A powerful storm system barreled across the South on Friday, unleashing severe winds, tornadoes and heavy rain, and killing five people, including a grandfather in Arkansas swept in a flooded roadway as he traveled to see his grandson, officials said.

“The thunderstorms are producing 80 mph winds — treat them like tornadoes,” Gov. Andy Beshear of Kentucky warned residents on Twitter on Friday afternoon.

Mr. Beshear said two tornadoes had swept through the western part of the state — the first in McCracken County and the second in Christian County.

Gov. Tate Reeves of Mississippi said in a statement on Friday that one person had died after severe weather lashed the state on Thursday night. He did not share more details about the episode.

Mr. Beshear said on Twitter that there were two storm-related deaths in his state: one in Simpson County and the other in Edmonson County. He also did not share more information.

A 70-year-old man was killed by a falling tree branch in Talladega County in Alabama on Friday while sitting in his truck, the local coroner said. In Scott County, Ark., a man who was trying to meet his grandson drove into a flooded roadway and was swept away into a nearby river, the authorities said.

About 380,000 customers were without power in Kentucky as of late Friday, according to poweroutage.us. Power was out for about 345,000 customers in Tennessee and 100,000 in Alabama.

The storms were shifting northeast on Friday, bringing an enhanced risk of severe weather across parts of the Ohio and Tennessee Valleys, as well as parts of the Southeast.

A tornado watch was issued on Friday for a large part of the country, including northeastern Mississippi, western Tennessee, western Kentucky, Virginia and parts of Georgia, North Carolina and South Carolina. The watch, which will remain in place until Friday night, means conditions are favorable for tornadoes.

The National Weather Service in Louisville reported a record-low pressure for the city of 977.1 millibars at Louisville Muhammad Ali International Airport, an indication of strength of the storm and its extreme winds.

One meteorologist described the expected combination of damaging winds, hail and tornadoes in Kentucky and Tennessee as a “gauntlet.”

“In my 21 years as a forecaster here in Nashville, I do not recall a gradient wind event like this one,” a meteorologist with the National Weather Service said in a forecast discussion.

Mr. Beshear signed a state of emergency on Friday morning to prepare the National Guard to be deployed.

“With everything we have been through, I want to make sure that everyone is safe today,” he said in a video. He added: “This could be a very dangerous day. We hope that it’s not, but we want you to be safe.”

Dozens of school districts in Kentucky and Indiana canceled classes on Friday.

As storms moved through the Dallas-Fort Worth area on Thursday night, strong wind gusts were recorded throughout.

Throughout North Texas, there were reports of wind damage to roofs and toppled power lines, according to the Weather Service. In McKinney, a suburb north of Dallas, the police said four tractor-trailers were blown over on a highway. Minor injuries were reported.

Images on social media showed uprooted trees and fences that had blown over. In Little Elm, a town northwest of Dallas, winds toppled the facade of a supermarket onto vehicles parked outside the store, a town spokeswoman said. No one was injured.

“We could hear the wind roaring outside,” Tom Bradshaw, a Weather Service meteorologist in Fort Worth, said. “This was a really impressive event.”

The Weather Service issued a number of tornado warnings throughout North Texas on Thursday, including one for Dallas. At least one tornado was recorded by a drone in Linden, east of Dallas, and another was spotted in Talco.

This region (sometimes called Ark-La-Tex) was under a “moderate risk” of severe storms, forecasters at the Storm Prediction Center said on Thursday. A moderate risk means forecasters believe that the storms that form in the area will be at a strength typically only experienced once a year, if that.

These outlooks are typically only issued about a dozen times a year on average. This is only the second one issued this year. The last was issued this past Sunday when damaging winds and tornadoes tore through parts of Oklahoma, injuring at least a dozen people.

Reporting was contributed by Jesus Jiménez, McKenna Oxenden, Derrick Bryson Taylor and Eduardo Medina.