Weather warnings upgraded as summer dampener closes in

Forecasters have upgraded a number of warnings for wet and windy weather expected to hit the South Island from today, with more than a month's worth of rain expected to fall in a day in some areas.

While the West Coast is set to be slammed by the incoming storm, inland Otago is also being warned of a sharp change in summer conditions towards the end of the week as rain tips over the main divide.

MetService has issued a red warning for a chunk of the West Coast, where up to 800 millimetres is expected to fall about the ranges and up to 200 millimetres about the coast.

The heaviest falls are expected from late Friday morning, when peak rates could reach 30 to 35 millimetres an hour.

The forecaster is warning that slips and floodwaters will likely disrupt school holiday travel as some roads become impassable and communities potentially shut off.

The heavy rain is expected to ease on the coast from early Saturday morning.

Meanwhile residents and visitors in Fiordland should also be on alert, with an earlier weather watch there upgraded this morning to an orange warning.

Up to 350 millimetres of rain is expected, with peak rates of up to 35 millimetres per hour, enough to cause surface flooding, slips, and rapid rises in streams and rivers.

There will be periods of heavy rain for the headwaters of Otago lakes and rivers, approaching warning criteria within 15 kilometres of the main divide.

Strong winds, approaching severe gale in exposed places, are forecast for Canterbury high country and Fiordland on Friday.

NIWA meteorologist Ben Noll describes "an atmospheric river of moisture" tracking towards the West Coast.

In a post to Facebook, he says it will make land on Friday, delivering up to a month's worth of rain in a day to Westland, possibly causing flooding and slips.

"The moisture plume extends back to northern Australia, where the monsoon is active. Trampers, campers and holidaymakers in Fiordland and the West Coast, take note!"