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MetService is urging Otago and Southland residents to watch for periods of heavy rain and westerly gales tonight.
An active front is moving east over the South Island, and a low pressure system is forecast to deepen just south of the South Island tomorrow.
A MetService spokeswoman said the front and low system were expected to bring periods of heavy rain and gales to exposed southern areas.
A heavy rain watch is in place for areas of Otago, north of Wanaka to Middlemarch.
Between 8pm tonight and 5am tomorrow, rainfall accumulations may approach warning criteria.
"People are advised to keep up to date with the latest forecasts in case part of these watches are upgraded to a warning or further areas are added."