A colossal winter storm, stretching nearly 1,200 miles across the northern United States, is causing havoc for 42 million Americans as they navigate post-Thanksgiving travel plans this Sunday, historically one of the busiest travel days of the year.

According to the National Weather Service, winter storm warnings and advisories are in effect from Montana to New York. A perilous mix of snow, rain, ice, and harsh cold is sweeping through the northern Plains, Great Lakes, and the interior Northeast, complicating travel efforts.

The timing of this storm couldn’t be more inconvenient. AAA projected that over 81.8 million people would travel 50 miles or more during the holiday period, with the storm’s peak coinciding with the huge rush to get home.

The Transportation Security Administration (TSA) has already issued a warning, anticipating that it will screen over 3 million travelers on Sunday alone.

Adam Stahl, the acting deputy TSA administrator, highlighted in a statement, “We are projecting that the Sunday after Thanksgiving will be one of the busiest travel days in TSA history.”

Airlines are preparing for the storm’s impact. Airlines for America anticipates that U.S. carriers will serve a record 31 million passengers from last Friday through next Monday, just as snow and ice threaten to disrupt major airports.

Transportation Secretary Sean Duffy said the TSA has finally returned to full staffing after the government shutdown that previously forced flight reductions – but staffing alone may not be enough to outrun the storm.

The system has already dumped snow across the northern Plains and Great Lakes and continued marching east on Friday. Iowa and Illinois were taking the brunt, with six inches to more than a foot of snow expected in west-central Illinois between Friday night and Saturday night.

Storm warnings and advisories extended from Montana to New York, the National Weather Service said meaning up to 42 million people could be impacted by the storm. Pictured, Drivers struggle in Barrie, Ontario

A snowplow operates outside a church in Lowville, New York on Friday

More than 81.8 million people were predicted to travel 50 miles or more during the Thanksgiving holiday period, AAA said

Forecasters say the storm does not yet meet official blizzard criteria – usually defined as winds over 35 mph, visibility under a quarter mile and sustained for more than three hours – but the impacts may feel blizzard-like for millions.

Chicago, one of the nation’s most critical air-travel chokepoints , could be buried under 8 to 12 inches of snow or more, according to meteorologist Andrew Kozak, who spoke with CBS News.

Heavy snow is expected Saturday, threatening a domino effect of airport delays across the country.

The deepest snow totals measuring a foot or more are forecast downwind of Lake Superior across Michigan’s northern Lower Peninsula and near lakes Erie and Ontario, the weather service said. 

Central New York could also be hit with a foot of snow, while parts of Iowa, Illinois, Wisconsin and Michigan may exceed that by Saturday.

Snow squalls may produce sudden whiteout driving conditions across the interior Northeast, and the NWS winter storm severity index has already warned of highly dangerous travel in eastern Iowa and northwestern Illinois from Friday afternoon into the night.

Out west, snow and rain are sweeping into the Pacific Northwest and Rockies Friday night, tapering in those regions Saturday, but the system will continue charging into the Midwest just as travelers pour onto highways and into airports.

Out west, snow and rain are sweeping into the Pacific Northwest and Rockies Friday night into  Saturday

To the south, the storm turns wet and volatile, with heavy rain and flash flooding possible Saturday across the western Gulf Coast

On Saturday, widespread snow will fall in the upper Mississippi Valley and Great Lakes leading to both snow-covered, slippery roads and significant flight delays, especially at Chicago-O’Hare. By nightfall, the wintry mess will spread eastwards

By Monday the storm will have moved into the East with rain and a wintry mix most likely along the east coast – but it could still lead to significant flight delays in the major East Coast hubs

Snow is expected to last well into the weekend in some areas with Iowa and Illinois getting the brunt of it. Pictured, A man uses a snowblower to clear a driveway in Lefroy, Ontario on Friday

A truck passes the local theater in Lowville, New York in Friday. The town is not far from the Canadian border

The Transportation Security Administration said the agency is planning to screen more than 3 million travelers on Sunday. Pictured, passengers at Chicago O’Hare on Wednesday

More than 81.8 million people were predicted to travel 50 miles or more during the Thanksgiving holiday period. Pictured, travelers wait in line at a TSA checkpoint in Houston

To the south, the storm turns wet and volatile, with heavy rain and flash flooding possible Saturday across the western Gulf Coast.

Temperatures are plunging nationwide. 

While highs on Friday hovered in the 20s and 30s across the Midwest, the 30s and 40s in New England and the Mid-Atlantic, and only the 40s and 50s in the Southeast – the bitter cold could worsen icy road conditions and create further travel misery.