NEW YORK (Reuters) - Millions of Americans are set to crowd into airports or hit the road on the Wednesday ahead of Thanksgiving Day which is shaping up to be one of the busiest travel days of the year.

On Tuesday, about 2.7 million passengers were screened at airport security checkpoints, the highest ever for a Tuesday before Thanksgiving and beating last year's record, according to the U.S. Transportation Security Administration. The travel industry expects even more travelers on Wednesday and Sunday, Dec. 1.

Industry group Airlines for America expects more than 3 million passengers on each day, respectively.

Some travelers may encounter inclement weather with portions of the East and West coast under winter storm warnings, said William Churchhill, meteorologist at the National Weather Service Weather Prediction Center in College Park, Maryland. In the upper mid-west of the United States, temperatures are expected to fall as low as -40 degrees during the holiday period.

"All things considered though, not too bad for a winter holiday," he said.

The Federal Aviation Administration said inclement weather and volume drive nearly all flight delays.

United Airlines said on Tuesday that an acute shortage of air traffic controllers was causing "significant disruption" for travelers at its busy Newark, New Jersey, hub outside of New York City.

For the Thanksgiving travel period, the agency will implement traffic flow management initiatives to address any staffing shortages as needed, a spokesperson said.

About 4,800 flights were delayed in, within and out of the United States on Tuesday while 82 flights were canceled, according to tracking website FlightAware.

A record 80 million Americans are expected to travel at least 50 miles for Thanksgiving with cruise companies expected to see a 20% increase in passengers, according to industry group AAA.

Royal Caribbean Groups said from Nov. 26 to Dec. 2, nearly its entire fleet will be sailing, representing nearly 150,000 berths, the number of passengers a cabin can sleep.