Violent storms causing at least 13 deaths have forced states of emergency to be declared in four states and the US capital.
Maryland, West Virginia, Virginia, Ohio and the District of Columbia have all declared states of emergency.
Three million people were left without power during an intense heatwave as the sudden storms ripped up trees and downed power lines.
Maryland Governor Martin O'Malley said the storms had been even more challenging than a hurricane, because unlike hurricanes they began without warning.
His Virginia counterpart Bob McDonnell said the state - where six people died from the storms - had had its largest non-hurricane power outage in history.
"This is a very dangerous situation,” he told the Associated Press news agency.
The storm damage also caused online disruption, with Netflix, Instagram and Pinterest services temporarily disabled, according to the BBC.
The US National Weather Service has warned of more possible thunderstorms, saying it has three areas of particular concern: the northern Rockies, the mid-Mississippi River to Ohio Valleys and portions of the Mid-Atlantic into the Southeast, particularly the eastern Carolinas.
"The primary threats will be large hail and damaging winds; however, an isolated tornado can not be ruled out across southern Minnesota and Wisconsin as well as northern Iowa and Illinois," it said.
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