Winter storm system brings heavy snow, rain and wind across the U.S.

A major storm system already bringing heavy snow, coastal rain and high winds in the mountains of California is continuing a slow cross-country journey that is expected to last all week, forecasters said Sunday.

In the Sierra Nevada, snow is expected to continue to fall at up to 3 inches an hour, blanketing roads and creating "nearly impossible travel" and "near zero visibility," the National Weather Service said Sunday.

In San Luis Obispo County, near the central coast of California, roadways were flooded and strong winds with gusts of more than 60 mph brought down power lines, said David Gomberg, a meteorologist with the National Weather Service in Oxnard, California.

In Grover Beach, which is in San Luis Obispo County, all phone and 911 lines were temporarily down Saturday night because of the winds. Service was restored early Sunday, according to the city's Police Department.

The storm is expected to reach the Great Basin and the Desert Southwest by Monday, the weather service said, adding that those areas could get up to 2 inches of snow an hour, which will also make travel difficult.

The storm system, which made its way across the Northern Pacific on Friday and pushed into California, is expected to develop by Wednesday into a major winter storm from the Central High Plains to the Upper Midwest.

This could result in "multiple days of significant impacts to travel and infrastructure due to snow, blowing snow and freezing rain," the weather service said Sunday. It emphasized that "travel may become impossible."

Wintry precipitation will extend from the Great Lakes region into the Northeast on Sunday. Heavy rainfall and thunderstorms could reach the South.

The storm system began to move ashore Friday evening, bringing strong winds to the California coast. But in the mountains, that moisture has been falling as heavy snow.

Snowfall totals could reach 6 inches to 1 foot through Monday in the mountains of the Great Basin, the Central Rockies, Arizona and Southern California, the Weather Prediction Center said Sunday, adding that the heaviest snowfall of up to several feet is expected for the Sierra Nevada.

Forecasters predicted "extreme impacts" — the gravest warning on the weather service's winter storm severity scale — across the Sierra Nevada.

More than 5 feet of snow is expected in parts of the Sierra Nevada, forecasters at the prediction center wrote.

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 1 to 3 feet across much of the higher terrain.

"We are increasingly confident that we will be dealing with a pretty significant Northern Plains blizzard" this week, said Greg Carbin, the chief of forecast operations for the 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 1 foot is likely, 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 expected across portions of the South.

It looks highly likely that severe storms, possibly capable of producing tornadoes, will form Tuesday across an area from eastern Texas to 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," Bunting said, "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."

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 EF-1" — rated on a 0-5 scale of 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 EF-1 or stronger tornado during the year."

Severe winter storms like the ones predicted this week can be more dangerous than ones that form during peak severe weather season, in May and June.

"Because days are shorter," Brooks explained, these storms are "more likely to occur after dark." This "makes them more dangerous" because people in harm's way cannot spot them as they approach, he said.

"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," he added. "Thus, the impacts can be greater."

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

Up to 6 inches of snow could accumulate in higher elevations of the Berkshires and the Catskills, the weather service said. Portions of western New England and central Pennsylvania could get 1 to 2 inches.

Snow accumulations could reach up to 4 inches in parts of Connecticut, the weather service in New York said.

"While locations in the mid-Atlantic closer to the coast should see mostly rain, some snow may mix in later Sunday evening," the forecast said.

Christine Chung, April Rubin and Eduardo Medina contributed reporting.