After powerful storms swept through the South on Friday, leaving at least 12 people dead and hundreds of thousands of customers without electricity on Saturday, officials urged residents to use caution as downed power lines and uprooted trees posed lingering threats.

Heavy rains, severe winds and tornadoes damaged homes and businesses in Alabama, Arkansas, the Carolinas, Kentucky, Mississippi, Tennessee and West Virginia.

Kentucky was hit particularly hard by the storms, which left at least five people dead in the state, Gov. Andy Beshear said at a news conference. About 240,000 customers in Kentucky remained without power as of Sunday. In Tennessee, about 40,000 customers did not have power, according to poweroutage.us.

Mr. Beshear said wind gusts reached up to 75 miles per hour in some areas and that there had been at least two small tornadoes. He said the power outages were affecting water systems, putting more than 1,800 residents under a boil-water advisory.

“When it comes to power, this is going to be a multiday event,” he said.

Clear skies and sunshine were forecast for the rest of the weekend, but Mr. Beshear warned residents to be careful because of threats from the storm damage.

He advised residents to only use generators outdoors and to treat every downed power line as if it were live. He said more trees could fall because of the “very soggy, very wet” ground.

Mr. Beshear said five people died in the storms: a 68-year-old man in Simpson County; a 23-year-old man in Edmonson County; a 63-year-old man in Logan County; an 84-year-old man in Bath County; and a 41-year-old woman in Fayette County. He did not say how they had died.

Several people died because of fallen trees, including three in Alabama.

A 70-year-old man was killed by a falling tree in Talladega County, Ala., while sitting in his truck, the local coroner said. A 43-year-old man in Lexington, Ala., died after a tree fell on him, said Kim Edgil Jones, the coroner in Lauderdale County, Ala. And in Huntsville, Ala., a man was cleaning up tree limbs when, shortly before 2 p.m., a tree fell on him, the police said. The man, whose name was not released, was pronounced dead at the scene.

In Tennessee, the state’s Emergency Management Agency confirmed two weather-related fatalities. The communications director of the agency, Maggie Hannan, did not say how they died.

In Arkansas, a man in Scott County who was trying to meet his grandson drove into a flooded roadway and was swept away into a nearby river, the authorities said.

A person was killed when a tree fell on their vehicle in Yazoo County, Miss., the Mississippi Emergency Management Agency said. Dozens of homes, a community college and an apartment complex were damaged by the severe weather, the agency said.

The Tennessee Highway Patrol said a sergeant was trapped momentarily in a patrol car after several trees collapsed on top of it in Meigs County. The sergeant escaped without injuries.

Wind gusts reached up to 79 m.p.h. in Tennessee on Friday, the National Weather Service said. “If you’ve ever wondered what it’s like in a tropical storm, this is it,” the service said on Twitter on Friday.

Zack Taylor, a meteorologist with the Weather Service’s Weather Prediction Center, said that the region would see much quieter and drier weather over the weekend.

“That would give time for the folks there to dry out and deal with the recovery efforts in terms of the downed trees and power lines,” Mr. Taylor said.

There is evidence that the United States can expect more unusual severe storms as the planet heats up, potentially striking in new places or at unexpected times of year.

While some questions are difficult to answer — such as whether that will mean more tornadoes in the future — scientists say the risks of increasingly wild weather make it all the more urgent that cities and states take steps to protect people and property.