United, Southwest and Frontier have canceled flights out of Denver International Airport because of the blizzard.The number of flights canceled out of DIA is expected to be in the hundreds.DIA spokesman Chuck Cannon said airlines are canceling a lot of flights but the airport is still open.United Airlines has canceled at least 100 flights."These are proactive cancellations. It is in no one's interest to have customers going to the airport to wait it out and find out whether or not flights have been canceled," said United Airlines spokesman Rahsaan Johnson.United is waiving rebooking/change fees for customers who bought tickets for travel Wednesday, Thursday and Friday. Travelers can rebook their flight without cancellation fess.These are for flights to Denver, from Denver, or through Denver.United advises customers to log on to united.com to make changes to their itineraries or to call United reservations at 1-800-UNITED-1 or their travel agency. All changes must be made by midnight of the customer's original travel date. Rescheduled travel may be subject to higher fares if it does not meet original rule and booking-code restrictions or is a new itinerary.The revised ticketing policies apply to all tickets purchased March 25, 2009, or earlier, for travel between March 25 to March 27, to, from and through Denver on any United, United Express or United code-share flight. Origin and destination cities must remain the same.Customers with canceled flights are eligible to apply for a full refund.United and United Express has 400 departures out of DIA, on average, for Thursdays.Southwest Airlines and Frontier have also canceled flights that depart after 11 a.m."We have made the decision to discontinue operating from 11 a.m. to 5 p.m. This means we will not be flying in and out of Denver between these times," said Southwest Airlines spokesman Chris Mainz.Southwest said it has canceled 82 flights."We will take a wait-and-see approach later to see if we can or should resume operations," Mainz said.Mainz said Southwest will relax its fare rules to allow them to rebook without penalties.Frontier spokesman Steve Snyder said that Frontier canceled 54 flights on large aircraft and 33 regional flights operated by Lynx Airlines. The airline will re-evaluate later on the day and may cancel other flights."It's going to be tough for folks to fly out to today if they haven't gone out already," Snyder said.Snyder said because Denver is a hub for Frontier, one of the challenges is to make sure that the aircraft and crew are in place to resume flights after the storm is over.To check the status of your flight, visit the DIA Web site at FlyDenver.com or check the Web site for your local airline.