Waves of intense weather were moving east across the South on Wednesday, bringing flash flooding and tornadoes to cities along the Gulf Coast, with the potential for more destructive winds still looming, forecasters and local officials said.

A flash flood emergency was declared in the New Orleans area, where the National Weather Service said many roads in and around the city were under water and impassable. In Slidell, La., a city northeast of New Orleans, the authorities were reporting that up to four potential tornadoes had ripped through buildings and streets, causing damage. A tornado was also reported in Southeast Texas and severe weather damaged homes across several counties in Mississippi.

More than a dozen flash flood warnings were issued early in the day, including one for New Orleans, forecasters said. Officials closed some roads in southeastern Louisiana because of the heavy rain and some roads were reported under water in Southern Mississippi. Dallas was also under a flash flood warning on Wednesday afternoon.

There was a substantial risk of tornadoes in the area, and tornado warnings and watches were issued, including in Mobile, Ala. Forecasters warned that more than seven million people across Louisiana, Mississippi, Alabama and Florida could experience extreme weather through the day

The Storm Prediction Center issued a tornado watch for Southeast Alabama, the Florida Panhandle and Southwest Georgia on Wednesday afternoon, noting that “a cluster of storms are expected to move into the region through mid/late afternoon.”

Daniel Seuzeneau, the chief administrative officer for the Slidell Police Department, said that there were reports of up to four tornadoes that came through the city, and that the impact was widespread, from the north end to the south, in a stretch of about five miles where sporadic damage was reported.

The damage was “extensive,” he said. Buildings, including an apartment building, were partially collapsed. Trees were down on houses, people were trapped and cars were flipped over, he said. The department had not reported any major injuries or deaths.

“We rescued about 50 people out of an apartment complex,” he said. “Usually we don’t see damage this wide and extensive unless it’s a hurricane.”

In Mississippi, Gov. Tate Reeves said on Wednesday that 23 homes were damaged and at least one was destroyed in Hinds, Neshoba, Warren and Yazoo Counties, according to an initial assessment. Downed trees blocked roads in Hinds and Yazoo counties. There were no fatalities reported.

In Yazoo County, after a heavy rain, a family of four was evacuated from a house in the Eastbrook subdivision, which was downstream from a 10-acre man-made lake that was close to bursting its banks, said Jack Willingham, the county’s emergency management director. Residents in seven to eight other houses were warned that access to their homes would be cut off if the water spills over the embankment.

The weather in the region began to deteriorate before dawn. The Weather Service issued more than a dozen tornado warnings and watches that covered towns from Texas to Mississippi. Early on Wednesday, a tornado struck Katy, a city west of Houston, Jeffry Evans, a meteorologist at the National Weather Service in Houston/Galveston, told reporters.

(A tornado warning is an urgent alert issued after a weather forecaster spots a possible tornado on a radar or a trained spotter sees a tornado. A watch means weather conditions are favorable for one to form in an area.)

Flooding began to inundate areas in East Texas early on Wednesday. Schools were closed and roads were shut down on Wednesday through the city of Kirbyville in Jasper County, Texas. The sheriff’s office said its deputies, local fire departments and emergency authorities were deployed for rescue efforts in the area around the city of more than 2,000.

In Kirbyville, Mayor Frank George sat in his truck in the central part of the city, where water had risen to several feet deep in some places. Across the street from him was the flooded Central Baptist Church. Water skimmed the roof of a parked Lincoln Continental vehicle, which was almost completely submerged.

Mr. George said that since before daylight, volunteers and a swift water team were circulating in boats, pulling stranded people out of homes and bringing them to a shelter. There were no fatalities or reports of injuries, he said.

Many more needed to be rescued, he said, and another shelter was expected to be opened on Wednesday. “We are getting calls as the water rises and we are expecting the water to rise,” Mr. George said in an interview. We are anticipating more calls.”

The cluster of storms was expected to move east through the evening, with the potential to cause more tornadoes and widespread, damaging winds in areas across southern Alabama, the Florida Panhandle and southern Georgia.

Derrick Bryson Taylor and Aimee Ortiz contributed reporting.