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More heavy rain is set to fall on the West Coast and Otago headwaters, which could trigger slips, flooding, road closures and detours tomorrow.
West Coast Civil Defence Emergency Management spokeswoman Claire Brown said the organisation was taking the MetService forecast seriously and was keeping a close eye on it.
Severe weather battered the region late last month, causing widespread flooding. South Westland's Waiho River bailey bridge was washed away in the torrential rain, cutting off access between Fox Glacier and Franz Josef. A new bridge is being built and was set to open on Friday.
A moist northerly flow is strengthening over the South Island today, ahead of an active front forecast to move across the South Island from the west tomorrow, a MetService spokesman says.
"This front delivers a period of heavy northerly rain to western parts of the South Island, with the heaviest falls expected in Fiordland, north of Doubtful Sound, and Westland, south of Otira, where a heavy rain warning is now in force.
"People are advised to stay up to date with the latest forecasts in case new areas are added to the warning.''
He said the heavy rain may cause streams and rivers to rise rapidly.
"Surface flooding and slips are also possible and driving conditions may be hazardous.''
Between 150mm and 200mm of rain was forecast to fall in Westland, south of Otira, while 80mm to 100mm was predicted elsewhere until 9pm tomorrow.
"Peak intensities of 20mm to 30mm per hour and thunderstorms are possible.''
In Fiordland, north of Doubtful Sound, 90mm to 120mm of rain was forecast to accumulate until 1pm tomorrow, and peak intensities of 20mm to 30mm per hour and thunderstorms were also possible.
The active front was also expected to deliver a period of heavy northerly rain to the headwaters of the Otago lakes and rivers until 3pm tomorrow.
"Rainfall amounts may approach warning criteria within 15km east of the main divide,'' he said.