A tornado watch was issued Sunday afternoon by the National Weather Service as Tropical Storm Debbie moved northward in the Gulf of Mexico parallel to Florida's Gulf Coast.

At 2 p.m. Sunday, the National Hurricane Center said Debby was still a tropical storm with winds of 65 mph and forecasted to bring severe weather, including heavy rain, to coastal communities Sunday afternoon.

A tornado watch is in effect for much of Florida through Sunday evening.

Debby is expected to become a hurricane Sunday evening and will likely make landfall sometime between 7 a.m. and 8 a.m. Monday in the Big Bend area of Florida's Panhandle.

Once Debby makes landfall, the storm is expected to slow down and essentially stall over the Southeast, dropping potentially historic amounts of rainfall as it moves into Georgia and South Carolina.

A hurricane warning is in effect for the west coast of Florida from the Ochlockonee River southwest of Tallahassee to the Suwannee River northwest of Gainesville.

-ABC News' Daniel Amarante

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

At least 1,613 flights within, into or out of the U.S. were canceled nationwide on Sunday and another 3,213 were delayed, according to , a website that tracks flights.

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