passes later in the week.
MetService said a heavy rain watch is already in force for the ranges of the Westland District, covering 36 hours from noon Wednesday to midnight Thursday, and will be upgraded to a warning tomorrow.
MetService meteorologist Devlin Lynden told 1News the whole event traces back to a single system, a low pressure zone currently sitting near Adelaide.
"It really arrives at the South Island and starts to slow down really rapidly as it gets cradled between these two areas of high pressure," Lynden said, describing a band of rain left "just sitting on the South Island for quite a prolonged period of time".
He said the rain would linger from early Wednesday through Thursday and would not give up until the early hours of Friday, when it finally begins to move north.
The heaviest falls are expected across the Southern Alps and West Coast, where the highest accumulations are likely to build in the ranges.
West Coasters are well used to downpours, but Lynden said the duration set this event apart.
"The prolonged nature means that the rivers will rise higher than normal just because it's a long period of time, and more surface flooding, and more of everything just because of the length of it," he said.
Colder southerly air then changes the picture from Thursday, driving snow to low levels inland.
NZTA has warned motorists that more than 30cm of snow could affect State Highways 8 and 80 in the Mackenzie Basin and South Canterbury on Thursday, with up to 20cm on State Highway 73 above 700m between Christchurch and the West Coast.
"Accumulations at this level are likely to impede travel and NZTA may close sections of highway or introduce restrictions to keep people safe and avoid vehicles becoming trapped," an NZTA spokesperson said.
The North Island escapes the worst of it, staying settled through the first half of the week with light winds and dry weather for most. Strengthening northwesterlies then drag a warmer airmass across the island as the week goes on, with temperatures climbing noticeably.
Full list of weather watches and warnings
Heavy Rain Watch
Ranges of the Westland District - 36 hours from 12pm Wednesday to 12am Thursday (will be upgraded).
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