The rain Hawke's Bay has wanted for so long is expected in bucket-loads tonight, when it's expected as much will fall in Napier in 12 hours as has fallen in past three months.

The forecast comes with a severe weather watch for Hawke's Bay and Gisborne, and last night MetService duty forecaster Claire Flynn said it could be upgraded to a severe weather warning as the day progresses.

Coastal areas, Napier in particular, were expected to be hardest hit.

The rain is due to start early in the afternoon and potentially more than 80mm will have fallen by midnight - considerably more than the 66mm MetService has recorded in Napier since the end of March.

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If it eventuates, Napier could go from potentially ending the month with less than a third of the average June rainfall to having more than the average in one day.

Similar levels of rain are forecast for Hastings, where, like Napier, the rain is expected to average more than 10mm an hour later in the evening, easing early tomorrow morning, although Hastings was not forecast to have more than 60mm.

About 30mm was being forecast for Dannevirke up to midnight, most of it in the early evening, and Gisborne was forecast to have about 10mm in the same time.

The likelihood of heavy rain today and tomorrow has increased as a low pressure system across the North Island is challenged by winds on the east coast turning onshore.