A major winter storm will unfold over the northern part of the United States this week, bringing a mixed bag of precipitation that will be “extremely disruptive,” forecasters said. The storm was taking shape in the West by Tuesday morning. More than 30 million people were under winter weather alerts in a nearly continuous stretch from the coast of California all the way to the coast of Maine.

Snow was beginning out West, with some falling at low elevations of 1,000 feet and even as low as 500 feet in Northern and Central California. The extreme weather threat then shifts toward the Northern Rockies and Northern Plains, where more than 13 million people were under a winter storm warning and 1 to 2 feet of snow was expected in the mountains.

The heavy snow and strong winds that develop over the Northern Rockies on Tuesday will spread south and east, according to an advisory from the National Weather Service. The weather system is expected to extend into the Plains by the evening, packing heavy snow, strong winds and freezing rain. The weather services warned of possible disruptions to power lines.

The Upper Midwest, Great Lakes and parts of the Northeast are expected to get a taste of the wintry mix on Wednesday, when weather conditions are expected to worsen. Wind gusts greater than 30 mph could cause blizzard conditions over portions of the Midwest, forecasters said.