The U.S. Just Had Its Warmest September on Record — And the Nights Were Even More Extreme
September 2026 was the warmest September on record across the contiguous United States, according to preliminary climate data, with temperatures averaging about 3.5 degrees Fahrenheit above normal nationwide. The month surpassed the previous September record set in 1998 by nearly half a degree.
But some of the most unusual heat happened after the sun went down. Overnight temperatures across the country averaged about 4.6 degrees above normal, breaking the previous September record for warm nights by roughly 1.5 degrees. Daytime high temperatures were also warmer than normal but ranked only ninth highest for September.
High humidity helped keep temperatures elevated overnight, particularly across parts of the Southwest, Plains and Midwest. Moist air can make it harder for temperatures to fall after sunset, leaving people with less relief from daytime heat. Several areas experienced unusually warm nights during the month.
Chicago offers one example. The city’s average low temperature during September was 63 degrees — 5.9 degrees above normal. Chicago also tied or broke records for its warmest daily low temperature on three consecutive days early in the month, according to the National Weather Service.
The nationwide figures come from the PRISM Climate Group at Oregon State University, whose monthly records extend back to 1895. Because September just ended, the latest data are preliminary and will undergo additional updates and quality checks before becoming final.
