How hot can September get?

 

A fall-like day in suburban Washington, D.C. (Photo Credit: Susan Granzow)

Temperatures in the nation’s capital have reached the 90s only once in September dating back to last year. That’s unusual since the nation’s capital has averaged between three and four September days with highs in the 90s over the last 30 years.

By comparison, five of DC’s 10 warmest September’s have occurred over just the last 15 years. During that time, there have been two September’s with at least nine days high temperatures reached the 90s (2010 and 2019). September 2023 even had a six-day heat wave in its first week, but didn’t rank among DC’s warmest September’s. 

NOAA’s Climate Prediction Center expects warmer than average temperatures to dominate much of the country, including the D.C. Metropolitan Area, for the remainder of this month. While there’s an outside chance the nation’s capital may log a second day of 90-degree heat, it gets less likely as the month wears on.  

Average daily temperatures fall as September wears on due to the diminishing sun angle. When the decreasing sun angle is combined with the falling amount of daily sunlight, it’s more difficult to get very warm as October draws closer. The average daily high temperature in the nation’s capital also falls to 75° by September 30.

There have been only six Octobers over the last 30 years that Washingtonians have felt temperatures in the 90s. The most recent occurrence was on October 2, 2019. Over the same 30-year time period, DC’s last 90-degree day of the year occurred in August nine times, most recently in 2024. 

Since the first half of September 2025 has been such a cool month (3.4° below average as of September 18), it wouldn’t be a surprise for it to finish cooler than average. That’s despite the warmer than average weather that’s expected by NOAA over much of the next two weeks.

DC’s Warmest September’s (Source: NOAA)

1. 78.2° (1881)
2. 77.1° (1980)
3. 76.5° (2019)
4. 76.0° (2016, 1930)
6. 75.5° (2018, 2010)
8. 75.4° (1998)
9. 75.0° (2005, 1970)