As of 5 p.m. ET on Sunday, Tropical Storm Debby still has winds of 65 mph and is forecast to undergo rapid intensification into a hurricane overnight, according to the National Hurricane Center.

Debby is expected to make landfall as a hurricane in Florida’s Big Bend region on Monday morning around 7 a.m.

Rain is expected to be the most impactful element from this storm, affecting a wide area over an extended period of time.

"Across portions of southeast Georgia and South Carolina, 10 to 20 inches of rainfall, with local amounts to 30 inches, are expected through Friday morning," the National Hurricane Center said. "This potentially historic rainfall will likely result in areas of catastrophic flooding."

An evacuation order was issued Sunday for residents of Alachua County, Florida, residing in mobile homes, manufactured homes and recreational vehicles ahead of the arrival of Debby, which is forecast to be a hurricane when it makes landfall on Monday.

People living in areas close to rivers and lakes, and low-lying areas that tend to flood in Alachua County are also encouraged to evacuate, according to a statement issued by county officials.

Gainesville is the largest city in Alachua County

"We encourage residents affected to find alternative housing with friends, family or short-term rentals," according to the county's statement.

Alachua County is opening three shelters for residents in need.

-ABC News' Bill Hutchinson

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