The MetService has issued a thunderstorm watch for the Bay of Plenty as spells of heavy rain move in across the region.

High humidity and onshore winds are creating the potential for thunderstorms this afternoon.

The combination of moist air, onshore winds and daytime heating over the land is expected to produce heavy showers and isolated thunderstorms in Bay of Plenty.

'Any thunderstorms that do form are likely to be slow moving and as a result have the potential to be severe and capable of producing localised downpours of 25-40mm/hr,” says a statement from the weather organisation.

'Rainfall of this intensity can cause flash flooding, especially about low-lying areas such as streams, rivers or narrow valleys, and may also lead to slips.”

Motorists are also advised to take care as driving conditions may be hazardous due to flash flooding and poor visibility.

Severe Thunderstorm Criteria

MetService uses the following criteria for severe thunderstorms in New Zealand:

Heavy rain (from thunderstorms):

Rainfall of 25 millimetres per hour or more.

Large hail:

Hailstones 20 millimetres in diameter or larger.

Strong wind gusts (from thunderstorms):

Gusts of 110 kilometres per hour (60 knots) or stronger.

Damaging tornadoes:

Fujita F1 (wind speeds greater than 116 kilometres per hour (63 knots)) or stronger.