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

President Joe Biden was briefed Monday morning on the administration’s ongoing response to Debby, according to a White House official.

On Saturday, Biden approved Florida’s request for an emergency declaration and deployed rescue personnel, meals and water, the official said.

The Biden administration is in touch with officials from Florida, Georgia and South Carolina. The Federal Emergency Management Agency has deployed staff to Florida, Georgia, South Carolina and North Carolina as the storm moves north, the official said.

-ABC News’ Justin Ryan Gomez

Debby, which made landfall in Florida as a Category 1 hurricane Monday morning, weakened to a tropical storm just before 11 a.m. ET.

Debby is barreling north with winds of 70 mph.

The storm is forecast to cause major flooding across the Southeast this week.

Rain totals have reached 19 inches in Florida so far.

In South Carolina, the rain total could topple the state’s all-time precipitation record for a tropical cyclone: 23.63 inches from Hurricane Florence in 2018.

Some of the heavy rain will make its way into Virginia by Friday and possibly to the coastal Mid-Atlantic by the weekend.