Storm: 21% of all U.S. flights canceled Thursday

Last update: 9 p.m. ET.

A miserable day for flight cancellations became even worse after a Delta Air Lines flight skidded off a New York LaGuardia runway around 11 a.m. ET. All flights were grounded there through about 3 p.m. ET, when one of the airport's two runways reopened to flights. Still, operations there were likely to remain sparse for the rest of the day.

Beyond LaGuardia, 4,892 flights have been canceled nationwide and another 4,000 delayed as of 9 p.m. ET, according to flight-tracking service FlightAware. That means more than 21% of all flights in the United States have likely been canceled Thursday. That's based on what FlightAware says is a typical winter weekday schedule of about 23,000 scheduled flights. Throw in Thursday's delayed flights, and that means about 40% of all of the day's flights have either been delayed or canceled.

It comes at the end bracket to a brutal 14-day stretch in which airlines have canceled more than 22,000 flights and delayed another 58,500.

And Friday appears set to add to the misery, with about 350 flights already preemptively canceled.

As with previous storms, airlines waived change fees for customers ticketed to fly into airports experiencing poor weather. The policies varied by airline, though they generally allowed customers to make one change without paying the customary change fee.

Unsurprisingly, cancellations at LaGuardia soared after the morning's incident. Nearly 75% of the day's entire schedule -- about 900 flights -- had been canceled there as of 9 p.m. ET.

Next on the list was Dallas/Fort Worth with more 800 cancellations, representing about 40% of the entire day's schedule there. That follows a similarly rough day on Wednesday for the airport.

DFW is the busiest hub for American, the nation's biggest airline.

But DFW was just one of many airports to suffer a major weather-related blow on Thursday.

Nearly two thirds of the flights at Washington Reagan National and more than half of the day's schedule at Philadelphia had been grounded, according to FlightAware. Both are hubs for US Airways, now part of American Airlines.

Also hard hit Thursday were Newark Liberty and Washington Dulles, both big hubs for United Airlines. About a third of the day's schedule had been axed at Dulles and more than 40% at Newark.

New York's JFK airport – a hub for American, Delta and JetBlue – appeared to be faring a bit better, with about 20% of the day's schedule lost to cancellations.

At Baltimore/Washington International, close to half of the Thursday schedule had been canceled as of 9 p.m. ET. BWI is one of Southwest's busiest bases.

And problems were widespread across large portions of the South and Ohio Valley, where near-record snow totals were reported in some areas.

Among airports with significant cancellation totals were: Boston; Charlotte; Cincinnati; Columbus, Ohio; Cleveland; Dallas Love; Detroit; Lexington, Ky.; Louisville; Memphis; Nashville; Pittsburgh; Raleigh-Durham and Richmond, Va.