Parts of the United States will see snowfall this Christmas Eve and Christmas Day, but the real winter weather this year could come after the holiday, forecasters say.

Across the country, more than 2 million people are under winter weather alerts through mid-Christmas Day, according to the National Weather Service and Weather.com.

In New England, parts of central and western New York and eastern Pennsylvania, a light dusting of snow is expected on Monday. It's unclear if the dusting will hold over until Christmas morning.

The heaviest snowfall in the region will happen near the shores of Lake Erie and Lake Ontario, where winter weather advisories are in effect until 10 p.m. ET. Accumulations in the area could reach 5 to 6 inches.

Farther northeast, snow squalls are possible Monday afternoon, which could inhibit some holiday travel due to low visibility.

Out west, the northern Cascades to central Sierra Nevada — stretching from Oregon into eastern California and northern Nevada — could receive 3 to 5 inches from a winter storm. The highest snowfall in this region will be at the Sierra Nevada's highest altitude, where 20 inches of new snow could fall through the afternoon of Christmas Day.

Heavy rain is also a threat out west. The Washington coast, northern Oregon coast and the Bay Area of California are under coastal flood advisories through 11 a.m. PST Monday.

That storm system is likely to move eastward through the Plains, upper Midwest and northern New England Wednesday through Friday, according to Weather.com.

Temperatures at the end of the week will be low enough on the storm's north side to generate snowfall from the northern and central Plains into the upper Midwest and northern Great Lakes region, Weather.com reported. That storm system, mixed with strong winds, could cause near-blizzard conditions.