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MetService meteorologist Claire Flynn said a ridge of high pressure would lie over New Zealand tomorrow, and a front approaching the South Island was forecast to send temperatures soaring to 30degC or more in the city.
"It's caused by the combination of two things. Firstly, there is quite warm, moist air coming across the Tasman Sea from Australia.
"And then that's coming up against the Southern Alps, and then we've got northwesterly winds across the Alps, so that causes all that moisture to turn to rain and get dumped on the western side of the Alps.
"Then as the wind crosses the Southern Alps, it dries out and heats up."
As the air flowed eastward across Otago, it would continue to heat up, she said.
"It's called the Foehn effect and that's what's causing the warm temperature for Dunedin," she said.
Dunedin was expected to reach 30degC because it would be mainly fine, whereas temperatures in other parts of Otago and Southland were expected to range between 27degC and 29degC because they were forecast to be cloudier.
The forecast is for rain and cooler temperatures on Sunday.
MetService has advised it is likely that rainfall will reach warning levels in Fiordland late tomorrow and throughout Sunday.
It is also likely rainfall will reach warning levels in the headwaters of the Otago lakes and rivers on Sunday.
"In addition, northwesterlies are expected to become gusty about eastern parts of the South Island on Saturday, and there will be gales in exposed places on Sunday.
"There is moderate confidence of severe northwest gales about eastern and southern Fiordland, Southland, Otago on Saturday and Sunday."