CNN  — 

A potent winter storm is poised to deliver a mixture of snow, sleet, freezing rain and possible flooding across much of the eastern United States through Thursday.

More than 117 million people in the country, many from the Plains and Midwest to the Mid-Atlantic and Northeast, are under some sort of winter weather watch, warning or advisory. Nearly 20 million are facing a flood warning, watch or a flash flood watch across the Southeast.

Warm moist air streaming north from the Gulf of Mexico will combine with a colder storm system exiting the Rockies to create an expected wintry mix Tuesday night into Wednesday.

Heavy rain fell in the South throughout Tuesday.

As of Wednesday night, much of the Pacific Northwest is still under winter weather warnings or watches. Las Vegas can expect an inch or two of snow, and those living in the mountains of Southern California should brace for heavy snow. Phoenix is under a freeze warning.

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Heaviest snow will hit from Omaha to the Twin Cities

• Winter weather advisories are out for Oklahoma City; Wichita, Kansas; the Kansas City area, St. Louis; Chicago; Milwaukee; Indianapolis; Louisville, Kentucky; Cincinnati and Columbus, Ohio; and Pittsburgh, where light to moderate, snow or ice is forecast.

• There are winter storm warnings for Omaha, Nebraska; Des Moines, Iowa; and Minneapolis, where up to 10 inches is expected to fall.

East Coast takes the brunt

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• Snow and ice will cause problems for the Baltimore and Washington areas. Four to 6 inches of snow and sleet are expected in both cities, with wintery weather starting overnight and lasting into Thursday. As warmer air mixes in, ice accumulations of a 10th of an inch are possible.

Snow or ice is expected from Philadelphia northward to Boston. A few inches of snow could fall in the region before switching over to rain.

• If you are a student in the School District of Philadelphia – the nation’s eighth largest by enrollment – you can stay home Wednesday. If you work in an administrative role, the decision whether to open offices will be made by 5 a.m. Wednesday. If you are a building engineer, the district says to come to work.

Waves of rain soaking the South, especially Tennessee

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Flood or flash-flood watches extend from northern Alabama and Mississippi northward to Ohio. In Tennessee, floodwater has begun covering some roads and authorities in Lebanon evacuated a campground as a nearby creek began rising.

• Parts of northern Alabama saw flash flood warnings on Tuesday night. The Birmingham office of the National Weather Service said some areas may get 5 inches of rain by Friday morning.

CNN’s Dave Hennen and Chuck Johnston contributed to this report.