Flash flood and wildfire warnings were issued on Tuesday, as severe thunderstorms and torrential rainfall were again expected to hit the Midwest and Northeast.
"Once again, thunderstorms are expected to fire in parts of the northern Plains to Lower Great Lakes on Tuesday," the National Weather Service said.
"A separate area of severe weather is possible in the Northeast. Large hail and damaging winds will be the primary threats. In addition, rainfall could be heavy enough to cause flash flooding and river flooding in the Midwest and Lower Great Lakes," it added.
In Chicago, there were at least 200,000 power outages on Monday as severe weather lashed the area.
The storms forced the temporary closure of O'Hare International Airport, NBC Chicago reported.
Fearing injury or worse at the city's Midway International Airport, officials cleared the ground and ordered people away from the windows.
Felled trees also led to traffic chaos across the city.
In Belmont, N.H., 23 scouts were hospitalized after being struck by lightning as they hid out from a storm.
Meanwhile, in Wisconsin there was almost a foot of rain over the weekend.
An area from the eastern Dakotas to northeast Iowa saw two to five inches of rain in the past week alone, according to weather.com.
The water-logged soil means additional rainfall is more prone to run off and cause rapid flooding of low-lying areas, officials warned.
Lightning strikes could also spark wildfires on the northern Plains, experts warned.
Embattled firefighters across the country have already tackled a number of huge blazes this year in Colorado, California and New Mexico.
The massive West Fork Fire in Colorado is expected to burn for some time to come.
But after more than a week the raging blaze in Doce, Ariz., was expected to be fully contained tomorrow.
More than 500 personnel and three helicopters have battled to bring in it under control.