MetService is forecasting a substantial drop in temperatures this week, as a cold front moves over the country from the Southern Ocean.

The arrival of the cooler airmass will bring rain to many of the parched areas of the South Island.

Several heavy rain watches have been issued for the southern areas before this system travels further north.

"The change in temperature will be quite dramatic for some locations. Plenty of places in Otago and Canterbury will see a fall of ten degrees from Tuesday to Wednesday," says MetService meteorologist Alwyn Bakker.

Alexandra is in for a particularly remarkable drop, with a forecast high of 30°C on Tuesday and a mere 15°C on Wednesday.

"This isn't the largest change in temperature they've ever recorded in 24 hours, but it is close," says Alwyn.

"And if Alexandra doesn't rise above 15.7°C on Wednesday, they will see their coldest maximum temperature this summer."

Along with the cooler temperatures comes the rain.

"The parched south and east of the South Island will be in for some relief, with some locations forecast to see 40mm of rain in 24 hours. That's as much rainfall as parts of inland Otago have had all year."

Heavy Rain Watches have been issued for Otago, Southland, northern Fiordland, and the ranges of Westland south of Otira.

Coupled with the cold temperatures, that rain may fall as snow dusting the peaks of the ranges down to 1000 metres.

As the front travels further north, cooler temperatures continue but rainfall amounts are expected to decrease.

"The arrival of cold air may not be welcomed, but decreased rain intensity will be a relief for those parts of the North Island affected by Cyclone Gabrielle last week. MetService advises keeping up to date with our forecasts, as the criteria to trigger Heavy Rain Warnings have been lowered in many areas."

Image: MetService.