The first half of September was the hottest on record for dozens of US cities

Nashville hit 100 degrees on Sept. 15, breaking a daily record that had stood since 1927, while Dallas reached 101 and Memphis 99 the same afternoon as a heat dome parked itself over the south-central United States. By the time the pattern broke, the first half of September had turned into the hottest on record for dozens of cities stretching from Colorado and New Mexico to Virginia and North Carolina, with many locations running 6 to 12 degrees above normal for the date.

More than 41 million people — about 12% of the country’s population — were under heat advisories or warnings on Sept. 15 alone, according to NASA’s Earth Observatory, which tracked the event using satellite-fed atmospheric modeling. The heat dome capped a summer that federal climate records already ranked as the warmest June-through-August stretch in 132 years of U.S. recordkeeping, meaning the Plains and Southeast moved from one record-setting stretch directly into another without much of a break.

A heat dome parked over the Southern Plains

The mechanism behind the spike was a heat dome: a zone of high pressure aloft that pushes hot air downward and traps it near the surface, suppressing cloud formation and letting the ground absorb more direct solar heating day after day. NASA’s imagery from its GEOS atmospheric model showed the darkest, hottest readings — areas approaching or exceeding 104 degrees — concentrated over Texas, Oklahoma, Arkansas, Missouri, and Tennessee on Sept. 15, with the dome’s influence stretching into the Mississippi Valley and Southeast through the following days.

Oklahoma City, Little Rock, and Nashville were forecast to flirt with triple digits as the pattern held, while Dallas, Austin, and San Antonio sat in the 90s alongside Indianapolis, Cincinnati, and Louisville further north, a spread wide enough that the same air mass was setting records on both sides of the Mississippi River at once.

Records that had stood since the 1920s fell in an afternoon

Nashville’s 100-degree reading did not just beat an ordinary daily average; it beat a specific mark set in 1927, one of the older entries in the city’s official climate record. That kind of gap between a modern heat event and a Jazz Age-era baseline is what pushed forecasters to describe the stretch in historic terms rather than routine late-summer warmth.

“One of the hottest and driest periods,” is how Sarah Barnes, a National Weather Service meteorologist, characterized the run of days, a description that captured both halves of the problem: the heat itself, and the parched ground underneath it that let temperatures climb even faster once the high pressure settled in. Little rainfall accompanied the pattern across the Southern Plains, removing one of the few natural checks — evaporative cooling from wet soil — that might otherwise have taken the edge off the highs.

Drought expanded under the same high-pressure dome

Oklahoma moved from partial drought coverage to full statewide drought status during the same stretch the heat records fell, a pattern climatologists describe as flash drought, where abnormally hot, dry conditions dry out soil far faster than a slow-building multi-year drought would. The U.S. Drought Monitor’s Oklahoma page showed the state’s most severe category, exceptional drought, spreading across a meaningful share of the state by late September, while Texas’s drought map showed a similar expansion across its northern and central counties over the same two weeks.

That feedback loop — heat drying soil, dry soil amplifying heat — is a documented driver behind why single-digit-above-normal anomalies in early September widened into double-digit departures by mid-month in several of the affected cities. The National Weather Service’s Dallas-Fort Worth office maintains the daily and monthly climate summaries meteorologists used to confirm just how far outside normal range the month’s readings fell once the full two weeks were tallied.

The pattern broke before the month was out

Relief arrived in stages rather than all at once. Forecasters expected St. Louis to fall to around 68 degrees by the following Tuesday, Atlanta to drop from the 90s into the low 80s by midweek, and the Deep South to settle into the upper 80s and low 90s by week’s end as a broader pattern shift pushed cooler air into the region. Kansas, Missouri, and Illinois were forecast to see highs retreat into the 70s and even upper 60s once the shift took hold.

None of that late-month cooldown erased the record already set. The first-half-of-September numbers were locked in as soon as the two-week period closed, regardless of how quickly the following days cooled off, which is why forecasters kept describing the stretch as a completed climate record rather than an ongoing emergency by the time cooler air finally reached the region.

The two-week stretch also left a mark beyond daily temperature logs. Utilities across the affected states reported higher-than-typical September electricity demand as air conditioners ran on days that would ordinarily have called for little more than an open window, and several city health departments extended cooling-center hours past their usual Labor Day cutoff to cover the unexpected extension of summer conditions into a month state climate offices normally file under early fall.

Cities that logged the anomaly now carry it forward in their own official records, the same way Nashville’s 1927 mark survived nearly a century before this September replaced it. Whichever cities eventually break the new 2026 highs will be measured against a benchmark set during a two-week run that started as a routine Plains heat wave and ended up rewriting climate books from Colorado to North Carolina.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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