Dunedin is experiencing hail storms, crashes of thunder and flashes of lightning while heavy snow and rain warnings are in place for some areas of Otago and Southland as the weather worsens in the South.

A broad complex trough is expected to cover the country next week through to Thursday, with embedded low centres.

MetService has issued a severe weather watch for Fiordland and coastal Otago.

"Cold southerlies are expected to affect the south of the South Island from this afternoon through to later Tuesday, bringing wintry conditions to the south of the South Island. "

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Snow is predicted to fall as low as 400m with significant accumulation at 800m in inland Southland, Central Otago and Clutha.

"This watch is for the possibility that the snow accumulation may approach warning amounts for eastern Fiordland, Southern Lakes, and inland Dunedin."

The weather watch is also in place due to the possibility that rainfall accumulation may reach warning amounts for coastal Otago.

MetService meteorologist Andrew James said the change would be felt in the South Island today.

"A lot of cold and wintry weather is on the way down south," he said.

"Tonight, Queenstown and Wanaka are looking like they will get down to about 2C, which is cold for this time of year."

"It's going to be a cold and showery week."

Outbreaks of rain and even hail are expected over much of the country during this period, with the more persistent rain setting in about eastern parts of the South Island.

James warned that South Island motorists needed keep an eye on the forecast for roads like Arthur's Pass, Porters Pass, and Lindis Pass over coming days.

MetService warned periods of snow were expected in the Lindis Pass, Crown Range Rd and Milford Rd.

Snow was expected about eastern parts of the South Island above 400 to 500 metres, and possibly about the North Island high country above 1000 metres.