Communities from Houston to New Orleans opened cooling centres to bring relief as steamy hot temperatures settled across a broad swath of the US South on Saturday (Sunday NZ), and beachgoers fled a waterspout that swept ashore on a Florida beach.

Governor Greg Abbott visited Perryton in the Texas Panhandle, where officials said more than 1000 customers were left without electricity after a tornado killed three people.

The Perryton Ochiltree Chamber of Commerce said it would open a cooling centre in the town of 8000 people, about 185km northeast of Amarillo, to counteract the effects of the high temperatures that followed the storm.

“At times of events like these, Texans come together,” Abbott told reporters as he signed a disaster declaration that he said would “trigger all the resources the state can bring to bear … to accelerate the ability to rebuild”.

The Republican governor said he was shocked to see how much of the town had been destroyed and praised what he called “non-stop heroic efforts by healthcare providers” who he said treated 160 injured people at the local hospital that has just 25 beds.

Texas' Republican Senator Ted Cruz holds up a small cowboy boot he recovered from debris in a trailer park that was damaged by a tornado. Photo / AP

W Nim Kidd, chief of the Texas Division of Emergency Management, warned more severe weather was forecast for the area,bringing rain, high winds and possibly more tornadoes.

The National Weather Service issued excessive heat warnings along the Gulf Coast from Brownsville, Texas, to Houston. It said heat indexes ranging from near 46C in Houston to near 49C at Brownsville and Corpus Christi in Texas. Cooling shelters were set up in cities along the coast and farther inland for residents left without electricity.

“What’s really going is the humidity,” said Allison Prater, a National Weather Service meteorologist in Fort Worth, Texas. “That’s making the heat index, or the ‘feels like’ temperature really skyrocket.”

Prater said the air temperature Saturday in the Dallas area could reach 34C, but high humidity would make it feel like 40.5C.

“The reason we’re having such heat is there is a lot of moisture being pulled up from the Gulf of Mexico,” Prater said. “That’s working with the warmer temperatures to induce … that ‘feels like’ temperature.”

Two women and an 11-year-old boy died when the tornado slammed into Perryton. On Saturday (Sunday NZ), authorities upgraded the intensity of the twister to EF-3, saying it packed winds of up to 225km/h.

Ochiltree County Sheriff Terry Bouchard told KVII-TV in Amarillo that missing people had been located.

Kari Kile wears a neck fan as she holds an umbrella over her daughter, Taylor, as they wait outside in the heat. Photo / AP

“It dropped down right on top of Perryton,” Bouchard said. “We’ve lost a lot of homes, businesses, rental properties. There’s just a lot of damage to our community and it’s going to take some time to get this cleaned up.”

Storm-related power outages were also reported in East Texas ahead of a weekend promising sweltering days of high temperatures and high humidity.

In Louisiana, the National Weather Service projected daytime temperatures through to Monday (Tuesday NZ) of about 34C with high humidity and heat index values as high as 44C.

The City of New Orleans opened cooling centres and hydration stations and advised residents to take extra precautions if they were spending time outside by wearing lightweight and loose-fitting clothing, taking frequent rest breaks in shaded or air-conditioned environments, and drinking lots of water.

Entergy New Orleans and the Sewerage & Water Board of New Orleans suspended electricity shut-offs for delinquent accounts.

In Florida, city officials in Clearwater said in an email that a waterspout came ashore Friday afternoon “sending beach-related items flying into the air” and injuring two people from Kansas.

Authorities said the 70-year-old woman and 63-year-old man were treated for minor injuries at a local hospital. Their identities were not made public.

Waterspouts develop over water, usually during severe thunderstorms or tornadoes and dissipate rapidly when they make landfall, according to the National Oceanic and Atmospheric Administration. Some can cause significant damage and injuries.

The National Weather Service in Miami issued a heat advisory for most of the South Florida area, where the combination of heat and humidity was forecast to reach a “feels like” temperature of 40.5C.

“Hot temperatures and high humidity may cause heat illnesses to occur,” the service reported. The air temperature in Miami and Fort Lauderdale, Florida, was expected to reach about 33C on Saturday (Sunday NZ).