Photo: Unsplash / Daoudi Aissa
Orange rain warnings are in force for the Westland and Tasman districts today, while the rest of the coast is under a heavy rain watch.
MetService says up to 140 millimetres of rain could fall in the Westland ranges south of Otira, which could cause surface flooding and slips.
In Tasman, 80 to 120mm of rain was expected in the ranges, and 50 to 80mm possible nearer the coast.
The warning for the Tasman district, west of Motueka, applies from 5pm.
From late Tuesday evening 15 to 25mm peak rates were expected.
The rain was not unusual for the area but a little heavier than normal, MetService meteorologist Alec Holden told Morning Report.
Heavy rain watches cover the ranges of Buller and Westland north of Otira, the headwaters of the Canterbury lakes and rivers south of Arthur's Pass and Fiordland, along with the Richmond and Bryant Ranges, the Rai Valley and northern parts of the Marlborough Sounds.
Holden said a bog-standard front had come out of the Tasman which was "currently being squished by a pair of high pressure systems on either side of it".
This was causing a narrow and intense band of rain to be pushed directly up and over the Westland Ranges, he said.
People would need to be mindful around rivers and lakes, Holden said.
"This is your bog-standard front that's come out of the Tasman" - MetService meteorologist Alec Holden
Warm nights
MetService has told those who have already taken out the winter blankets that they might need to go back in the cupboard this week.
Warm muggy nights were expected thanks to warm air from the north.
"We should be seeing that more northerly flow ahead of the front be swapped out by a southwesterly as the front pushes through, though we're not expecting that to last particularly long as another ridge of high pressure moves in its wake," Holden said.