A large storm capable of producing "significant" tornadoes and grapefruit-sized hail could be heading for north Texas, forecasters said.
The most dangerous weather - heavy winds, tornadoes and giant hail - are likely to take aim at a 102,000sq mile area stretching from central Texas to southern Nebraska, including the Dallas, Oklahoma City and Wichita, Kansas, areas, according to the Storm Prediction Centre in Oklahoma.
Severe thunderstorms and strong wind gusts are also predicted for Mid-Atlantic states where voters are casting ballots in primary elections.
George Eischen, 51, spent Tuesday morning moving vehicles off the forecourt at his Chevrolet dealership in the small town of Fairview, about 100 miles north west of Oklahoma City. Mr Eischen said he has been lining the new vehicles "bumper to bumper" in the shop to protect them from the hail.
"We've never been hit by a tornado here in town, amazingly," Mr Eischen said. "But yeah, we've had hail. And that's the real enemy of the car dealer."
In all, more than 53 million people from the Rio Grande in south Texas to Omaha, Nebraska, and parts of Missouri, Arkansas, Iowa, Illinois, Indiana and Kentucky are at a slight risk or higher of experiencing severe weather.
That tally also includes Washington, DC, Philadelphia and Baltimore, where a separate storm system could bring strong winds and thunderstorms to Mid-Atlantic states.
"We shouldn't assume that we're going to have a lot of information - you know, a lot of lead time," Storm Prediction Centre meteorologist Matt Mosier said. "We may or we may not."
Winds gusting up to 60mph downed trees, damaged buildings and knocked out power in parts of northern and central Missouri.
The National Weather Service reported that storms early on Tuesday brought torrential rains and hail ranging from 1in to 1.5in in Kansas City and other north-west Missouri towns.
Storms are expected in Delaware, Maryland and Pennsylvania, where voters are casting ballots in primary elections, though forecasters are not expecting a severe weather outbreak there.
Some schools in the Oklahoma City area called off classes on Tuesday, while others said students would be sent home early to avoid the worst of the weather.
Mid-Del Public Schools, in the Oklahoma City suburb of Midwest City, cancelled classes late on Monday. It said in a statement that the safety of students and staff is a priority, noting that it reworked its tornado safety plan three years ago after a storm killed seven schoolchildren in the neighbouring suburb of Moore.
In recent years, authorities have been able to predict storm conditions like these several days in advance with greater confidence, Mr Mosier said, though he noted that the weather does not always pan out as expected.
"It's never straightforward when you're sitting here talking about (predicting) large tornadoes," Mr Mosier said. "We're trying to be as confident or as accurate as we can."
Residents of affected areas should develop a plan to take shelter from a quick-forming storm without driving in severe conditions, Mr Mosier said.