A system of severe weather is forecast for parts of the Midwest and Mid-Atlantic on Wednesday and Thursday.
Around 74 million people live in the areas that could be affected.
Tornadoes, harsh winds, large hail and powerful thunderstorms are all possible, according to the Storm Prediction Center.
The storms could also include a derecho, fierce blasts of wind that are up to 80 mph or more.
The Storm Prediction Center issued a "High Risk" warning Wednesday for Northern Illinois and Northern Indiana. Other cities at a greater risk include, Milwaukee, Chicago, Louisville, Indianapolis, Columbus, Cleveland, Cincinnati, and Pittsburgh.
Overnight Wednesday and into Thursday, damaging winds, large hail and a few tornadoes are possible in the Mid-Atlantic region, including Philadelphia, Baltimore and Washington, D.C., the Weather Channel forecasts.