Beware, holiday air travelers. 'Tis the season for delays.

Winter storms have wreaked havoc and forced flight cancellations to and from Europe ahead of Christmas, with more snow forecast for overseas.

US Airways Group Inc., whose primary international hub is Philadelphia, canceled its Philadelphia flight to London Heathrow airport on Sunday, and the return flight Monday, but was scheduled to operate its 9:10 flight to Heathrow Monday night.

Philadelphia's busiest airline scrubbed its Philadelphia-to-Brussels flight on Monday, as well as the return Brussels flight Tuesday, and a Paris-to-Charlotte, N.C., flight Monday, US Airways spokesman Todd Lehmacher said.

Severe winter weather in Europe is expected to continue to make air travel difficult this week.

"We are just taking it one day at a time," Lehmacher said. "Right now, we are hoping to operate [to Europe] Tuesday, but, obviously, more decisions will need to be made whether that's going to happen."

British Airways operated only one of its two Monday evening flights to Heathrow from Philadelphia, and had a terse message for customers on its website: "Please consider whether your travel is essential. Significant weather continues to cause significant disruption to our operations."

If you do not need to travel, you can cancel your flight, get a refund, or change to another date in the next 355 days, the airline said.

US Airways issued a travel advisory Sunday night relaxing ticketing policies for flights to Brussels, Paris, Frankfurt, Amsterdam, Heathrow and Gatwick in London, and Manchester in the United Kingdom.

US Airways said it would allow one itinerary change, without penalty, for customers traveling to, from, or through those European cities through midnight Friday.

United and Continental also announced travel advisories, allowing customers to reschedule their itineraries to Europe with no penalty or change fee.

Southwest Airlines Co., Philadelphia's second-busiest carrier, canceled 24 flights because of heavy rain in California on Monday, including one morning flight from Philadelphia to Los Angeles. Two later Philadelphia flights to Los Angeles left on time, spokesman Paul Flaningan said. "It was pretty minimal disruption."

US Airways had no weather-related flight cancellations on the West Coast, but "there were extensive delays in the San Francisco area due to heavy rain," Lehmacher said.

"Last year, it was pretty intense. We had a lot of proactive cancellations," Flaningan said. "The Northeast was a continuous challenge for us."

Philadelphia International had no weather-related problems Monday. "We have been normal all day," airport spokeswoman Victoria Lupica said.

Elsewhere, according to the Federal Aviation Administration website, San Francisco, Minneapolis, Boston, Chicago, Newark, N.J., and New York's JFK airport had delays caused by weather, ranging from low-ceiling visibility to snow, ice, and wind.

Besides the weather, packed planes and inexperienced, once-a-year travelers toting children, extra bags, and packages can cause long lines at security checkpoints during the holidays.

The number of passengers traveling from Tuesday through Jan. 3 is expected to be up 3 percent over last year, to 43.6 million travelers, according to the Air Transport Association, the trade group for U.S. airlines.