Astronomical fall begins on Tuesday, Sept. 22, but for much of the south-central United States, September has felt like midsummer. A persistent heat dome made the first half of the month the hottest on record for dozens of cities, including Austin, Dallas, Oklahoma City, Memphis, Nashville and Louisville, with many running 6 to 12 degrees above normal, according to a Weather.com analysis of long-term station records through Sept. 16.

Relief is arriving unevenly. Cooler air is spreading into parts of Kansas, Missouri, Illinois and the Tennessee Valley this week, but the southern Plains have the best odds of staying warmer than average through the end of September, according to NOAA's Climate Prediction Center outlook cited in the same forecast.

For households, that split matters. Residents of Memphis, Nashville and the Ozarks can expect a real break from heat stress and cooling costs, while families in Texas and Oklahoma should keep hot-weather precautions in place for at least another week.

A Heat Dome at the Edge of Summer

NASA's Earth Observatory captured the event in a modeled temperature map for 4 p.m. Central Time on Sept. 15. The NASA heat dome map, built by combining satellite observations with the agency's GEOS weather model, showed high temperatures spanning large parts of Texas, Oklahoma, Arkansas, Missouri and Tennessee, with the hottest areas approaching or exceeding 104 degrees Fahrenheit (40 degrees Celsius). More than 41 million people, about 12 percent of the U.S. population, were under an NWS extreme heat advisory, watch or warning that day.

A heat dome forms when high pressure in the upper atmosphere pushes hot air toward the surface and traps it there. That sinking air suppresses clouds and rain, letting sunshine heat the ground further. The pattern is common in summer. What stood out this time was its timing, arriving when normal temperatures are beginning to fall.

The heat followed the warmest June through August on record for the contiguous United States in NOAA's 132-year dataset. NASA noted the three-month period was 0.4 degrees Fahrenheit warmer than the previous records set in 1936 and 2021.

City Records from Dallas to Nashville

North Texas logged one of the clearest records. On Sunday, Sept. 13, Dallas-Fort Worth reached 103 degrees, its 12th triple-digit day of the month, the most ever recorded in September, KERA News reported. "It's definitely one of the hottest and driest periods," National Weather Service meteorologist Sarah Barnes told the station. The area had also logged 49 days of 100 degrees or more this year, the sixth-most on record so far, and had gone 60 days without measurable rain.

Daily records kept falling as the heat dome settled in. On Sept. 15, Dallas hit 101 degrees, Memphis 99 and Nashville 100, all at least 12 degrees above normal, according to NASA. Nashville's high broke a daily record dating to 1927, a day after the city set a record-warm low of 75 degrees. Warm nights matter for health, because bodies cannot fully recover when overnight temperatures stay high.

The heat also reached the Ohio Valley. In Evansville, Indiana, Sunday, Sept. 20, marked the seventh straight day of highs in the mid to upper 90s, according to local station 14 News.

Whether climate change influenced this specific heat dome has not been formally assessed. A single event is not proof of a trend, though NOAA's record summer places it within a broader pattern of warming.

Where the Cooldown Reaches and Where It Stalls

The forecast breaks down by region. Parts of Kansas, Missouri and Illinois are expected to see highs in the 70s, and even some upper 60s, for several days this week. Northwestern Oklahoma, northwestern Texas and northern Arkansas were forecast to dip into the 80s starting Monday, with relief reaching the Tennessee Valley by midweek, according to Weather.com.

In Memphis, a cold front Monday evening was expected to knock temperatures down about 10 degrees into Tuesday, with highs settling in the 80s from Wednesday through Friday, according to Action News 5 in Memphis. Evansville was forecast to fall into the upper 70s as fall began.

The Deep South gets only a partial break. Highs from Texas to the lower Mississippi Valley may ease only to the upper 80s or low 90s later this week, and the Climate Prediction Center outlook favors continued above-average temperatures for the southern Plains from this weekend through the end of the month. Monthly outlooks are probabilities, not guarantees, and individual days could still run cooler or hotter.

Drought is part of the story. As of Sept. 15, Drought.gov reported that flash drought was worsening across the southern Plains and the lower Mississippi River Basin and was spreading into parts of the Southeast. Dry ground heats up more easily than moist soil, because less of the sun's energy goes into evaporating water, which can help sustain high temperatures.

Staying Safe as the Heat Lingers

Those at greatest risk include older adults, young children, people with heart or lung conditions, outdoor workers, athletes practicing in the afternoon, and households without working air conditioning. Late-season heat can catch people off guard because they assume the worst is over, and school sports seasons are underway.

Watch for warning signs of heat illness, including heavy sweating, dizziness, nausea, headache and muscle cramps. Confusion, a very high body temperature, or loss of consciousness can signal heat stroke, which is a medical emergency requiring a 911 call. Check on older neighbors and relatives, and never leave children or pets in parked vehicles.

Cost is a real barrier for some families. Many utilities and state programs offer bill assistance, and some cities keep cooling centers open during advisories. Residents should check with their county or city for current cooling center hours rather than assuming summer schedules still apply.

Texans and Oklahomans should keep watching NWS heat products through the end of the month, while Tennessee Valley residents can expect a more fall-like pattern by midweek. The Climate Prediction Center's updated October outlook, issued at the end of the month, will show whether warmth is favored to continue into mid-autumn.

What Readers Want to Know

What is a heat dome?

It is a strong area of high pressure in the upper atmosphere that pushes hot air toward the ground and traps it there. It suppresses clouds and rain, allowing temperatures to climb day after day.

Which cities set September heat records?

Weather.com reported record-warm first halves of September in Austin, Dallas, Oklahoma City, San Antonio, Little Rock, Memphis, Nashville, Indianapolis, Cincinnati and Louisville, among others.

How hot did it get?

NASA's modeled map for Sept. 15 showed temperatures approaching or exceeding 104°F in the hottest parts of the south-central U.S., and more than 41 million people were under heat alerts that day. Dallas-Fort Worth recorded 12 triple-digit days in September by Sept. 13, a monthly record.

When will it cool down?

Parts of Kansas, Missouri and Illinois cool this week, with relief reaching the Tennessee Valley by midweek. The southern Plains are favored to stay warmer than average through the end of September.

Was this heat caused by climate change?

No formal attribution study has been released for this event. It followed the warmest U.S. summer in NOAA's 132-year record.

What are the warning signs of heat stroke?

Confusion, a very high body temperature, hot skin, and loss of consciousness. Call 911 immediately and move the person to a cooler place.