A total of 1,339 flights were canceled, and 606 flights were delayed, as of 6:45 a.m. ET, according to flight-tracking website FlightAware.

Southwest Airlines canceled 265 flights, the most among carriers, followed by American Airlines at 176. Reagan National Airport in Washington, DC, canceled 234 flights, the most among airports and constituting 58% of all flights scheduled there, according to the data.

Major US airlines issued travel advisories between Saturday and Monday, including Delta, American, United Airlines and Southwest.

As the storm moved toward the mid-Atlantic region on Monday, more than a dozen states from Kansas and Missouri to New Jersey were under winter storm warnings and advisories.

“For locations in this region that receive the highest snow totals, it may be the heaviest snowfall in at least a decade,” the National Weather Service said.

Washington, DC, Mayor Muriel Bowser declared a snow emergency until at least the end of the day on Tuesday, while Maryland Gov. Wes Moore declared a state of emergency and warned residents to avoid traveling.

Reuters contributed to this report.