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

A major storm system capable of delivering multiple feet of snow across the West and blizzard conditions across the Northern Plains will approach the West Coast on Friday evening and begin a slow traverse of the country lasting all week and into the following weekend, forecasters said Friday morning.

The storm system was diving down across the Northern Pacific on Friday and will push into California over the weekend, forecasters with the Weather Prediction Center of the National Weather Service said in a short-term forecast on Friday.

By the middle of next week, the storm is forecast to strengthen with blizzard conditions in the Northern Plains and tornado-producing storms across the South. The following weekend, the same system could generate a coastal low-pressure system, bringing rain and possibly some snow to the Northeast.

Computer modeling has given forecasters confidence in predicting the types of risks that will happen at the beginning of next week as the storm sweeps across the central United States. Here is what forecasters believe is likely to happen over the next week.

Extreme winter impacts are highly likely in the West this weekend.

As the storm system begins to move ashore Friday evening, it will bring strong winds to the California coast, with widespread rain in the lower elevations that could lead to flooding.

In the mountains, though, that moisture will fall as heavy snow.

“Numerous winter-weather-related advisories are in effect for the mountains as well as some of the interior valleys in the Pacific Northwest, Northern Rockies, and northern Great Basin,” the Weather Prediction Center forecasters wrote Friday morning. “The highest snow totals are currently expected for the Sierra Nevada in California, where several feet of snow is forecast.”

Forecasters are predicting “extreme impacts” — the gravest warning on the Weather Service’s winter storm severity scale — across the Sierra this weekend.

“Heavy snowfall rates of one to three inches an hour are likely at times, primarily on Saturday,” they warned. “This will result in extremely dangerous travel, especially across mountain passes. If you must travel, prepare for rapidly changing conditions, and carry winter driving supplies.”

As this low-pressure system moves ashore, it will tap into an atmospheric river — an area of moisture that flows through the sky like a river at a level of the atmosphere near where planes fly. The combination will allow for the snowfall total to reach one to three feet across much of the higher terrain.

More than five feet is 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.” 

Winter severe storms like the ones predicted next week can be more dangerous than ones that form during peak severe weather season (May and June).

“Because days are shorter,” Mr. Brooks explained, “they’re more likely to occur after dark, which makes them more dangerous” because people in harm’s way cannot spot them as they approach. “They also are more likely to occur in the mid-South and southeastern United States, which have greater rural population density than the Plains and have a higher fraction of manufactured housing and poverty. Thus, the impacts can be greater.”

The severe weather next week is likely to last through Tuesday night and into Wednesday. Deeper into the week, the forecast for what the storm might bring to the East is less certain. A coastal low is likely, but probably won’t bring snow to the major Northeast cities, Mr. Carbin said.

“It doesn’t look like a blockbuster at this moment,” he said, speaking of the possible effects on the Northeast. However, “it could look completely different,” he said, chuckling, knowing that this far out in a forecast, a lot can change.