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The MetService is urging Southlanders to be prepared for heavy rain tomorrow as an active weather system passes.
A front is forecast to move on to the far south of the South Island from the Tasman Sea today and then move slowly northwards across the island over the weekend.
A MetService spokesman said the front was preceded by strong, warm and moist north-to-northwesterlies, then followed by cooler southerlies.
Periods of heavy rain (which is defined as up to 60mm falling within a 12-hour period) were expected across Southland between 9am tomorrow and 9am on Saturday.
An Orange weather warning was now in force for Fiordland, which was expected to receive between 150mm and 200mm of rain between 1am tomorrow and 1am on Saturday, with peak rainfall rates of up to 20mm per hour.
"People in these areas are advised to stay up to date with the latest warning alerts and forecasts.
"Heavy rain may cause streams and rivers to rise rapidly. Surface flooding and slips are also possible and driving conditions may be hazardous."
Further heavy rain was expected in Fiordland after this period, and the Orange warning was likely to be extended, he said.
"There is high confidence of rainfall accumulations exceeding warning amounts in Fiordland south of George Sound on Friday and Saturday, and then for the remainder of Fiordland and Westland south of about Otira on Saturday and Sunday.
"This has the potential to be a significant heavy rain event."
For Westland, south of Bruce Bay, between 100mm to 140mm of rain was now expected from 6pm tomorrow until 6pm on Saturday.
And for the central part of the West Coast (Bruce Bay northwards), currently 200mm to 250mm of rain was expected from midnight tomorrow until noon on Sunday, but larger accumulations were possible - particularly about the ranges - of up to 350mm.
On Saturday and Sunday, the spokesman said it was possible a heavy rain warning would be issued for the headwaters of the Canterbury and Otago lakes and rivers, and also for Clutha and Southland, including Stewart Island, from late Saturday into early Sunday.