USA - Thousands of flights were canceled across the US on Monday, continuing an ugly weekend for holiday travel.

Of more than 2,500 planes grounded globally as of noon Eastern time on Monday, almost 1,000 were within, into, or out of the US, according to Flight Aware.

Since Friday, more than 4,000 flights within the US have been canceled over Christmas weekend.

The first and familiar explanation is bad weather, particularly in the Western US, where winter storms have shut down roadways and airways.

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The National Weather Service warned Monday of "significant snowfall" along the West Coast mountain ranges and a pair of storm systems that will bring snow and "an icy wintry mix" to the Upper Midwest, Great Lakes, and Northeast.

"Travel delays due to icy conditions are possible in these regions, both on the ground and in the air," forecasters said.

And in a more common refrain for 2021, Omicron has also wreaked havoc on the airlines as well, with hundreds of flights being canceled due to crew members calling out sick with Covid-19.

Airlines, many of which have mandated vaccines for their staff and crew, have been pressing the CDC to lower the quarantine window for vaccinated workers from ten days to five – similar to healthcare employees – but the labor unions have pushed back.

"We recognize that how long a person should isolate is not a 'one-size-fits-all' number of days," Sara Nelson, the president of the Association of Flight Attendants, said in a statement last week.

"We also note that fully vaccinated people may be less infectious for less time than unvaccinated people. Still, we consider your current 10-day recommendation to represent a prudent middle ground.

"The current climate in the passenger cabin is highly stressed. We are experiencing a record high number of aggressive passenger incidents, many of which are fueled by alcohol and refusal to comply with onboard mask rules. Staffing flights with crew members who may still be symptomatic, infectious, or both by shortening them on necessary isolation time will only make this situation worse."

The chaos of the canceled flights comes during Christmas weekend, one of the busiest travel times of the year.

On Sunday, more than two million people passed through TSA checkpoints, almost 800,000 more than the same day in 2020.