By Aatreyee Dasgupta and Shivansh Tiwary

Dec 29 (Reuters) - Winter Storm Ezra disrupted holiday travel across the U.S. Northeast, Midwest and Great Lakes for a third straight day on Monday, causing thousands of flight delays and hundreds of cancellations as airlines scrambled to recover and meteorologists warned of a brewing "bomb cyclone" that could further snarl trips ahead of the New Year's holiday.

Nearly 6,000 flights were delayed and 751 canceled as of 3:25 p.m. ET, according to flight tracking website FlightAware. Since Friday, weather disruptions have canceled over 3,600 flights and delayed more than 30,000 others.

The storm hit during one of the year's busiest travel periods, when airlines operate near capacity with limited flexibility to rebook passengers. Holiday travelers faced long waits, rebooking difficulties and accommodation challenges as airlines coped with severe winter weather.

AccuWeather meteorologists warned the powerful storm would intensify into a "bomb cyclone" through Monday night, bringing blizzard conditions, dangerous ice, flooding rain and strong winds from Wisconsin to Maine.

A bomb cyclone occurs when rapidly dropping atmospheric pressure produces hurricane-force winds and heavy precipitation. The arctic cold front driving the storm brought with it wild swings in temperatures. In Philadelphia, temperatures, which soared close to 60 degrees on Monday, were expected to drop into the 20s overnight.

Major travel disruptions, regional power outages and hazardous conditions were expected through early Tuesday.

The harsh weather also disrupted road travel, with poor visibility, icy surfaces and blowing snow contributing to several multi-vehicle pileups and prompting authorities in parts of the region to urge drivers to avoid non-essential travel.

The Federal Emergency Management Agency warned travelers that driving could be dangerous as blizzard-like conditions, high winds and ice descend across the Upper Midwest and Great Lakes.

The Federal Aviation Administration issued a ground stop at Washington's Dulles International Airport until 4:15 p.m. ET due to high winds. Delays at airports in Boston and Newark, New Jersey, which serves New York City, were imposed due to low visibility and windy conditions.

Detroit-bound flights faced additional disruption after the FAA ordered a ground stop at Detroit Metropolitan Wayne County Airport through 8 a.m. ET on Monday. The halt, affecting only Delta Air Lines DAL.N flights, was imposed for undisclosed operational reasons, and delays were expected to continue until midnight.

Delta had the highest share of cancellations and delays on Monday, and its shares fell nearly 3% in afternoon trading.

Shares of United Airlines UAL.O, American Airlines AAL.O and Alaska Air Group ALK.N were each down about 2%.

The FAA also said departures from Albany, Bangor, Burlington and Minneapolis-St. Paul were delayed as ground crews cleared snow and ice from planes.

Airline operations are tightly interlinked, meaning canceled flights can leave aircraft and crews out of position, complicating efforts to restore normal schedules.

American Airlines, United Airlines, JetBlue Airways and Delta told Reuters they had waived change fees for passengers affected by weather-related disruptions.

(Reporting by Aatreyee Dasgupta, Shivansh Tiwary and Abhinav Parmar in Bengaluru. Edited by Dawn Kopecki and Shilpi Majumdar)

((Aatreyee.Dasgupta@thomsonreuters.com;))