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A moist northwest flow was forecast to strengthen over New Zealand on Monday - Otago's anniversary day - ahead of an active cold front that should move across from the west during Tuesday and Wednesday.
A MetService spokesman said the front would deliver a period of significant northwesterly rain into Fiordland and the Otago headwaters, from Monday until Wednesday.
"There is high confidence of rainfall accumulations far exceeding warning criteria in these areas during this time.
"This is likely to be a significant heavy rainfall event, and people in these areas are advised to keep up to date with the latest forecasts."
Dunedin is forecast to be wet but warm on Monday, with a high of 25degC, while rain was also on the cards for Queenstown, Wanaka, Oamaru and Invercargill and Alexandra. Temperatures, though, will also be in the 20s, including Alexandra which was predicted to reach 26degC.
MetService said the front was also expected to bring severe northwesterly gales to Central Otago and Southland on Monday and Tuesday.
"There is moderate confidence of northwesterly winds rising to severe gale in exposed parts of Fiordland, Southland, Otago west of Alexandra, and the Canterbury High Country."