One of the busiest U.S. travel days of the year may be a bust for some as a mix of rain, snow and ice threatens to delay flights in Chicago and Denver this afternoon.

On the eve of the Thanksgiving holiday, light snow was falling in Denver, with 1 to 4 inches forecast. In Chicago, rain may turn into as much as 3 inches (8 centimeters) of snow later this evening, according to the National Weather Service.

"A sharp cold front will allow a changeover to snow to take place from northeastern Kansas to northern Illinois, including Chicago," Kristina Baker, a meteorologist with, said. "Snow and frozen wet spots will lead to hazardous travel toward evening as temperatures plummet."

About 207,000 travelers will pass through Chicago's O'Hare Airport today and 72,000 through its Midway Airport, the city's Aviation Department said. O'Hare is the biggest U.S. hub for UAL Corp.'s United Airlines.

"All of our parking lots are open, roadways are clear, and security checkpoint lines are moving," Gregg Cunningham, the media relations coordinator at O'Hare, said. "We're taking a wait-and-see approach with the weather. We're definitely prepared and ready."

Cunningham said fliers getting to the airport before 5 p.m. local time will have the best chances for flights leaving on schedule, as the storm is forecast to reach Chicago about then.

The Air Transport Association projects 27 million passengers—a 4 percent rise over last year—will fly from Nov. 16 through Nov. 27, the busiest time of year for airlines.

"We're looking pretty good today despite the high volume," Diane Spitaliere, a spokeswoman for the FAA, said. "We're diverting some flights into Canadian airspace to keep them moving, and as always, we're keeping our eyes on the weather. For the most part, we're in good shape."

At 1 p.m. New York time, Teterboro, White Plains, LaGuardia and Newark airports in the New York area were listed as having delays of as long as 57 minutes, according to the U.S. Federal Aviation Administration. All other airports were running on time.

President George W. Bush opened military airspace to commercial planes and suspended some nonessential maintenance this week to help increase on-time arrivals. Through September, about 73 percent of flights arrived on time. The tie-ups are worst over New York, New Jersey and eastern Pennsylvania.

Amtrak expected more than 115,000 riders today, about a 70 percent increase over a typical Wednesday, spokesman Cliff Cole said. Everything was running smoothly and on time, he said.

The American Automobile Association expects a record 38.7 million Americans will travel 50 miles (80 kilometers) or more this week, with about 80 percent of those trips by car. That would top the previous mark of 38.1 million travelers last year.

Drivers should prepare for road congestion today. The 225- mile stretch of Interstate 95 between Washington and New York has been identified as the most-congested in the U.S. for holiday travel, according to Metro Works, the traffic-reporting unit of Westwood One Inc.

Air and road traffic may become a mess in the Northeast on Nov. 25 and 26, when a weather system moving north through the Gulf of Mexico is expected to bring more of an inch of rain throughout the Northeast, the National Weather Service said.

"On the way back, you're bound to be stuck in traffic," AAA's Sean Comey said. "Everyone returns on Sunday afternoon."

Thanksgiving ranks behind the Fourth of July and the Christmas-New Year's period for holiday travel, according to AAA, whose reporting period for Thanksgiving runs from today through Nov. 25.