Airline schedules took a hit Sunday from the latest winter storm to snarl flights. And things were likely to get worse on Monday.

More than 1,500 flights had been canceled nationwide as of 1:20 p.m. ET Sunday, according to flight-tracking service FlightAware.

Throw in the count from Saturday -- as well as Monday's preemptive cancellation count -- and airlines had grounded more than 3,100 flights since the storm first began affecting flights Saturday. That figure seemed likely to grow significantly by Monday morning.

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Most big airlines eased rebooking rules for passengers scheduled to fly through airports in the storm's path. Airlines advised customers scheduled to fly Sunday or Monday to check ahead on the status of their flights before heading to the airport.

On Sunday, American Airlines and its regional affiliates were the hardest-hit, cancelling more than 640 flights across the AA network. AA operates its busiest hub at Dallas/Fort Worth, where wintry weather was forecast for the region. More than 540 combined arrivals and departures had been canceled at that airport alone as of 1:20 p.m. ET.

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But an even bigger headache loomed for fliers on Monday. Airlines had already preemptively canceled more than 1,100 of Monday's flights as of 1:20 p.m. ET on Sunday, according to FlightAware.

The bulk of Monday's cancellations – so far – came in the greater Washington, D.C. area, though problems were popping up at airports throughout the populous Northeast.

The storm was forecast to dump up to a foot of snow around the nation's capital by late Monday afternoon. Nearly 290 flights – or more than 30% of Monday's schedule – had already been canceled at Washington's Reagan National Airport. The Monday percentages were lower but still significant at the region's other two big airports, Washington Dulles (more than 20% canceled) and Baltimore/Washington (more than 10%).

But poor weather also was forecast Monday for New York and Philadelphia, metro areas that are home to four of the nation's busiest – and most delay-prone – airports.

More than 170 of Monday's flights had been canceled at New York LaGuardia as of 1:20 p.m. ET, a figure that represented nearly 15% of the airport's daily schedule, according to FlightAware. More than 10% of Monday's flight schedule had already been canceled at New York JFK and Newark Liberty airports.

In Philadelphia – a big hub for American merger partner US Airways – about 15% of Monday's flight schedule had been grounded as of 1:20 p.m. ET on Sunday, according to FlightAware.

A number of other major airports were being affected across as well. Among those reporting significant preemptive cancellation numbers for Monday as of 1:20 p.m. ET on Sunday were: Boston; Norfolk, Va.; Raleigh/Durham; Richmond, Va.; and Westchester County, N.Y.