Storms are already messing with holiday travel plans during what is poised to be a record-setting week at airports nationwide.
Floods have ravaged the West Coast this week, with more on the way, and caused more than 5,000 flight delays Monday.
A plane crash due to foggy skies in Galveston, Texas, killed at least five people, also Monday. The U.S. Coast Guard has said the cause of the crash remains under investigation, but visibility in the area was down to as little as a quarter-mile amid dense fog.
But most of the country will be spared from messy weather Tuesday, save for the Northeast and the West Coast.
Winter weather alerts are in place for 11 million people across the interior Northeast and New England.
Snow was falling early Tuesday in New Jersey and a wintry mix of cold rain and snow was expected in the New York City area, causing slowdowns and slick roads. The entire Northeast, including New England, can expect snowy conditions Tuesday, stopping by the evening.
This will not be a major winter storm in terms of snow totals, but will likely cause delays on the roads and in the air, including at the major hubs in Washington, D.C., Baltimore, Philadelphia and New York City.
The largest snow accumulations of 4 to 8 inches will fall across Maine and the mountains of New England. Areas closer to Interstate 95 can expect from a dusting to up to 2 inches, including for New York City.
By Christmas Eve, the snow is expected to move out of the Northeast, leaving behind a sunny and dry Christmas Day in the region.
Storms continue to plague the West Coast, specifically in California, where airport delays are expected in San Francisco on Tuesday. By Wednesday, another big storm is set to hit Central and Southern California, including gusty winds up to 35-55 mph.
Over the past 72 hours, up to 20 inches of rain has fallen across portions of the northern Sierra. Beginning Tuesday, the strong atmospheric river shifts south toward Southern California. A total of 41 million people are now under flood watches across nearly the entire state of California, as well as portions of southern Nevada and western Arizona.
Rainfall rates of up to 1½ inches per hour are expected to cause widespread urban flash flooding, as well as debris flows and mudslides over the wildfire burn scar areas, including those from the Palisades and Eaton fires.
The heavy rain will come in two rounds, the first Tuesday night through Wednesday evening, then after a lull Wednesday night, another round will come in Thursday.
In addition to the long-duration intense rainfall, thunderstorms capable of strong damaging winds and isolated tornadoes will also be possible, especially with Thursday's round of rain.
Snow will return to the Northeast after Christmas on Friday and heavy rain will dump on the Great Lakes region, while flood threats remain out West.
The Transportation Security Administration is preparing for what could be the busiest holiday travel season ever. The agency is anticipating more than 44 million people at its security checkpoints during the holiday week.
Sunday is expected to be the busiest day to fly this holiday season. Travelers should give themselves ample time to catch their flights.
American Airlines said it is set to operate 6,600 flights carrying more than 675,000 customers Tuesday. On Wednesday, the airline said it expects almost 700,000 customers across 6,400 flights.
A number of holiday travelers told NBC News that Chicago O'Hare International Airport was bustling Tuesday morning, just two days before Christmas.
"It is chaotic because it's Christmas break," said Eli Hibler, from Muskego, Wisconsin, who is traveling to California then Hawaii. "It's a ton of fun. It's just really crowded, though."
Others offered up their best travel tips.
Amy Beischeo, a Chicago resident traveling to Charleston, said she books an Uber ahead of time and tries to pack the night before.
Andrew Garcia, who lives in Gurnee, Illinois, and traveling to Laredo, Texas, to see family, suggested travelers should "have everything ready. Be prepared. You know, don't be waiting 'til the last minute for everything."
Lea Krivinskas and Spencer Shepard, who are heading to Cancun, Mexico, said "definitely give yourselves a little extra time and a little extra patience."
As for travel by car, AAA suggests getting up and hitting the roads early for those driving to their destinations — the best time to be on the roads is before 10 a.m. Avoid driving between 1 p.m. and 7 p.m., AAA suggests, and be aware of weather that could complicate trips.
