An oppressive heat wave blamed for at least 13 deaths in Texas and one in Louisiana blanketed the South on Thursday, as authorities warned of dangerous, triple-digit temperatures.

Forecasters said temperatures could rocket up to 20 degrees above average in parts of the region as a heat dome that has taxed the Texas power grid spread eastward. Officials urged people to stay hydrated, shelter in air-conditioned rooms and check on each other as humidity combined with high temperatures to make conditions even more dangerous.

The National Weather Service issued an excessive heat warning for parts of Arkansas, Louisiana, Mississippi and Tennessee for Thursday and Friday. The heat index, which indicates how hot it feels outdoors based on the temperature and relative humidity, was expected to reach 115 degrees (46 Celsius) in several cities. The heat dome was expected to be center over the mid-South by the weekend.

The heat added to weather-related woes for some Tennessee residents cities who still had no power after strong storms Sunday knocked down trees and power lines.

To get some relief, John Manger, 74, and his wife were sitting in shady spots outside their sweltering home in the Memphis suburb of Bartlett and taking cold showers.

"I just suck it up, with a washcloth, towel, whatever. I just sit in my chair by the window, and maybe get a breeze," said Manger, who is retired.

Their house was among more than 20,000 homes and business in Shelby County that were without electricity as of Thursday morning. Local utility Memphis Light, Gas and Water said dozens of crews were working to restore power.

Louisiana already has been plagued by hot weather over the past month. Between May 12 and May 24, more than 680 went to the hospital for heat-related illness, based on the most recent figures from the state Department of Health. These illnesses can range in severity from mild, such as heat rash and heat cramps, to severe, such as heat exhaustion and heatstroke.

A 49-year-old man died Sunday in Shreveport after he was found lying on a sidewalk.

"This is very real and we need people, to not only take care of themselves, but also to look after their neighbors — especially those who are older," Gov. John Bel Edwards said Wednesday afternoon.

In Florida, National Weather Service offices forecast heat index levels of up to 112 degrees (44 Celsius) over the next several days. Forecasters in Jacksonville said the extreme heat would persist through the Independence Day holiday.

Temperatures in St. Louis were expected to reach around 100 degrees Fahrenheit (38 degrees Celsius) on Thursday and Friday, and 95 degrees Fahrenheit (35 degrees Celsius) on Saturday before a slight cool-down Sunday. Smoke from Canadian wildfires has combined with the heat and humidity to worsen air quality.

Gentry Trotter, who runs Cooldownstlouis.org, said volunteers were taking donated window air conditioners to the elderly and needy.

Trotter recently went into the home of an 83-year-old woman, measured the indoor temperature and found it was 105 degrees Fahrenheit (40.6 degrees Celsius). Still, she refused to accept an air conditioner.

"Somebody needs to convince her that if she doesn't have a blasting air conditioner, she's going to die," Trotter said.

Eleven of the Texas heat-related deaths occurred in Webb County, which includes Laredo. The dead ranged in age from 60 to 80 years old, and many had other health conditions, according to the county medical examiner, Dr. Corinne Stern. Also, two Florida residents died while hiking in extreme heat at Big Bend National Park.

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Sainz reported from Memphis, Tennessee; Goldberg from Jackson, Mississippi; Salter from St. Louis, Missouri. Associated Press reporter Curt Anderson contributed from Miami, Florida; Sara Cline from Baton Rouge, Louisiana.