A pre-Christmas blizzard that is battering at least eight states in the middle of the nation could trip up travelers headed home in the coming days.
As of this afternoon, nearly 20 inches of snow had been reported in Colorado just west of Denver. Six to 10 inches had been reported in Nebraska. Between 3 and 8 inches had accumulated in Iowa and more was possible. Snow was falling 2 inches per hour in Wisconsin.
No planes were able to land at Iowa's Des Moines International Airport late this afternoon. All flights were cancelled until at least 11:45 a.m. Friday.
Chicago may prove most problematic for travelers. Rain had cancelled 400 flights into and out of Chicago O'Hare by late afternoon today, according to data from FlightAware.
Snow and wind expected tonight was likely to further complicate travel and likely cancel more flights.
American was cancelling all flights to and from O'Hare at 9 p.m. ET.
Southwest Airlines was cancelling all departures and arrivals at Midway Airport as of 4 p.m. local time. On a typical day, the airline has between 200 and 220 flights in and out of Midway.
Southwest was also planning to cancel all arrivals and departures from the Milwaukee Airport as of 6 p.m. local time. Southwest has 35 flights in and out of Milwaukee.
Several airlines issued flexible travel policies today, allowing travelers with flights into, out of and through affected areas to change their plans without penalty.
Airlines for America, an airline industry trade group, estimated that 42 million passengers will fly on U.S. airlines for the 21-day holiday travel period from Dec 17 to Jan 6. Daily passenger volumes were expected to range from 1.5 million to 2.3 million people.
The busiest days of the Christmas travel season were expected to be Dec. 21, 22, 23 and 26, and Jan 2. Foul weather in major hub cities, particularly on those days, would most certainly cause travel headaches on the roads and in the skies.
When bad weather grounds flights at major airports, delays pile up around the nation, stranding travelers even in places where the weather is good. And because planes fly so full around the holidays, it's difficult for the airlines to find empty seats to accommodate fliers whose flights have been cancelled.
Passengers are entitled to a refund if their flight is cancelled.
Travelers should confirm their flight is taking off as planned on their carrier's website before leaving their homes.
If you are at the airport by the time you find out, use every avenue available to get re-accommodated. While you stand on line to talk to a customer service agent, also call your carrier and use Twitter to get in touch with your airline. Many airlines are faster to respond on Twitter than on the phone. Delta Airlines and JetBlue are particularly active.
A few Twitter handles to know:
Travelers who find themselves stranded and in need of a hotel room should use apps such as HotelTonight, Travelocity's LastMinute.com hotel booking app and the Priceline app to find deals on last-minute hotel stays.
The Midwest storm was expected to move east tonight, spreading rain into the Northeast, with some areas from Washington, D.C., to Boston getting as much as 2 inches.
Behind the storm, cold air was forecast to come in and change rain to snow in western Pennsylvania, Maryland, New York and West Virginia, where 3 to 14 inches (in the highest elevations) could accumulate.
The severe weather was also impacting drivers in the Midwest Thursday afternoon. In Iowa, there were more than 88 crashes, with hundreds of motorists calling for assistance. Most of Wisconsin was reeling, with hundreds of emergency calls placed. In Missouri, emergency responders were being dispatched to everything from slide offs and minor crashes to sites where injuries had occurred.
ABC News' Max Golembo and Ginger Zee contributed to this report.