More than 41 million people in the United States — about 12 percent of the population — were under a National Weather Service extreme heat advisory, extreme heat watch or extreme heat warning on September 15, 2026, days before the autumnal equinox. NASA’s modelled air temperature map of the contiguous U.S. shows the high temperatures spanning large portions of Texas, Oklahoma, Arkansas, Missouri and Tennessee, with the darkest reds marking areas where readings approached or exceeded 40 degrees Celsius (104 degrees Fahrenheit).
The warmth arrived at the tail of a summer that had already broken the national record, and it did not let go quietly. Meteorologists warned that high humidity, limited cloud cover and light winds could push the apparent temperature above thermometer readings and raise the risk of heat-related illness across the affected states. NASA’s Earth Observatory published the map as its Image of the Day for September 17, 2026, under the flat summary that temperature records toppled across the central and southern U.S. as the autumnal equinox approached.
September 15, 2026 Heat Dome Impacts 41 Million Americans
Dallas, Texas recorded 38 degrees Celsius (101 degrees Fahrenheit) on September 15, 2026, establishing a new daily high temperature record amid an expansive heat dome affecting the south-central United States. Dallas was one of three cities that ran at least 12 degrees Fahrenheit above normal that day, a measure of the unusual warmth gripping the region as astronomical summer concluded.
Memphis and Nashville, Tennessee also experienced record-breaking heat, reaching 37 and 38 degrees Celsius respectively, with Nashville additionally setting a record-high minimum temperature of 24 degrees Celsius the previous day. Nashville’s daily high broke a record established in 1927, highlighting the extremity of the event and the longevity of the previous benchmark.
The daily records sat on top of a seasonal one. NOAA’s National Centers for Environmental Information reported on September 9 that June through August 2026 was the warmest such stretch in the contiguous United States in a 132-year record, beating the previous marks set in 1936 and 2021 by 0.4 degrees Fahrenheit. The mid-September heat dome arrived less than a week after that announcement, extending a warm season that had already closed the books as the hottest on record. AccuWeather had forecast on September 11 that the heat dome would linger across the south-central and south-eastern United States through mid-September, and it did.
GEOS Model Maps Record Temperatures Across South-Central U.S.
The map is not a photograph of heat. It shows air temperatures across the contiguous United States on September 15, 2026, at 4 p.m. Central Time — 21:00 Universal Time — modelled at 2 metres (6.5 feet) above the ground, the height at which a weather station thermometer sits. It was produced by combining satellite observations with temperatures predicted by a version of the Goddard Earth Observing System model, which uses mathematical equations to represent physical processes in the atmosphere.
That distinction matters for reading the picture. A satellite radiometer senses radiation from the top of the atmosphere and from the land surface, not the air temperature a person standing in Dallas would feel, and ground stations report only where they happen to be sited. Blending the two against a physical model produces a continuous field instead of a scatter of station dots, which is what makes the shape of the dome — its edges and its centre over the south-central states — legible at a glance. It also means the map is a best estimate rather than a direct measurement, and NASA labels it as modelled for that reason.
Heat Dome Formation Suppresses Convection and Traps Warm Air
The sustained heat involves a suppression of typical atmospheric convection. A heat dome develops when high-pressure systems in the upper atmosphere force air downwards, compressing it and trapping heat near the ground. This process inhibits the rising air currents necessary for cloud formation and precipitation, allowing solar radiation to reach the surface with minimal obstruction and further increasing temperatures.
Picture a lid pressed down over a pan. The air beneath cannot rise, so it cannot form the clouds that would otherwise shade the ground or the showers that would otherwise cool it, and each day of sunshine adds to what the previous day left behind. That single structure is why the Dallas, Memphis and Nashville records fell on the same date rather than arriving piecemeal across the week. NASA’s account does not extend the explanation to Nashville’s record-high overnight minimum, and this article does not either. The Earth Observatory story was written by Lindsey Doermann; the map is a NASA Earth Observatory image by Michala Garrison, using GEOS-FP data from the Global Modeling and Assimilation Office at NASA GSFC.