A powerful storm that slammed southern California Thursday will crawl across the southern tier of the United States over the next several days, delivering a nasty mix of snow, ice, heavy rain, floods and even a few tornadoes.

Ahead of the storm, Oklahoma Gov. Mary Fallin declared a state of emergency for all 77 counties in her state.

As of midday Thursday, about 20 million people were under some level of weather alert. Many tens of millions more should be added to this total over the next few days.

Rain and snow pelted southern California throughout the day Friday, leading to floods, mudslides and nightmarish traffic. The rain also caused a plane to skid off a wet runway at Hollywood Burbank Airport.

Snow was confined to the higher elevations, forcing the closure of I-5, a major north-south route.

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On Friday, the storm is forecast to ramp up in intensity as it moves across the southern U.S.: "Accumulating snow and ice appears likely from eastern New Mexico to western Oklahoma on Friday with snow totals of 4 to 8 inches, with locally higher amounts possible especially across the Texas Panhandle," weather service forecaster Allison Santorelli said.

"A swath of potentially significant ice is also forecast from roughly Lubbock, Texas, to near Oklahoma City, with accumulations in excess of 0.10 to 0.25 inches possible," she added. The Weather Channel warned that the ice accumulations could cause extensive power outages and tree damage.

Further to the south, very heavy rainfall and flooding will be a big threat with this storm, according to AccuWeather meteorologist Ryan Adamson. The heaviest rain and flood threat Friday is across southern Texas, where up to 10 inches of rain could swamp the flood-prone Houston area.

Some severe thunderstorms could also rumble through the region: “The greatest risk of severe weather that includes the possibility of a few isolated tornadoes is over portions of central and South Texas on Friday afternoon and evening,” AccuWeather meteorologist Alex Sosnowski said.

By Saturday, the rain and flood threat will shift to most of the Deep South and Southeast. Along the northern edge of the storm, from Missouri to the Carolinas, more freezing rain and ice is likely.

The storm could save its worst for last: Late Saturday and through the day Sunday, it's forecast to paste the central and southern Appalachians with heavy snow, potentially crippling the region. "From central and western North Carolina to parts of southern West Virginia and southwestern and south-central Virginia, this will be a major storm with the potential for a foot or more of snow," according to AccuWeather meteorologist Brian Wimer.

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Asheville, North Carolina, is forecast to pick up a foot of snow, which would place the storm on the city's list of biggest snowstorms on record.

After blasting the South, the storm should finally move east and off the East Coast by Monday and Tuesday, with little to no impact expected for the big cities of the Mid-Atlantic and Northeast. However, that forecast could still change as more recent weather information is fed into the computer models that help predict storm paths.

The Weather Channel has named the storm Winter Storm Diego.

Contributing: The Associated Press