Torrential rainfall is expected to "super soak" tens of millions of people across much of the Southeast and Mid-Atlantic over the next 48 hours, forecasters said early Tuesday as the deadly severe storms pumped moisture into many states.
Baltimore, Maryland, Washington, D.C. and parts of New Jersey and New York were among dozens of areas under a National Weather Service flash flood watch until Thursday, said The Weather Channel's lead meteorologist Mark Ressler.
Worst-affected areas in the South have already been hit by floods, and relentless rain could spread the misery further afield as the storms track north and eastward.
The rain - caused by the slow-moving storm currently producing the deadly tornadoes across the South - is going to "super soak" the wider region, The Weather Channel's Bill Karins predicted.
Hailstones were recorded and downpours left streets submerged in downtown Mobile, Alabama, early Tuesday – the aftermath of a storm front that lashed the state with tornadoes.
Even in areas likely to escape flooding, heavy rain is forecast - with wet weather stretching across a huge area from the Plains down to the Gulf Coast and east to the Atlantic coast.
"By the time this whole severe event is done, this could be a very big deal," Ressler said. "Some of the rain could by very heavy and prolonged. Parts of the Appalachians could see anything from six to 10 inches of rain and there are watches in place from the Carolinas up to the New Jersey coast."
Many counties in Virginia were also under a flood watch, the National Weather Service said.