The active front is dropping the heavy rain to Fiordland, the headwaters of the Otago lakes and rivers (warnings), and the ranges of Westland (watch), Bellam said.
“The foehn effect [which occurs when strong winds sweep across mountain ranges] is boosting the temperatures in the east of both islands, where the highest temperatures are expected this week.”
Strong winds are expected to develop across inland Southland, Otago and Canterbury ahead of the rain, potentially reaching gale force in the most exposed areas.
Bellam said the wind pattern is familiar for this time of year.
Strong or gale-force northwesterly winds are not unusual for early summer and spring over the South Island, and are occurring ahead of the active front over the Canterbury High Country (warning), and other southern parts (watch), he said.
Auckland and Wellington are forecast to reach highs of 27C and 20C today respectively, while Hamilton and Tauranga are expected to reach 29C.
The bad weather should ease as it moves across central New Zealand tomorrow, with most watches and warnings in place until early tomorrow morning.
The rain watch for the Westland ranges extends until 10am tomorrow.
“A narrow ridge of high pressure brings a clearing trend to the South Island through Monday afternoon and evening,” Bellam said.
Meanwhile, the warm and windy conditions are expected to persist into the new week.
“Warm north to northwesterly winds affect the South Island on Tuesday and Wednesday, with Canterbury and Marlborough reaching the high twenties.”
Looking further ahead, Bellam said there’s some uncertainty over the direction of midweek weather.
MetService’s weather outlook has a low confidence of northwest winds reaching severe gale in parts of southern Fiordland, western Southland and Steward Island late Tuesday and Wednesday.
There is also low confidence of rainfall reaching lower warning criteria in North Otago on Wednesday and Thursday.