Millions of Americans across the Midwest were facing damaging winds, large hail, flooding and a possible tornado outbreak Wednesday.
Chicago, Indianapolis, Cincinnati, and Columbus were all under the threat area for severe thunderstorms through the day, according to forecasters.
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"This area is densely populated which increases the odds of significant damage if tornadoes do form," NBC News meteorologist Bill Karins said. He added there was a "mini-tornado outbreak possible this afternoon [or] evening."
Heavy rain was already falling across the Iowa, Illinois and Wisconsin borders as of 4:30 a.m. ET, prompting a patchwork of flash flood watches and warnings from Lake Michigan into South Dakota.
Twisters were most likely to touch down in northern Illinois but were also possible in south Michigan, northern Indiana.
The "Rockford [Illinois] to Chicago area will need to be watched closely between 4 p.m. and 8 p.m. central time [5 p.m. to 9 p.m. ET]," Karins said.
The storm was likely to come in two waves, a morning cluster of thunderstorms in the Upper Mississippi and Ohio valleys followed by a single squall line pushing east across Indiana, Ohio and Michigan by the evening, according to The Weather Channel.
The forecaster added that "we can't rule out" this squall line surviving until the mid-Atlantic states on Thursday, causing wind gusts and severe weather in Washington, D.C., and Philadelphia.
It would be the second time in three days that the mid-Atlantic has been targeted by severe weather, after heavy rain, winds of up to 80 mph and golf-ball-sized hail battered the region Tuesday.
At least 35,000 homes and businesses lost power in New Jersey following the storm and 127 flights were canceled at Baltimore-Washington International Airport, according to FlightAware.
The same day an 80 mph tornado with a preliminary EF0 rating touched down for more than 12 miles in Howard County, Maryland, causing damage to a house, the National Weather Service said.
There was also severe weather in the High Plains on Tuesday, and three other tornadoes were reported in North Dakota, including one that tore the roof and garage away from a home, the NWS said