Areas from the eastern Great Lakes to the Tennessee River Valley are at risk Thursday.
Large hail, damaging winds and tornadoes will be possible as these storms develop.
Cities under the greatest risk on Thursday include Pittsburgh and Buffalo, as well as portions of northeast Ohio.
The focus for the stronger storms will persist Thursday from the northern Great Plains into other portions of the Midwest.
Showers and thunderstorms also will pop up over South Florida.
The northern Plains will be the target for severe storms Friday. Monsoonal showers will also move into the Central and Southern Rockies this weekend.
Heat from South begins to spread North
Much of the country is feeling summertime heat, and that will only grow in the days ahead.
The really oppressive conditions will expand from the South into the Eastern U.S., and then begin to move into the northern and central Plains this weekend.
Heat advisories were posted across the lower Mississippi River valley.
Once the heat builds this weekend, it will last well into next week.
Fox News' Travis Fedschun contributed to this report.