A severe thunderstorm watch has been issued for the Bay of Plenty today.

This morning's drizzly rain and strong winds are expected to continue to lash the region, with heavy falls with thunderstorms likely this afternoon.

Metservice has warned the period of heavy rain is expected to last until tonight with rain possibly reaching warning criteria, especially about the ranges of eastern Bay of Plenty.

A front is expected to move onto the northwest of the South Island in the early hours of Friday morning, onto the west of the North Island from about dawn and then cross the upper North Island during the afternoon.

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There is a high risk of thunderstorms moving through these areas from west to east during the period with locaised rainfall rates generally expected in the 10 to 25mm/hr range.

There is a moderate risk that some of these thunderstorms may become severe with localised downpours of 25 to 45mm/hr possible.

Such rainfall rates may last one to two hours and have the potential to cause flash flooding, especially about low-lying areas such as streams, rivers or narrow valleys, and may also lead to slips. Driving conditions will also be hazardous with surface flooding and poor visibility in heavy rain.