More than 1,500 flights have been canceled within, into or out of the U.S. on Monday, and that number is expected to continue to climb.

American Airlines is seeing the biggest impacts because of its hubs in Miami and Charlotte, North Carolina.

American Airlines said it canceled operations at airports in Gainesville, Sarasota and Tallahassee until noon on Monday.

Sunday was one of the worst air travel days of the year with more than 2,400 cancellations.

Storm surge and flooding are major threats to Florida as Hurricane Debby slams the state, Gov. Ron DeSantis warned Monday.

Significant flooding has already impacted the Bradenton and Sarasota County area up to North Florida, he said.

The National Guard and Florida Highway Patrol are on standby, DeSantis said.

-ABC News’ Victoria Arancio

Hurricane Debby made landfall near Steinhatchee, Florida, on Monday morning, the National Hurricane Center said.

-ABC News’ Max Golembo

Hurricane Debby is "very near" landfall along Florida's Gulf Coast, the National Hurricane Center said at 5 a.m. ET.

"Expected to bring life-threatening storm surge in portions of Florida and major flooding in the southeastern United States," the center said in an update.

The Category 1 storm had maximum sustained winds of an about 80 mph as it approached Big Bend, a northern area near the Panhandle, the center said.

-ABC News' Max Golembo and Kevin Shalvey