September Heat Dome: How the South Got a 104-Degree Wake-Up Call

A September heat dome parked over the south-central United States in mid-September, pushing temperatures past 100 degrees in cities that normally start cooling off by then. On September 15, more than 41 million people, roughly 12 percent of the country's population, were under National Weather Service extreme heat advisories, watches or warnings stretching from Texas to Tennessee.

The timing is what made the event stand out. Astronomical summer was days from ending, and the region was supposed to be sliding toward milder weather. Instead, a dome of high pressure in the upper atmosphere pushed warm air toward the surface and held it there, suppressing the clouds and storms that would normally break the cycle. Meteorologists warned that high humidity, limited cloud cover and light winds could make it feel even hotter than the thermometer readings, raising the risk of heat-related illness.

Records fell from Dallas to Nashville

Several cities set daily high-temperature records on September 15. Dallas reached 101 degrees, Memphis climbed to 99, and Nashville hit 100. All three cities ran at least 12 degrees warmer than normal for the date, according to NASA Earth Observatory's Image of the Day coverage of the event.

Nashville's mark carried extra history. The 100-degree reading broke a daily record that had stood since 1927, a span of 99 years. The day before, the city set another record for the warmest low temperature on that date, when the overnight low fell no further than 75 degrees. Warm nights matter during heat waves because the body normally uses cooler overnight hours to recover, and that recovery window shrank or vanished for people without air conditioning.

Satellite data captured the scale of it. NASA Earth Observatory's map of conditions at 4 p.m. Central Time on September 15 shows air temperatures across the contiguous United States at about six and a half feet above the ground, built from satellite observations combined with NASA's GEOS atmospheric model. The darkest red areas mark temperatures approaching or exceeding 104 degrees.

A record summer behind it

The September heat dome did not arrive out of nowhere. It followed the warmest meteorological summer ever recorded for the contiguous United States. In a September 9 report, the National Oceanic and Atmospheric Administration said the June-through-August average reached 74.4 degrees, three degrees above the 20th-century average and enough to top the previous records set in 1936 and 2021. August alone was the hottest August in records dating back to 1895, with average daytime highs of 88.6 degrees.

In Nashville, the heat never really let go. The Tennessean reported that the city set four daily high-temperature records between July and September, with the season's hottest reading, 101 degrees, landing on July 3. After the mid-September dome arrived, the city tied or broke records again on September 15, 18 and 19, each time reaching 100 degrees. Starting August 27, Nashville logged at least 25 consecutive days with temperatures above the normal high for the date.

What a heat dome actually does

The weather pattern behind the event works like a lid. A persistent area of high pressure pushes warm air downward and keeps it pinned near the ground. It also suppresses convection, the upward movement of warm air that helps clouds and thunderstorms form. With fewer clouds and less rain, strong sunlight reaches the ground for longer stretches, the surface heats up, and the air just above it warms in turn. The same pattern that creates the heat also preserves it, which is why domes can linger for days.

Humidity raises the stakes. People cool themselves mainly by sweating, and sweat sheds heat best when it evaporates. When the air is already loaded with moisture, evaporation slows and the body has a harder time getting rid of excess heat. That combination of heat and humidity is what turns a hot day into a dangerous one, especially for older adults, young children, outdoor workers and people with certain health conditions.

Relief arrived with the equinox

The dome finally showed signs of weakening as September wore on. The Tennessean reported that September 21 was expected to be the last day of well-above-normal temperatures in the Nashville area, with a cooler pattern arriving September 22 and bringing more seasonable weather plus chances for showers and thunderstorms through September 24. Forecast highs from September 22 through 27 were expected to settle in the mid-80s, with overnight lows dropping into the low to mid-60s, and temperatures projected to dip as low as 57 on the night of September 25.

Still, the episode closed a summer that rewrote the record books. Drought had covered 59.1 percent of the contiguous United States by September 1, according to the U.S. Drought Monitor cited in NOAA's monthly climate report, and the agency's September outlook favored continued above-average temperatures across much of the country. The calendar said summer was over. The atmosphere took a few extra weeks to agree.

Read the NASA Earth Observatory coverage via Earth.com, the Nashville follow-up in The Tennessean, and the NOAA summer-record breakdown in USA Today. More on this beat: the climate emergency section, and this week's UN declaration on sea level rise and island statehood.