Overnight thunderstorms rumbled across eastern Massachusetts early Wednesday, briefly flooding roadways and waking residents with heavy downpours and frequent lightning. Meteorologists said the worst of the rain would clear by 9 a.m., leaving lingering showers before skies turn mainly cloudy with occasional afternoon sunshine. Temperatures in Boston are forecast to reach only the mid-70s—about 20 degrees cooler than Tuesday—while communities west of Interstate 495 may touch the low 80s. Humidity will stay elevated, and forecasters expect a second, better-organized band of rain to sweep across the region before dawn Thursday, potentially dropping another 0.5 to 1.5 inches and slowing the morning commute. Farther south, the National Weather Service warned of oppressive heat along the Gulf and Atlantic coasts. Tampa Bay should top out near 92°F Wednesday with heat-index readings of 100 to 104°F amid scattered afternoon thunderstorms, while New York City is bracing for indices in the low- to mid-90s as sultry air lingers. Conditions are more comfortable in the Upper Midwest, where high pressure is keeping the Twin Cities sunny and in the low- to mid-80s. Storm chances will increase there late Wednesday night and persist through Friday, bringing the risk of locally heavy downpours, gusty winds and frequent lightning.