(NEXSTAR) – Over 11,000 flights within, into, or out of the U.S. have been canceled Sunday as much of the eastern portion of the country is battered by a severe winter storm. As of 5 p.m. ET, more than 3,600 flights within, into, or out of the U.S. have been delayed, according to the FlightAware tracking service. Another 11,082 flights have been canceled. Hartsfield-Jackson Atlanta International Airport has seen the most cancellations so far, with more than half of Sunday's flights originating at the Georgia airport being canceled, according to FlightAware. The 10 U.S. airports with the most outbound flights canceled as of 5 p.m. ET are: Mid-morning Sunday, officials at Reagan Airport said all flights at the airport were canceled as crews worked to clear the airfield. According to FlightAware, cancellations at LaGuardia and Philadelphia comprise 91% and 94% of each airport's schedule for the day, respectively. It's the delayed flights that have been steadily growing through the afternoon. At Chicago O'Hare, more than 370 flights have been delayed, the most in the U.S. as of Sunday afternoon. At Dallas-Fort Worth, delays have ticked over 200, while at Hartsfield-Jackson, more than 170 flights are delayed. More than 1,600 of the cancellations are American Airlines flights, while more than 1,400 are Delta Air Lines flights. Cancellations for Southwest and United have surpassed 1,300 and 1,000, respectively. For both American and Southwest, more than 500 flights apiece have been delayed. For SkyWest and Delta, delays total more than 450 flights each. If you’re already at the airport, get in line to speak to a customer service representative. If you’re still at home or at your hotel, call or go online to connect to your airline’s reservations staff. Either way, it helps to also research alternate flights while you wait to talk to an agent. Most airlines will rebook you on a later flight for no additional charge, but it depends on the availability of open seats. Delta announced last week that it would waive the fare difference for select flights for impacted travelers, for example. You may be able to get booked on another airline, but companies aren’t required to put you on another carrier’s flight. Some airlines, including most of the biggest carriers, say they can put you on a partner airline, but even then, it can be a hit or miss. If your flight was canceled and you no longer want to take the trip, or you’ve found another way to get to your destination, the airline is legally required to refund your money — even if you bought a non-refundable ticket. It doesn’t matter why the flight was canceled. The airline might offer you a travel credit, but you are entitled to a full refund. You are also entitled to a refund of any bag fees, seat upgrades or other extras that you didn’t get to use. A massive winter storm is continuing Sunday, dumping sleet, freezing rain and snow across the South and up through New England, bringing frigid temperatures, widespread power outages and treacherous road conditions. The ice and snowfall were expected to continue through Monday in much of the country, followed by very low temperatures, causing “dangerous travel and infrastructure impacts” to linger for several days, the National Weather Service said. Heavy snow was forecast from the Ohio Valley to the Northeast, while “catastrophic ice accumulation” threatened from the Lower Mississippi Valley to the Mid-Atlantic and Southeast. “It is a unique storm in the sense that it is so widespread," weather service meteorologist Allison Santorelli said in a phone interview with The Associated Press. "It was affecting areas all the way from New Mexico, Texas, all the way into New England, so we’re talking like a 2,000 mile spread.” As of Sunday morning, about 213 million people were under some sort of winter weather warning, she said. The number of customers without power was approaching 800,000, according to poweroutage.us, and the number was rising. Even once the ice and snow stop falling, the danger will continue, Santorelli warned. “Behind the storm it’s just going to get bitterly cold across basically the entirety of the eastern two-thirds of the nation, east of the Rockies," she said. That means the ice and snow won't melt as fast, which could hinder some efforts to restore power and other infrastructure. President Donald Trump had approved emergency declarations for at least a dozen states by Saturday, with more expected to come. The Federal Emergency Management Agency pre-positioned commodities, staff and search and rescue teams in numerous states, Homeland Security Secretary Kristi Noem said. The Associated Press contributed to this report.