Tropical Storm Debby and thunderstorms in the Northeast were causing airlines to cancel or delay flights on Sunday.

American Airlines said it canceled 601 flights, or about 16% of its flights, on Sunday, the most of any airline.

Debby, now a tropical storm in the Gulf of Mexico, is expected to make landfall around the Big Bend area of the Florida Panhandle on Monday morning as a Category 1 hurricane.

Also causing delays and cancellations were severe thunderstorms moving up the East Coast. Parts of the mid-Atlantic states are under a severe thunderstorm watch until 9 p.m. ET Sunday, including the cities of Washington, D.C., Baltimore, Philadelphia, and New York City.

The most flights canceled on Sunday were at Charlotte Douglas International Airport, where 214 flights were canceled and another 172 were delayed, according to FlightAware.

In New York City, LaGuardia and John F. Kennedy international airports reported a combined 363 flights canceled and 258 delayed on Sunday.

Miami International Airport reported 209 canceled flights and 151 delayed fights. Airports in Orlando, Tampa, Philadelphia, Newark, Washington, D.C., and Dallas-Fort Worth all reported more than 100 flight cancellations on Sunday.

-ABC News' Clara Mcmichael

About 10 million people along Florida's west coast and up through Georgia and South and North Carolina were under a tropical storm alert Sunday as Debby took aim at the area and is forecast to make landfall in the Panhandle region on Monday.

Debby, now a tropical storm in the Gulf of Mexico about 190 miles southwest of Tampa, is expected to make landfall sometime between 7 and 8 a.m. in Big Bend on the Florida Panhandle as a Category 1 hurricane, state emergency officials said.

Debby is expected to create a storm surge of 4 to 7 feet in Cedar Key and Crystal River on the Florida west coast and a 2- to 5-foot storm surge farther south in Tampa and Sarasota.

The storm is forecast to move into the cities of Tallahassee and Jacksonville, bringing up to 20 inches of rain to some parts, before causing potential flooding in Georgia and the Carolinas on Tuesday, Wednesday and Thursday.

-ABC News' Kenton Gewecke

Members of the Florida National Guard have been advised to be prepared to conduct search-and-rescue missions once Tropical Storm Debby makes landfall on Monday, according to Florida Gov. Ron DeSantis.

Widespread flooding is expected for the Florida Panhandle region, particularly in the Big Bend area where Debby is expected to make landfall Monday morning as a category 1 hurricane, DeSantis said during a news conference Sunday morning.

DeSantis said the Florida National Guard and the state's Emergency Response Team are prepared to conduct search-and-rescue and provide humanitarian assistance.

"The Florida National Guard is standing by with 3,000 service members ready to assist the state Emergency Response Team, which includes search-and-rescue, route clearance, commodity distribution and protection of critical infrastructure," DeSantis said.

The governor said at least 12 swift boat crews and flat-bottom jon boat crews are also standing by to help with rescues.

He said that more than 30,000 bottles of water, more than 160,000 meals and nearly 14,000 tarps have been pre-staged in parts of Florida that are expected to be hit hard by the storm.

DeSantis said Florida utility companies have notified up to 17,000 linemen to be ready to spring into action once it is safe to repair any damage.

-ABC News Bill Hutchinson

Florida residents in the Big Bend region of the Panhandle were warned Sunday that Debby is going to bring "catastrophic rain to the area" that will cause flooding and power outages.

Kevin Guthrie, director of the Florida Division of Emergency Management, said during a news conference on Sunday that once the Debby makes landfall as a possible Category 1 hurricane on Monday morning, winds will subside quickly, but the slow-moving storm will produce an abundance of rain.

"We're going to be in a catastrophic rain situation where we have situations in Florida that will receive 15, maybe as high as 20 inches of rain," Guthrie said.

Florida Gov. Ron DeSantis told residents who are in the path of the storm to expect widespread power outages.

"It's possible that you could have serious intensification between now and landfall. It could get up to 85, 90 and 95 mph sustained winds. That is absolutely possible, particularly in parts of the state like here in Tallahassee. There's going to be a lot of trees that are going to fall down and you're going to have debris. You are going to have power interruptions," the governor said.

-ABC News' Bill Hutchinson