Fort Myers, Florida - Hurricane Debby made landfall on Florida's Gulf Coast early Monday, knocking power out for hundreds of thousands of people as the southeastern US braced for potentially historic levels of rain and major flooding.

Debby barreled ashore in Florida's Big Bend region as a Category One hurricane, the lowest on a five-stage scale, thanks to an unusually warm Gulf of Mexico. The National Hurricane Center said its maximum sustained winds were 80 miles (130 kilometers) per hour.

More than 250,000 customers have lost electricity so far, according to tracker poweroutage.us.

The National Hurricane Center warned there is a danger of life-threatening storm surges along Florida's Gulf Coast with six to 10 feet of inundation above ground level in some areas.

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The storm will probably cause catastrophic flooding with "potentially historic heavy rainfall" when Debby moves northeast across Georgia and South Carolina over the next few days, the NHC said.

"We are looking at potentially really, really significant flooding that will happen, particularly in north-central Florida," Governor Ron DeSantis told an emergency briefing on the storm Sunday.

He and NHC deputy director Jamie Rhome had urged Floridians to quickly prepare for the storm.

Mandatory evacuations were ordered for part of Citrus County, Florida, with eight other counties under voluntary evacuation orders, local media reported.

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.

The governors of Georgia and South Carolina have declared a state of emergency ahead of the storm's arrival.

Authorities have predicted multiple days of heavy rainfall, possibly record-breaking levels, and likely severe flash flooding.

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

DeSantis has activated the state's National Guard, with 3,000 service members on standby to help with storm response.