MetService has issued severe weather warnings for some South Island areas as a large trough, preceded by a strong northerly flow, bears down on the country.

This trough is expected to bring heavy rain to Fiordland and the ranges of Westland, and could spill over into the headwaters of the Otago lakes and rivers.

MetService said Fiordland could expect 120mm of rain in a nine-hour period starting this evening, while the ranges of Westland could cop 180mm from tonight until tomorrow afternoon.

Thunderstorms were possible in some areas as well.

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More heavy rain was likely later this week, which could lead to some streams and rivers rising rapidly, making conditions difficult for trampers.

Further north, severe northwest gales gusting 130kmh were likely in exposed parts of the Canterbury high country from tonight through to tomorrow afternoon.