The Bay of Plenty's weather has been unpredictable of late, with sunny days suddenly turning grey and heavy rain failing to eventuate.

MetService meteorologist Tom Adams says the nature of the weather at the moment means thunderstorms and other severe weather events tend to be isolated.

'There was some heavy rain overnight in the Bay of Plenty. The heavy falls were inland on the Kaimai Range, with 25mm of rain falling on State Highway 29. In Tauranga there were only a few millimetres at the airport.

'These are spots of heavy rain surrounded by areas with very little. That's the nature of the weather at the moment – it's very convective.

'You get thunderstorms or ‘heat showers' – smaller version of a thunderstorm, without the thunder, basically – popping up in places, but you could be half a kilometre away and get nothing.”

Thus, when a severe thunderstorm watch is issued by the MetService – as is currently in force across the Bay of Plenty and much of the North Island – it means there is a risk of thunderstorms in the region.

But it doesn't necessarily mean they will last long, or cover vast areas. Nor can the MetService pinpoint precisely where they may take place.

'It's like microwaving popcorn and trying to anticipate which piece of popcorn will explode first,” says Tom.

'If there's a front moving over the country, we know there will be rain all along that front, and it will move from here to here, so you can be more precise.

'However, with thunderstorms and convection, it's impossible to say where and when they will happen. We can only say there is a chance of thunderstorms in a certain area.”

The heavy rain watch for the Western Bay of Plenty has been lifted, but is still in place for areas east of Matata.