Airlines canceled over 5,000 U.S. flights on Friday as massive winter storms snarled airport operations around the United States and frustrated tens of thousands of holiday travelers.

The cancellations followed nearly 2,700 canceled flights on Thursday and almost 500 flights have already been canceled for Saturday, according to flight tracking website FlightAware.

Passenger railroad Amtrak has canceled dozens of trains through Christmas, disrupting holiday travel for thousands.

Many highways in the Midwest faced lengthy delays because of snowy weather or crashes and police in Indiana areas urged motorists to avoid nonessential travel in the northwest part of the state.

The Federal Aviation Administration (FAA) imposed ground stops or delays for de-icing at a number of U.S. airports because of winter weather.

Transportation Secretary Pete Buttigieg told CNN the U.S. aviation system “is operating under enormous strain” and two different storms and high winds are affecting airports around the country. About 10% of U.S. flights were canceled on Thursday, Buttigieg said.

Another 8,000 U.S. flights were delayed on Friday – including more than 32% of those operated by American Airlines (AAL.O), United Airlines (UAL.O), Delta Air Lines (DAL.N) and Southwest Airlines (LUV.N).

Southwest canceled 951 flights on Friday, nearly a quarter of all its scheduled flights, while Alaska Airlines (ALK.N) canceled 445, or 57%, of its flights.

Seattle-Tacoma International Airport had 303 flights, or 54% of departures canceled as of 2:30 p.m. PST. The FAA early on Friday issued a ground stop there due to snow and ice.

More than 40% of departing flights at Detroit Metro were canceled, 67% at Portland, 36% at New York’s LaGuardia, 27% at Boston and 25% at Chicago O’Hare.

Chicago was facing dangerously cold temperatures with the National Weather Service warning wind chill could hit minus 30 Fahrenheits (minus 34 Celsius)