Roaring flash floods struck Maryland Sunday, turning downtown streets into raging rivers that swept away parked cars and reached the first floor of some buildings in Ellicott City.

Maryland Gov. Larry Hogan declared a state of emergency. One person was declared missing early Monday in Howard County, which is just west of Baltimore.

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Meanwhile, subtropical storm Alberto was expected to make landfall sometime Monday on the northern Gulf Coast, where white sandy beaches emptied of their usual Memorial Day crowds.

In a 5 a.m. ET update, the National Hurricane Center said that Alberto was centered about 125 miles south of Destin, Florida. The storm had maximum sustained winds of 65 mph with higher gusts.

"Little change in strength is expected before Alberto reaches the northern Gulf Coast later today," it added.

The NHC also warned of life-threatening surf conditions, the possibility of a tornadoes in much of Florida and parts of Georgia, South Carolina and Alabama, as well as up to a foot of rain in some areas.

In the mid-Atlantic, a separate storm system was causing what the weather service called an "extremely dangerous and potentially catastrophic situation" in Ellicott City.

Howard County fire and rescue officials said there were reports that a building had collapsed.

During a news conference, Howard County Executive Allan Kittleman said he was “heartbroken for the people in Ellicott City” and recalled a flood two years ago that struck the city's, when 6 inches of rain — or a month's worth — fell in two hours, killing two people and requiring more than 100 rescues.

“I can tell you right now, the devastation is worse I think than it was in July of 2016, which is kind of hard for me even to imagine, because back then we all said that it looked kind of like the set from a disaster movie,” he said.

Much of that devastation had been repaired when Sunday’s storm struck, Kittleman added, and more than 90 percent of its businesses had reopened.

A resident, Isabelle Levine, worried that this wouldn’t happen again.

“If Old Ellicott City comes back after this, I will be amazed,” she said. “I would give up as a business owner.”

Across the Gulf Coast, meanwhile, residents were preparing for the worst of Alberto's powerful winds and heavy rains to arrive Monday.

 Jo Newton of Panama City said she was filling up sandbags "to hopefully keep the water from coming in my front door."

"I'm actually terrified of the amount of rain that is predicted to come in," she said.

The storm's approach also triggered mandatory evacuations of some small, sparsely populated Gulf Coast barrier islands in one Florida county. The Florida Division of Emergency Management said in a statement Sunday that a mandatory evacuation has been issued in Franklin County for all barrier islands there and those in the county living directly on the coast in mobile homes or in recreation vehicle parks.

Florida Gov. Rick Scott said Sunday that such storms were "unpredictable," but he appeared to be preparing for the worst.

On Saturday, he declared a state of emergency in 67 counties. More than 5,000 National Guard members were ready for deployment, while 165 state troopers were set to begin 12-hour shifts and dozens more had been assigned to "quick response" teams.