More than 253,000 customers are without power in Florida, according to PowerOutage.us, which tracks energy outages.

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

Tornadoes are possible Monday afternoon in Florida, including in Tampa, Orlando and Jacksonville, as well as in the coastal cities of Savannah, Georgia, and Beaufort, 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. Conditions will deteriorate in Savannah, Georgia, throughout the day.

-ABC News’ Dan Peck

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

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