Debby is moving inland across north Florida Monday afternoon after dumping more than 1 foot of rain in the state.

Tornadoes are possible Monday night in the coastal cities of Savannah, Georgia, and Charleston, South Carolina.

The biggest concern this week is flash flooding from the widespread, relentless rain. A flash flood emergency remains in effect in Suwannee County, Florida, northwest of Gainesville, where 10 to 15 inches of rain has fallen so far.

Debby will slowly churn into southeastern Georgia throughout Monday afternoon and evening. Through the night, the storm will be focused on Jacksonville, Florida; Savannah; and southern South Carolina.

-ABC News’ Dan Peck

More than 1,700 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.

At least four people have been killed from Debby, authorities said.

A 13-year-old boy was killed when a tree fell on a mobile home Monday morning in Levy County, Florida, which encompasses Cedar Key, according to the Levy County Sheriff’s Office.

"Our thoughts and prayers are with this family as they deal with this tragedy," the sheriff’s office said in a statement. "We encourage everyone to use extreme caution as they begin to assess and clean up the damage. Downed powerlines and falling trees are among the many hazards."

In Hillsborough County, Florida, which encompasses Tampa, a driver of a semi was killed when the truck went into a canal off Interstate 275 Monday morning, according to the Hillsborough County Sheriff's Office.

Deputies recovered the semi and found the driver dead inside the cab, authorities said.

"It’s just heartbreaking," Tampa Mayor Jane Castor told ABC News Live. "We really ask everyone, one, to be aware of the weather situations in your area, and then heed the warnings. Especially the high wind and the water."

In Dixie County, Florida, a driver “lost control due to the inclement weather and wet roadway” on Sunday night, crashing into the center median guardrail and then overturning the car, the Florida Highway Patrol said.

The driver, a 38-year-old woman, and a passenger, a 12-year-old boy, were killed, authorities said.

Another passenger, a 14-year-old boy, was seriously injured, the highway patrol said.

A flash flood emergency has been issued in Suwannee County in north Florida, northwest of Gainesville, where officials are reporting that water is entering structures.

Ten to 15 inches of rain has inundated the county so far and another 3 to 6 inches of rain is expected.

-ABC News’ Dan Peck