1 killed and almost 2 dozen injured in overnight storms in Mississippi, officials say

At least one person was killed and nearly two dozen injured in storms that swept through Mississippi on Sunday night, and more than 50 million across the Southeast face the threat of more severe weather, including tornadoes, on Monday.

A storm system spawned a reported tornado late Sunday in Mississippi, leaving injuries and structural damage around Bay Springs and Louin in Jasper County, preliminary reports from the National Weather Service indicate.

Now, the Mississippi Emergency Management Agency is assessing damage from overnight storms, the agency said, noting more than 49,000 customers have no power in the central part of the state. They are among nearly a half million from Texas and Oklahoma to Tennessee who remain in the dark amid record-breaking heat after several rounds of storms last week.

And the severe weather threat persists Monday, with a Level 2 of 5 slight risk of severe weather across parts of the Gulf Coast and the Southeast, including New Orleans and Baton Rouge in Louisiana, along with Jacksonville, Florida; Mobile, Alabama; and Savannah, Georgia. The main threats are damaging wind gusts, large hail and isolated tornadoes.

On Monday morning, a tornado was confirmed near Florida's Santa Rosa Beach, according to the weather service. The service issued a tornado warning until noon for all of southwestern Walton County.

The service urged all residents to "take cover now."

"Flying debris will be dangerous to those caught without shelter," wrote the service. "Mobile homes will be damaged or destroyed. Damage to roofs, windows, and vehicles will occur. Tree damage is likely."

A Level 1 of 5 marginal risk stretches from central Texas to southern Florida and north to western North Carolina, leaving cities like Atlanta, Charlotte, North Carolina; Austin, Texas; and Tampa, Orlando and Miami in Florida under the threat of large hail and damaging wind gusts.

A shelter "for all those displaced from the recent destruction of tornado activity" was set to open Monday morning, the Jasper County Sheriff's Department said in a Facebook post. South Central Regional Medical Center in Laurel received 20 storm victims from the Louin area, including one who died, spokesperson Becky Collins told CNN Monday. The other patients are stable, she added, and most have been discharged from the hospital.

That followed several days of severe weather across the region, including a tornado last Thursday that devastated the Texas Panhandle community of Perryton, where three people - including an 11-year-old boy - were killed and more than a hundred others injured.

On Monday, the threat of excessive rainfall moves eastward to the southeastern parts of the country, bringing the threat of thunderstorms and flooding over parts of the Southeast, southern mid-Atlantic and Southern Appalachians.

The threat Monday follows more than 70 storm reports across the Southeast on Sunday, according to the Storm Prediction Center, including up to eight tornado reports, mostly in central Mississippi. Hail two inches wide or larger was also reported Sunday in Kerr County, Texas, about 65 miles northwest of San Antonio.

Meanwhile, around 35 million people are under heat alerts from a blistering heat wave that has settled across much of Texas, Louisiana and southern New Mexico and Mississippi, according to the National Weather Service.

Many are facing the heat without air conditioning with 465,000 customers without power in the South Monday morning after severe weather in recent days - including 215,000 in Oklahoma and about 85,000 in Texas and another 83,000 in Louisiana, according to PowerOutage.us.

The National Weather Service is advising residents to stay inside during the hottest part of the day, drink plenty of water and not leave children or pets in vehicles.

"In case we haven't said it enough," the weather service in Midland, Texas, tweeted, it's going to be "HOT. Try to spend as little time as possible outdoors, but if you must be outside, take frequent breaks in the AC, drink plenty of water & spend as much time as possible in the shade."

As the heat wave lingers, over 40 daily records could be tied or broken across Texas this week. The worst of the heat is expected from Monday through Wednesday.

The combination of temperature and humidity - or the heat index - could climb to 113 to 122 degrees in cities including Houston, San Antonio, Brownsville and Dallas.

Several daily heat records were already broken on Sunday. Del Rio, Texas, recorded a temperature of 111 degrees Sunday, breaking the previous daily record of 106 degrees set in 2011. Camp Mabry, in Austin, Texas, tied its record of 106 degrees set a dozen years ago and McAllen, Texas, reported a record-breaking 105 degrees.

"Temperatures in the 100s will not only rival daily high temperature marks for the nation but may tie or break existing records," the weather service said. "There will be little relief (with) overnight with lows in the upper 70s and 80s."

Cities across the South - some still cleaning up from last week's storms - are preparing for hot weather by opening cooling centers.

Houston will have cooling centers open from 3 p.m. to 7 p.m. Monday as the city braces for high temperatures. Caddo Parish in Louisiana has opened additional cooling centers as it grapples with power outages and storm cleanup.

New Orleans' emergency preparedness campaign is working with the New Orleans Fire Department to set up hydration stations to provide water and sunscreen on Sunday and Monday.

CNN's Jamiel Lynch, Mitchell McCluskey, Devon Sayers, and Zoe Sottile contributed to this report.