Air travelers faced major headaches Monday afternoon as a long line thunderstorms blocked flight paths from New England to the Gulf Coast.

Adding to the problems: near-record heat was slowing some flights in the Southwest while California flights were being slowed in Los Angeles and San Francisco.

Nationwide, more than 1,300 flights had been canceled and another 4,900 had been delayed as of 3:45 p.m. ET, according to flight-tracking service FlightAware.

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The biggest problem came in the East, where a long line of storms was creating headaches at airports throughout the region.

The three delay-prone airports serving the New York City area were among the hardest hit. At LaGuardia, about 1 out of every 4 flights had been canceled as of 3:15 p.m. while a similar number were running behind schedule. Combined, that meant about half of the entire day’s flight schedule was either delayed or canceled.

At Newark Liberty, about 20% of Monday’s flights were canceled (more than 250 total) and a similar number delayed. The numbers were slightly better at New York JFK, though delays and cancellations were a major problem there, too. Delays of up to three hours or more were being reported across the three airports.

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Significant back-ups also were being reported a dozens of airports from New England to Texas to the Great Lakes.

In the Northeast, Philadelphia, Boston and all three of the Washington-Baltimore area airports were seeing delay and cancellation numbers far above average. The line of storms was just moving into those…