Heavy Snow Hitting States From California to the Upper Midwest

A weather system strangling more than a dozen states from coast to coast will strengthen its grip on Wednesday, including in California, where a winter-storm warning is in place in the southern part of the state and tens of thousands of people in the San Francisco Bay Area are without power.

Almost all of the country is experiencing some form of notable weather this week, meteorologists said. By Wednesday morning, tens of millions of Americans were under winter weather alerts from California to Maine. A storm that began on Tuesday promises a mixed bag of wintry weather — strong winds, heavy snow, frigid temperatures and freezing rain — and will likely disrupt travel on the roads and in the skies.

The National Weather Service in Los Angeles issued a winter storm warning for the Ventura County Mountains and the Los Angeles County Mountains until early Friday. A blizzard warning, for the same areas, will take over from Friday through Saturday.

Forecasters in Los Angeles described the storm affecting that region as cold and dangerous and predicted up to seven feet of snow in areas more than 6,000 feet above sea level. Lesser amounts, between one and four inches, were expected in elevations of less than 2,500 feet. Areas along the coast and valleys could see a few inches of rain.

“This is shaping up to be a very unusual event in certain places, especially, and the impacts are probably going to be really substantial,” Dr. Daniel Swain, a climate scientist at U.C.L.A., said in a video on Tuesday.

He said that the weather will escalate on Thursday and Friday, when colder air moves in over the state. Snow is expected across low elevations “all the way from the Oregon border to the Mexican border,” he said, forecasting that many Californians will be able to see snow on the nearby hills on Friday morning.

Strong winds were causing power failures across the state. About 140,000 electricity customers in California — more than half of them in the San Francisco Bay Area — were without power on Wednesday morning. The utility Pacific Gas & Electric said that winds were affecting electrical equipment. The Weather Service office in San Francisco reported that powerful winds had downed trees and damaged roofs.

The heavy snow and strong winds that developed over the Northern Rockies on Tuesday will push east across the Northern Plaines and Upper Midwest on Wednesday, meteorologists said. Over a foot of snow is forecast from South Dakota through parts of Minnesota, Wisconsin and Michigan.

Snow totals could reach two feet in southern Minnesota, where the heaviest snow and blizzard conditions could develop late Wednesday. Gov. Tim Walz said on Monday night that he had directed several state agencies to begin preparing, and he asked residents to plan ahead and to limit travel.

Significant ice accumulations were expected in parts of Michigan, according to the Weather Service office in Detroit. Freezing rain, sleet and snow were also predicted to fall in the region.

While the storm was expected to slam the Upper Midwest, it was also forecast to bring snow to a portion of the Northeast later in the week. A winter storm watch was in effect for parts of Maine, New Hampshire, New York and Vermont.

Heavier snow totals of up to a foot were likely for higher elevations, like the Adirondacks and the Green and White Mountains, the Weather Service said. Lighter accumulations of up to four inches were predicted for the lower elevations across upstate New York and central New England.

Areas along the East Coast appear to be left out of the winter-weather equation this time, as they have been much of this winter. New York City and other major cities along the Interstate 95 corridor are experiencing some of their least snowy seasons in the past 50 years. On Thursday, temperatures are expected to be more than 40 degrees above average in the Mid-Atlantic region and will feel more like June than February. In Richmond, Va., the mercury is expected to reach into the low 80s.