Hurricane Debby made landfall on Florida’s Gulf Coast early Monday, knocking power out for hundreds of thousands of people as the U.S. Southeast braced for potentially historic levels of rain and major flooding.

Debby barreled ashore in the state’s Big Bend region as a Category One hurricane, the lowest on a five-stage scale, fueled by an unusually warm Gulf of Mexico.

“We have seen significant storm surge, we have seen inundation, we have seen and we’ll continue to see flooding in various parts of the state of Florida,” Governor Ron DeSantis told reporters Monday.

He warned there was a threat of “significant flooding events” in the coming days.

More than 300,000 customers have lost electricity so far, according to tracker poweroutage.us.

“We have a lot of restoration personnel ready to go to get it back on,” DeSantis said.

The National Hurricane Center warned of life-threatening storm surges along the Gulf Coast with six to 10 feet (1.8 to 3 meters) of inundation above ground level in some areas.

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.

But it said Debby was weakening. The storm’s maximum sustained winds were 80 miles (130 kilometers) per hour at landfall but dropped to 75 mph shortly after.

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.

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

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

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

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.