NEW YORK, Nov 27 (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.