The historic F5 tornado in Charles County was born in a "supercell" thunderstorm that crossed Northern Virginia ahead of a cold front that also packed heavy rain, hail and high winds.
Twisters also spiked other thunderstorms that struck from Missouri to southern Virginia, and from western New York state to South Carolina.
At least six people were killed in three states, and many more were injured. High winds damaged or demolished thousands of vehicles, homes, businesses and other structures. Heavy snow fell in the upper Midwest.
The center of the storm system formed in southeastern Wyoming and Colorado. Saturday, it moved across the Central Plains, reaching the Great Lakes region by Sunday morning, according to Joe Schaefer, director of the federal Storm Prediction Center in Norman, Okla.
Air behind the front was cold and dry. The air in front of it was warm and heavy with moisture from the Gulf of Mexico.
It was the perfect setup for tornadoes: A difference in wind speed and direction between the colliding air masses creates a spinning effect. Rising warm, moist air within a thunderstorm cell tilts the rotating air into a rotating vertical column. And a tornado is born.
The fearsome F5s pack winds of 261 mph and more. They can last more than an hour and are defined as strong enough to lift well-built homes off their foundations, hurl automobiles hundreds of yards, and rip the bark off trees. That's what the damage teams found in La Plata.
Only 51 other F5 tornadoes have been recorded in the United States since 1952. This was just the second F5 ever seen in the weather service's eastern region.
"Historic and tragic," said Susan Weaver, spokeswoman for the Sterling, Va., forecast office.
Schaefer said a 12-year-old boy was killed and several homes were damaged when a twister touched down near Marble Hill, Mo., shortly after midnight. Another fatality and dozens of injuries were reported in Dongola, Ill., an hour later.
Two strong twisters, with winds of 153 to 206 mph, touched down in Paducah, Ky., and Murfreesboro, Tenn. Nearly four dozen people were hospitalized, and more than 125 homes were damaged or destroyed.
More tornadoes, with damage and some injuries, struck Sunday in Stark County, Ohio; Erie and Allegany counties in New York; Blacksburg and Emporia, Va.; and Westminster, S.C.
Maryland got its first alert on the brewing storm in a tornado watch issued at 3:05 p.m. Sunday by the Storm Prediction Center.
The watch said that tornado formation was possible across most of Virginia and Maryland between 3:30 p.m. and 9 p.m.