A winter storm warning has been issued in the Northeast as up to 9 inches of snow is forecast; A deadly atmospheric river traveled across California

More than 60 million people across the US are under a winter weather alert as a powerful storm system targets the Northeast after Christmas, while a separate atmospheric river batters California with torrential rain, high winds and dangerous conditions.

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

Severe weather is expected to unfold over several days, with impacts extending from the west coast to the northeast. Forecasters say conditions on the ground can change quickly, affecting everything from short drives to long-distance trips. Here’s what to know about the most affected areas.

Snow and ice are expected in the Northeast

The tristate area is expected to be hit by a major winter storm on Friday, with snow, sleet and freezing rain forecast to create hazardous travel conditions. Freezing snow is expected in New York, New Jersey and Connecticut Friday through Saturday morning. Some forecast models suggest that parts of New York and New Jersey could see up to 9 inches of snow, depending on the storm’s track.

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

Snowfall amounts of about 1 to 6 inches are expected in northeastern Pennsylvania, much of New Jersey, much of New York state and parts of southern and central New England, with higher amounts possible north and west of New York City.

The system is expected to bring cold 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, DC

California: Record rainfall, flooding and ongoing hurricane threats

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

On Wednesday, Governor Gavin Newsom declared a state of emergency in several counties, including Los Angeles, Orange, Riverside, San Bernardino, San Diego and Shasta, as the Atmospheric River storm brought heavy rain, strong winds and the threat of continued flooding.

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 coldest Christmas Eve or Christmas-week totals in decades.

Prolonged rains have increased the risk of mudslides, especially in areas recently burned by wildfires. Officials have urged residents to avoid floodways and be prepared to evacuate if conditions worsen.

Sierra Nevada: Heavy snow, avalanche risk and dangerous travel

At higher elevations, the same storm system brought dangerous winter weather to the Sierra Nevada, making mountain travel dangerous in 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 accumulate in some high-elevation areas, significantly increasing the risk of avalanches and making weekend travel especially dangerous.

The storms are coming 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 year-end holiday period, raising the possibility of widespread delays as severe weather moves across the country.