"The thunderstorms are expected to build on the Kaweka Range and, if they move, they will move off the ranges and towards the plains, so they're likely to be present right across Hawke's Bay including the towns and all the way offshore."
Kerr said it would be a clear and dry start to Tuesday morning, but showers brewing in the ranges will make a drift over the towns from the north, affecting both Hastings and Napier as they day continued.
"Unfortunately that might develop more persistent rain that will develop during Tuesday night and into Wednesday morning.
"By the time we get into Wednesday morning, the flow around Hawke's Bay turns south-east again and you could see some persistent rainfall for much of Wednesday through the morning and into the afternoon. So it's a wet start to the week."
Despite the rain sticking around, Kerr said it was not a cold weather event with Napier reaching a high of 22C on Monday with the overnight minimum sitting at a warm 15C.
"Temperatures should hover around the 20C mark all the way through the middle part of the week and we're still going to see 19C highs as we get into Thursday.
"We're not going to get a lot of swing when it comes to temperature changes between day and night and that's because it's all socked in with the cloud cover."
Federated Farmers president Jim Galloway said he had recorded about 54mm of rain at midday on Monday and said the wet weather was great for stock farmers, but not so much for crop growers.
"It's great for growing grass for the stock, but if it carries on much longer then it might start affecting them a little bit."
Although farmland has started to dry out in rural areas, Galloway said it was nothing unusual for this time of year, but the rain would help.
"Some areas have been getting showers and thunderstorms, so it can be quite localised where it's dry and not so dry."
NZTA communications manager Jasmine Higginson said the rain had been steady but there were no reports of serious surface flooding around the region.
