The chaos of winter weather is expected to make cross-country running

A large storm system that could bring several feet of snow to the West and blizzard conditions to the Northern Plains approached the West Coast Friday night, the start of a slow journey across the country that lasted all week and into next weekend, forecasters said Saturday.

The storm system moved across the North Pacific on Friday and will push into California over the weekend, forecasters at the National Weather Service’s Weather Prediction Center predicted said on Saturday. The storm will continue to bring mountain snow and rain along the coast to the west.

Wintry precipitation will extend from the Great Lakes region to the northeast on Sunday. Heavy rain and thunderstorms may reach the south.

Computer modeling has given forecasters confidence in predicting the types of risks that will occur early in the week as the storm sweeps across the central United States. Here’s what forecasters think will happen.

Extreme wintry impacts are very likely across the West this weekend.

The storm system began moving ashore Friday evening, bringing strong winds to the California coast. Heavy rain in low-lying areas may cause flooding.

In the mountains, however, this moisture began to fall as heavy snow.

“Impacts will be widespread from north to south with multiple winter weather advisories in effect,” The Weather Forecast Center said on Saturdayadding that “the heaviest snowfall is expected for the Sierra Nevada,” with several feet expected.

Forecasters predicted “extreme impacts” — the weather service’s most severe warning winter storm force scale — through the Sierra Nevada this weekend.

In the Sierra Mountain region, snow was falling at a rate of six inches per hour as of Saturday afternoon, according to the California Department of Transportation. “Travel is difficult to impossible in daylight conditions,” the department announced on Twitter.

A portion of Interstate 80 near the Sierra Nevada was closed Saturday afternoon due to drifting snow that created “near zero visibility” on the roads, the Department of Transportation said on Twitter. After the reports of “multiple vehicle spinouts,” the department said drivers were being asked to detour and that there was no estimate of when the highway would reopen.

More than five feet of snow is expected in parts of the Sierra Nevada, write forecasters from the Center for Predictions.

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

Forecasters believe blizzards will occur in the plains.

“We are increasingly confident that we will be dealing with a fairly significant blizzard in the Northern Plains 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 for heavy snow and very strong winds by Wednesday across the Northern Plains. A winter blast is possible from Colorado, including Denver, and northeast across the Northern Plains. On the other side of the Dakotas, at least a foot is likely, Mr. Carbin said.

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

Snow in the mountains of the Central Rockies and Arizona could reach a foot by Sunday night.

Severe storms, possibly with tornadoes, are expected in parts of the south.

Strong storms, possibly capable of producing tornadoes, appear very likely to develop Tuesday in an area from east Texas through 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 tend to have several common features, including a low pressure system near or north of the affected area, a southerly flow of increasingly moist air from the Gulf of Mexico moving north ahead of 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 in 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.

“On average, we have about four days in December per year with at least one EF1” — an estimate of scale from 0 to 5 tornado damage — “or a stronger tornado,” said Harold Brooks, senior scientist at 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.”

Severe winter storms like the one forecast next week can be more dangerous than those that form during the peak severe weather season in May and June.

“Because the days are shorter,” Mr. Brooks explained, “they’re more likely to occur after dark,” he said of the storms. That “makes them more dangerous” because people in danger can’t spot them when they get close, he said.

“They are also more likely to occur in the mid-South and southeastern parts of the United States, which have a higher rural population density than the Plains and have a higher proportion of manufactured housing and poverty,” he added. “That way, the impacts can be bigger.”

Snow is expected in the northeast.

Some snow is expected Sunday for southern New England, the interior Northeast and the central Appalachians, forecasters said Saturday.

“Moderate amounts of snow between 3-4 inches are possible inland for higher elevation locations such as the Berkshires and Catskills, and 1-2 inches are possible further south in central Pennsylvania,” the forecast said.

Rain and snow are expected across inland northeastern New Jersey, the lower Hudson Valley and southern Connecticut through Sunday evening. New York could see snow on Thursday.

April Rubin and Edward Medina contributed reporting.