Florida - Some residents on Florida's Gulf coast have been ordered to evacuate as tropical storm Debby approaches hurricane force, with a likelihood of dumping "potentially historic" rainfall in the southeastern US, officials said Sunday.

Debby, now strengthening rapidly in an unusually warm Gulf of Mexico, is expected to slam into Florida's Big Bend region at hurricane strength sometime late Sunday.

It is expected then to slowly weaken, but to continue dumping huge amounts of rain on Florida, southeastern Georgia, and South Carolina as it rumbles slowly through over the next five days, a National Hurricane Center (NHC) advisory said Sunday.

Mike Brennan, an NHC briefer, said Sunday that the entire west coast of Florida is being placed under a tropical storm warning.

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He urged locals, particularly on the northwestern coast and Florida panhandle, to "find a safe place to be in by nightfall."

As of 8:00 AM, the storm was about 155 miles southwest of the major city of Tampa, carrying maximum sustained winds of 60 miles per hour and moving northwest at 13 miles per hour, the advisory said.

As locals rushed to prepare, mandatory evacuations were ordered late Saturday for part of Citrus County, Florida, with eight other counties under voluntary evacuation orders, local media reported.

"Take the situation seriously," Paul Hasenmeier, Hernando County fire chief and public safety director, told reporters late Saturday. "We know the water's going to come up as the storm passes."

Debby is expected to dump six to 12 inches of rain in parts of Florida, and as much as 20 to 30 inches in coastal Georgia and South Carolina before the week is over, the NHC said.

Brennan predicted "multiple days of very, very heavy rainfall" – at possibly record-breaking levels. He said there likely will be severe flash flooding "in areas that don't normally flood."

Meteorologists warned of possibly "life-threatening" storm surges of up to 10 feet and said Debby could spawn tornadoes late Sunday or early Monday across both Florida and Georgia.

President Joe Biden on Sunday approved an emergency declaration for Florida, allowing federal aid to be expedited.

Florida Governor Ron DeSantis has activated the state's National Guard to help with storm response.