He said during this 33-hour period between 200 and 250mm could accumulate, but predominantly in the hills and ranges - water which will flow downstream.
Mr Adams added with rainfall of this intensity and the scattered nature of thunderstorms, the isolated downpours could happen anywhere in the region.
"Some areas are more sensitive than others, but there is definitely a chance there could be some flooding in places," he said.
"Watch out for the heavy downpours and be careful on the roads.
"Keep checking the warnings and watches as this could be quite a serious one, really, so it's best to keep checking the latest forecasts.
"Those [rainfall] numbers are quite big."
While the heavy rain warning will lift this evening, Mr Adams said don't expect it to dry up too soon.
"The bad thing is it is going to stick around for a wee while yet," he said.
"We are expecting to see rain in Hawke's Bay until Tuesday it just won't be as heavy, then there is going to be occasional rain all week."
He added the rain may pick up again on Friday.
Looking to the rainfall numbers for this month, Mr Adams said the long-term mean rainfall for September, with averages taken from sensors in Napier since 1991, is 55.4mm. He said that the rain over the next week could mean this month could be wetter than average.
"In terms of what we have had so far we are only actually at 47.2mm for September," said Mr Adams. "But we are only two-thirds of the way through this month and we are already nearly at the long-term mean. So it looks extremely likely that this September will be wetter than average, although it hasn't actually occurred yet."
Moreover the meteorologist said that Napier typically has 11.7 wet days that see more than 0.1mm of rain.
"So out of 30 days, typically 11 of them are wet. So far this September we have only had five," he said. "But we are looking at getting probably another five or seven days at least in a row coming up so that number is going to bump up pretty rapidly."
Despite this downpour, Mr Adams said this was a one-off instance, with the El Nino system currently in place not throwing as many easterlies our way. "So events like this should be less common this year," said Mr Adams. "In this one we are getting a decent chunk of rain in one hit so definitely although it does have dangers associated with it, there are positives to be had from it as well," he said, adding that the farmers will be happy.
"This event is not a trend that you can expect more of.. It's a one-off event. It is not a sign of any long-term change."
In addition to the heavy rain warning, Mr Adams said the MetService has issued a warning for the Napier-Taupo road and other high range roads due to snow falling down to around 600m.
"We are expecting snow to fall to around 600m with the heaviest of it above 800m," he said.
-For the latest weather information, visit metservice.com.
