CHICAGO (NewsNation) — Thousands of flights are canceled Friday as one of the most treacherous holiday seasons in the U.S. has seen in decades is underway, with temperatures plummeting, coinciding with travelers trying to their destinations before Christmas.

As of 5 a.m. CT Friday, more than 4,100 flights within, into or out of the U.S. are canceled and more than 4,600 flights are delayed, according to the tracking site FlightAware.

A total of 2,681 flights within, into or out of the U.S. were canceled and 10,399 were delayed Thursday, according to FlightAware.

Airports in Chicago, Denver, New York, Boston, Detroit and Cleveland are reporting the most cancellations.

U.S. airlines are waiving change fees and fare differences for passengers in a range of affected areas.

The frigid air was moving through the central United States to the east, with windchill advisories affecting about 135 million people over the coming days, forecasters said Thursday. Places like Des Moines, Iowa, will feel like minus 37 degrees, making it possible to suffer frostbite in less than five minutes.

Forecasters are expecting a bomb cyclone — when atmospheric pressure drops very quickly in a strong storm — to develop late Thursday and into Friday near the Great Lakes. That will stir up blizzard conditions, including heavy winds and snow, Cook said.

Amtrak canceled several dozen scheduled train trips in the Midwest through Christmas because of the weather conditions, including trains in Michigan, Illinois and Missouri and trains between New York and Chicago.

“These actions are with abundant caution and in consultation with state transportation departments, host railroads, emergency managers and weather forecasters,” Amtrak said in a service announcement. 

Railroad officials said on Thursday that its Northeast Corridor, the nation’s busiest railroad line, will still continue on schedule unless the ongoing weather conditions continue to get worse, according to The Washington Post. 

In the seven days ending Wednesday, the Transportation Security Administration said it screened nearly 16.2 million passengers, slightly below the 16.5 million screened in the same period in 2019, pre-COVID pandemic.

All modes of transportation have seen an increase in travelers, with nearly 113 million people estimated to travel at least 50 miles from home this year between Dec. 23 and Jan. 2 – 3.6 million more than last year, according to AAA. Of those, about 102 million will be driving. 

The National Weather Service says travel may be difficult because of icy roads and reduced visibility caused by blowing snow.

Wind chills are between -30 and -40 degrees early Friday. Frostbite can happen in as little as 10 to 20 minutes, so limiting outdoor exposure is encouraged.

Aside from the dangerously cold wind chills, icy roads and blowing snow is a threat to commuters.   

Exposure to severe windchill can lead to frostbite, hypothermia and death, meteorologists warned — and will be made even more dangerous in some areas by the prospect of blizzard conditions.

Last year’s holiday period was marred by an outbreak of COVID-19 among staff that forced airlines to cancel thousands of flights.