Weather warning upgraded, polar blast on the way

There's bad news for farmers and those on school holidays, with forecasters upgrading warnings about bad weather.

MetService says a polar outbreak will make its way up the South Island over the next couple of days, before moving on to the North Island.

Heavy rain is forecast for many places, with warnings issued for Fiordland and Westland south of Ōtira.

MetService forecaster Angus Hines says many areas will be affected by the weather pattern, and the cold temperatures will see snow down to low levels.

"Snow is expected for Fiordland and inland Southland from 4am Wednesday morning, and then inland Otago and south Canterbury."

Mr Hines says the heavy rain may cause streams and rivers to rise rapidly. 

Heavy rain warnings and road snowfall warnings are now in place for many parts of the South Island.

Heavy rain watch

Area: Ranges of Buller and northwest Nelson

Valid: 24 hours from 11pm on Wednesday to 11pm on Thursday

Forecast: Periods of heavy rain.

Area: Ranges of Westland north of Ōtira.

Valid: 17 hours from 3pm on Wednesday to 8am on Thursday

Forecast: A period of heavy rain is expected.

Heavy snow watch

Area: Inland parts of Otago and south Canterbury

Valid: 12 hours from 11am to 11pm on Wednesday

Forecast: Snow is expected to lower to around 400 metres during the day. Heavy snow is likely above 800 metres, affecting the high country farmers and roads. Accumulations may approach warning criteria.

Area: Fiordland and inland Southland

Valid: 11 hours from 4am to 3pm on Wednesday

Forecast: Snow is expected to lower to around 200 metres during the day, with heavy falls above 400 metres where snow accumulations may reach warning criteria.