A pre-Christmas blizzard that is battering at least eight states in the middle of the nation could trip up travelers headed home for Christmas in the coming days.
Nearly 20 inches of snow have been reported in Colorado just west of Denver. Nebraska has reported 6-to-10 inches so far. Between 3 and 8 inches have accumulated in Iowa already and more is possible. Snow is falling 2 inches per hour in Wisconsin.
No planes were able to land at Iowa's Des Moines International airport. All flights were cancelled until at least 11:45 a.m.
But it's Chicago that will prove most problematic for travelers. Rain has cancelled 300 flights into and out of Chicago O'Hare today so far, according to data from FlightAware. Snow and wind that are expected tonight will further complicate travel and likely cancel more flights.
Several airlines have already issued flexible travel policies, allowing travelers with flights into, out of and through affected areas to change their plans without penalty. For example, travelers headed to O'Hare today on American Airlines can change their flight to any day Dec. 21 to Dec. 25. Delta, United and others have similar policies.
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Airlines for America, an airline industry trade group, estimates 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 are expected to range from 1.5 million to 2.3 million.
The busiest days of the Christmas travel season are expected to be Dec. 21, 22, 23 and 26; and Jan 2. Foul weather in major hub cities, particularly on these days, will 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 also 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 online 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:
@JetBlue
@DeltaAssist
@AmericanAir
@United
@SouthwestAir
@FlyFrontier
@USAirways.
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.
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The Midwest storm moves east tonight, spreading rain into the Northeast with some areas from Washington, D.C., to Boston getting up to 2 inches. Behind the storm, cold air comes in and changes rain to snow in Western Pennsylvania, Maryland, New York and West Virginia, where 3 to 14 inches (in the highest elevations) could accumulate.
ABC News' Max Golembo and Ginger Zee contributed to this report.