A winter storm delivering snow, sleet, and ice across the United States is set to pummel everywhere from the High Plains to the Northeast and expected to affect roughly 90 million people, with 20 million facing possible flash flooding in the Southeastern states. 

Winter Storm Petra is expected to pick up on Tuesday night and last until Thursday, according to a forecast issued by the National Weather Service.

The heaviest snowfall will initially barrel through the Southern Plains to the Midwest to the Mid-Atlantic, taking place between Omaha and the Twin Cities, with some areas expected to brace for at least 9 inches of snow. 

Winter weather advisories have been set for states between Kansas and Pennsylvania, including cities like Wichita, Kansas City, St. Louis, Chicago, Indianapolis, Louisville, Cincinnati, Columbus, and Pittsburgh, with a forecast of light to moderate snow and possibly ice. 

Near the capital, snow is expected to affect those in the Washington, D.C.-Baltimore area, with an accumulation of 4-to-6 inches predicted to last until Thursday. Possible ice accumulations of nearly a 10th of an inch are also possible. 

Petra is anticipated to somewhat taper off in the Northeast, with Philadelphia and New York City projected to receive 2-to-3 inches of snow Wednesday before transitioning into rainfall. 

Nearly 20 million people in the Southeastern corridor have been issued flood or flash flood warnings, spanning across Alabama, Mississippi, and West Virginia.

Tennessee will also be affected by a northward trajectory, with a predicted 5-to-7 inches of rain in the Memphis and Nashville areas.

Northern parts of Georgia will also incur rainfall from Petra, with 2-to-3 inches around Atlanta and the state's mountainous regions.