Winter Weather Havoc Is Expected to Make a Cross-Country Run

More than five feet are expected in parts of the Sierra, forecasters at the prediction center wrote.

Forecasters believe blizzard conditions will happen across the Plains next week.

“We are increasingly confident that we will be dealing with a pretty significant Northern Plains blizzard next week,” said Greg Carbin, chief of forecast operations for the Weather Prediction Center.

The system will move out of the Rockies and begin to strengthen, increasing the chance of heavy snow and very strong winds through Wednesday across the Northern Plains. The wintry blast is possible from Colorado, including Denver, and northeast across the Northern Plains. Across the Dakotas, at least a foot is likely, Mr. Carbin said.

“The potential does exist there for some really impressive amounts,” he added, as he expects this storm system will most likely slow down.

Severe storms, possibly with tornadoes, are likely across portions of the South.

Forecasters are still trying to nail down the exact impacts and timing of the severe weather deeper into next week. Still, it is looking likely that severe storms, possibly capable of producing tornadoes, will form on Tuesday across an area from eastern Texas across Arkansas, Louisiana and much of Mississippi, said Bill Bunting, chief of forecast operations at the Storm Prediction Center.

“Most fall and winter severe weather events typically have several features in common, including a low-pressure system near or north of the area of concern, a southerly flow of increasingly moist air from the Gulf of Mexico moving northward prior to the event, and a cold front moving east towards the area,” Mr. Bunting explained.

“A similar setup is expected early next week,” he said, “which gives us confidence regarding the potential for a focused area of severe thunderstorms and possibly tornadoes.”

Tornadoes are not uncommon this time of year, but they are less likely than in the spring and early summer. “We average about four days in December per year with at least one EF1” — on a 1-to-5 scale of ranking tornado damage — “or stronger tornado,” said Harold Brooks, a senior scientist with the National Severe Storms Laboratory, part of the National Oceanic and Atmospheric Administration. “There are about 100 days with an EF1 or stronger tornado during the year.”