More than 55 million people across the Ohio, Tennessee, and Mississippi valleys are under a threat for severe storms on Sunday, and nearly as many across the south from Arizona to Louisiana are under "oppressive" heat.
There is a Level 3 of 5 risk for severe storms for portions of Arkansas, Mississippi, Alabama, Tennessee, Kentucky, Ohio, and Indiana, including the cities of Indianapolis, Columbus, Cincinnati, Nashville and Memphis.
A Level 2 of 5 risk surrounds the Level 3 risk, including Birmingham, Alabama and Jackson, Mississippi. Other notable cities under a marginal threat include Detroit, Cleveland, Atlanta, and Pittsburgh.
"The hazards associated with these thunderstorms are frequent lightning, severe thunderstorm wind gusts, hail, and a few tornadoes," according to the Weather Prediction Center, also noting "there is a threat of EF2 to EF5 tornadoes over the Ohio Valley."
In addition, more than 50 million people are under heat alerts Sunday, according to the National Weather Service, including much of Texas as well as parts of Arizona, New Mexico, Oklahoma, Arkansas, Louisiana, Mississippi, and Tennessee.
Dangerously high temperatures are forecast to expand into the central Plains and Middle and Lower Mississippi Valley by the middle of the week.
"The heat will be oppressive with widespread high temperatures exceeding 100 degrees and heat indices as high as 120 degrees," according to the Weather Prediction Center, noting "heat indices may exceed 120 degrees in some locations in South Texas."
The persistent heat will become "increasingly dangerous and potentially deadly" to those in south and south-central Texas, the weather service warned, as "many locations in those parts of Texas have already experienced a yearly record number of hours of dangerously high heat index readings."
This "significant" heat wave is forecast to continue through the beginning of the July 4th holiday week and potentially break several high temperature records across the region.
Heat leads to at least two deaths in Texas
The extreme heat is particularly affecting Texas and contributed to at two deaths Friday at the remote Big Bend National Park.
A father and his two stepsons were hiking the Marufo Vega Trail in extreme heat Friday -- with temperatures of 119 degrees -- when the 14-year-old stepson fell ill and lost consciousness, the National Park Service said in a news release.
The father, 31, hiked to their vehicle to find help, while the other stepson, 21, tried to carry his brother to the trailhead, the service said.
Emergency responders later reached the 14-year-old dead along the trial, the service said. The father's vehicle was found crashed over an embankment, and he was pronounced dead at the scene, the service said. No further details were provided about the 21-year-old.
The heat has also raised concerns about the capacity of the state's power authority, the Electric Reliability Council of Texas, known as ERCOT.
ERCOT said a "Weather Watch" will run through June 30, according to a Facebook post.
"Grid conditions are expected to be normal. Check our social media for daily updates and learn more about #TXANS," the post said.
ERCOT issues a weather watch about 3-5 days ahead of "significant weather and high demand," the company's website said. TXANS, otherwise known as the Texas Advisory and Notification System, is ERCOT's early notification system, designed to put out alerts "ahead of periods of higher demand," the power authority's website said.
In Austin, the state capital, multiple Parks and Recreation centers serving as cooling centers are extending their normal operating hours to 8 p.m. CT, the city's website said. All public libraries, except for three, are also extending their hours until 8 p.m., the city added.
The Travis County Community Centers are also serving as cooling centers on Sunday during normal business hours, the city said. Those without transportation to one of the cooling centers can catch a free ride on a city bus, Austin's Emergency Medical Service public information officer told CNN in an email Sunday.
The city's Community Health Paramedic program, "has been visiting and checking in with ... unhoused neighbors, especially those who are particularly isolated," and "handing out cases of water for people and pets alike," the spokesperson told CNN.
"Every Pop-Up Resource Clinic our CHPs are holding this summer will be indoors, with access to air conditioning, cold water, restrooms and a provided lunch," the spokesperson added.
Austin is under a heat advisory until 9 p.m. CT Sunday according to a weather outlook from the National Weather Service, with "heat index values up to 112."