The threat of flash flooding on Sunday afternoon was focused over parts of the Midwest, where flood watches have been posted from Missouri to West Virginia, including the cities of Indianapolis and Cincinnati.
Rounds of heavy rain and thunderstorms are forecast to sweep across the Midwest region over the next 24 hours.
Strong, slow-moving thunderstorms could produce torrential rain and trigger flash flooding in areas receiving the heaviest precipitation.
Earlier on Sunday, flash flooding hit parts of Iowa and prompted a flash flood warning in Des Moines Sunday morning that has since expired.
The new threat of flash flooding comes on the heels of a "flash flood emergency" that was issued on Saturday for northern Washington, D.C.
Saturday's downpours also hit Maryland, causing flooding in Chevy Chase, Silver Springs and Bethesda. At times on Saturday, some cities in Maryland reported getting 1 to 2 inches of rain in 30 minutes.
On Sunday, the flash-flood threat extended from Davenport, Iowa, and into northern Georgia.
Parts of northeast Missouri and central Illinois are bracing for a significant risk of dangerous flash flooding on Sunday afternoon and into the evening, with numerous flash floods possible.
In addition to the flash flood threat, scattered severe thunderstorms could also impact parts of the Midwest and Northeast later Sunday and into the night.
Parts of the Dakotas, the Ohio River Valley and the Northeast, including New York City and New England, are also expecting strong, potentially damaging wind gusts. Isolated large hail and tornadoes cannot be ruled out for the areas.
Meanwhile, heat alerts were issued on Sunday afternoon for more than 50 million people from Kansas to the Carolinas. The heat index, which factors in high humidity, is expected to make it feel like 108 degrees in Little Rock, Arkansas, 110 in St. Louis and 107 in Nashville.
The heat index could reach 105 to 110 degrees on Monday afternoon for cities such as Raleigh, North Carolina; Charleston, South Carolina; and Jacksonville, Florida.
The dangerous heat is expected to expand across a large part of the Midwest in the coming days and impact Chicago by midweek.
Extreme heat watches have been posted for parts of northern Kansas and Missouri and into central Illinois beginning late Tuesday as the dangerous heat is forecast to expand northward by midweek.