More than 60 million people across the U.S. are under winter weather alerts as a powerful storm system targets the Northeast after Christmas, while a separate atmospheric river continues to batter California with flooding rain, high winds and dangerous conditions.

Forecasters warn the systems could disrupt flights and road travel as millions of Americans head home after the holidays.

The severe weather is expected to unfold over several days, with impacts stretching from the West Coast to the Northeast. Forecasters say conditions on the ground could change quickly, affecting everything from short drives to long-distance travel. Here’s what to know about the most affected regions.

Snow and ice expected across Northeast

The tristate area is expected to bear the brunt of a major winter storm on Friday, with snow, sleet and freezing rain forecast to create hazardous travel conditions. Accumulating snow is forecast to hit New York, Connecticut and New Jersey — where a state of emergency has been declared — beginning Friday through early Saturday. Some forecast models suggest that parts of New York and New Jersey will see up to 9 inches of snow, depending on the storm’s track.

The storm is already disrupting air travel in the region. As of Friday morning, at least 1,300 flights across the U.S. had been canceled, according to flight-tracking service FlightAware, with delays expected to grow as the storm intensifies.

Snowfall totals of roughly 1 to 6 inches are expected across northeastern Pennsylvania, much of New Jersey, large portions of New York state and parts of southern and central New England, with higher amounts possible in elevated areas north and west of New York City.

The system is also expected to bring sleet and freezing rain to parts of Pennsylvania, Maryland, West Virginia and northern Virginia. A wintry mix is likely in cities including Philadelphia, Baltimore and Washington, D.C.

California: Record rainfall, flooding and ongoing storm threats

On the West Coast, California is facing dangerous conditions from a powerful atmospheric river system bringing flooding rain, high winds and heavy mountain snow to the state.

On Wednesday, Gov. Gavin Newsom declared a state of emergency in several counties, including Los Angeles, Orange, Riverside, San Bernardino, San Diego and Shasta, as atmospheric river storms brought intense rainfall, strong winds and ongoing flooding threats.

The storm has already turned deadly, with at least three deaths reported in Southern California.

Parts of Southern California saw record-breaking rainfall earlier this week, with some locations seeing their wettest Christmas Eve or Christmas-week totals in decades.

The prolonged rainfall has increased the threat of mudslides, particularly in areas recently burned by wildfires. Officials have urged residents to avoid flooded roadways and to be prepared to evacuate if conditions worsen.

Sierra Nevada: Heavy snow, avalanche risk and dangerous travel

At higher elevations, the same storm system has delivered dangerous winter weather to the Sierra Nevada, making mountain travel hazardous across parts of California and western Nevada.

Avalanche warnings were issued for parts of the Sierra as heavy snow and strong winds created unstable conditions in the mountains. Forecasters say 5 to 10 feet of snow could pile up in some high-elevation areas, significantly increasing avalanche risk and making travel especially dangerous through the weekend.

The storms are hitting during one of the busiest travel periods of the year. The American Automobile Association estimates that more than 122 million Americans are expected to travel at least 50 miles during the end-of-year holiday period, increasing the potential for widespread delays as severe weather moves across the country.