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."

More than 1,600 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.

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.