Storms from the middle Mississippi and Ohio valleys to the mid-Atlantic and southern New England will unleash torrential downpours and gusty winds into the end of the week.
A surplus of moisture will be wrung out of the atmosphere in the form of towering clouds, showers and thunderstorms through Friday.
"With the amount of moisture available, the slow-moving and repeating nature of the storms will greatly elevate the risk of flash and urban flooding," according to AccuWeather Senior Meteorologist Dave Dombek.
This is the type of setup that can leave people stranded for a while, where rain quickly inundates small communities, portions of cities and stretches of major highways.
Airline delays are likely as storms approach the major hubs.
A smaller number of the storms will produce strong wind gusts and/or hail. A couple of the strongest storms may produce a brief tornado.
A large portion of Ohio, Indiana and Illinois will be in the risk for severe weather into Thursday night.
This risk includes portions of western Pennsylvania and the northern West Virginia panhandle. In part of this area, a mere 1-2 inches of rain in as many hours can trigger flash flooding.
"Where the sun is out for a few hours ahead of approaching storms, the storms can get a boost, which can add to their severity," Dombek said.
Storms may become locally severe over a heavily populated area from Philadelphia to New York City, Hartford, Connecticut, and Providence, Rhode Island, during Thursday afternoon and evening.
"For some areas of the Central and Northeastern states, this will be a two-day severe weather and flash flood event," Dombek said.
On Friday, the main threat for locally violent storms will focus over part of the mid-Atlantic region.
Areas from the Chesapeake and Delaware bays to the Susquehanna and Delaware valleys to the Pocono Mountains will be at an elevated risk of severe weather Friday.
In some cases through Friday, streets will become fast-moving rivers. Storm drains may be overwhelmed and small streams could rapidly overflow their banks. Torrential rainfall could even flood some subway tunnel entrances.
Motorists are urged to not to drive through flooded roadways.
Sporadic power outages are possible, while tree limbs could break and trees could be uprooted by highly-localized strong gusts.
At the first clap of thunder, it's important to move indoors and away from windows.
Slightly cooler and less humid air will continue to advance eastward and push southward and mark an end to the threat of severe thunderstorms and flash flooding into this weekend.
By this weekend, the threat will end across the Ohio Valley, New England and the mid-Atlantic.
The next risk of severe storms for the Great Lakes region will increase on Sunday as a new push of cool air approaches from Canada.