Invercargill is in for a muggy Tuesday, with MetService forecasting a high of 24C and showers from the afternoon that could turn heavy and thundery.

That would be nine degrees warmer than Monday, when the city topped out at a forecast 15C under easterlies. Gore is expected to reach 22C on Tuesday, with cloud building through the day and the chance of a shower from the afternoon.

MetService's outlook for the week, issued on Monday with commentary from meteorologist Maddy Neal, said moist subtropical air would reach the South Island on Tuesday and picked out Invercargill and Gore as places in for a sticky day. Rain is expected to spread over much of the island.

The city forecast is for a partly cloudy start and northeasterly winds, then occasional showers in the afternoon and evening. Overnight lows of about 10C mean it will not cool off much after dark.

The same humid air is causing more serious trouble in the north. Until 9pm on Monday MetService had a severe thunderstorm watch in force over Auckland, Northland and Great Barrier Island, warning of localised downpours and flash flooding, and heavy rain watches from Northland to Taranaki, and inland to Taupo, running from Monday night into early Wednesday. The Monday outlook did not list any watches or warnings for Southland or Otago.

The warmth will not last. A change to southwesterlies arrives on Wednesday afternoon, bringing morning showers to Invercargill and a high of 18C, and occasional rain to Gore, where 19C is forecast.

From there MetService expects a westerly flow to take over, with cooler and less humid nights and generally settled weather to finish the week. Invercargill's forecast highs slide to 16C on Thursday and 14C on Friday, when the overnight low is put at 6C. Gore is tipped to dip to 5C on Friday and Saturday mornings.

Queenstown misses the worst of the humidity, with 19C and a few showers on Tuesday, rain for a time on Wednesday and fine weather returning by Saturday.

Saturday looks like the pick of the coming weekend in the south at this stage, with a partly cloudy 16C in Invercargill and 17C in Gore under light winds.

The swing is a sharp one for early November: a 24C afternoon on Tuesday followed three days later by a 14C high and a single-figure night.

Anyone planning to be outdoors on Tuesday afternoon should keep an eye on the sky. Thundery showers can bring sudden downpours, and MetService updates its severe weather watches and local forecasts through the day at metservice.com.