Map: Here’s how much rain could fall through Thursday morning

The National Weather Service has issued a flash flood warning for parts of eastern Massachusetts.

29weather - July 28, 2014 - A car splashes through water at an on-ramp to Storrow Drive in Brighton during floods caused by a severe rain storm. (Justin Saglio for The Boston Globe)
A car splashes through water at an on-ramp to Storrow Drive in Brighton during floods caused by a severe rain storm in 2014. –Justin Saglio for The Boston Globe

Isolated thunderstorms and torrential downpours are in the forecast for Boston and eastern Massachusetts on Wednesday, and the National Weather Service issued flash flood watches for large swaths of the state.

Weather service meteorologist Bill Simpson said the rain will not be an all-day event, but cautioned that commuters should plan for extra travel time due to possible heavy downfalls at times.

“It could rain pretty hard for about 20 minutes or so,” Simpson said.

Showers and isolated thunderstorms are expected during the day with more widespread rain during the evening, with some areas potentially seeing as much as 2 inches of rain an hour, the service advised. The service said some parts of Massachusetts, including Boston, could see between one and two inches of rain through early Thursday.

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—National Weather Service

The flash flood watch includes Suffolk, Norfolk, Middlesex, Essex, and Plymouth counties and extends through late Wednesday night.

Parts of New Hampshire and Maine could also see similar weather.

In the Boston region, some rain may continue Thursday and Friday with scattered showers, according to the service.

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