DC’s hot start to September

 

A warm day in the nation's capital

The nation’s capital had a memorable start to September with highs in the 90s on the first four days of the month, which hasn’t happened since 2015. That’s not surprising since the meteorological summer (June, July and August) that just ended finished as DC’s eighth warmest on record. Nine of DC’s 10 warmest summers have now occurred in the last two decades. 

This summer’s most extreme heat occurred in early July when the nation’s capital experienced it’s warmest July 4 with a record high of 103°. Triple-digit heat also occurred on June 12. The nation’s capital has also experienced summer’s that rank among DC’s Top 10 warmest without any triple-digit heat such as in 2019, 2020 and 2021. 

What’s helped make recent summers hotter in the nation’s capital has been an increased urban heat island effect. Increased greenhouse gas emissions have also contributed to a changing climate that’s made recent summers hotter. According to a recent study by Climate Central, “…human-caused climate change was the leading cause of summer warming — accounting for at least 50% of observed warming since 1970.”

While triple-digit heat is unusual in the nation’s capital and often a sign of a very hot summer, that isn’t always the case. For example, the nation’s capital experienced a total of five days of triple-digit heat during the summer of 1997, but the summer finished nearly two degrees below average. That helps illustrate that a few hot days aren’t enough to produce a warmer than average season.

On the other hand, a very hot summer can sometimes linger into September. Longtime Washingtonians might remember the summer of 1980. The nation’s capital experienced triple-digit heat in July, August and September of 1980. DC’s record high of 101° on September 2, 1980, was one of only four days the nation’s capital has experienced triple-digit September heat and the most recent.

Not only was September 1980 DC’s second warmest on record, but was warmer than June 1980. DC’s warmest September in 1881 was the warmest month of the entire year. Although no triple-digit heat occurred or is likely to occur in the nation’s capital this September, NOAA’s Climate Prediction Center expects this month to be warmer than average. 

DC’s warmest September’s (NOAA)

1. 78.2° - 1881 (hottest month of year)
2. 77.1° - 1980 (warmer than June)
3. 76.5° - 2019 (warmer than June)
4. 76.0° - 2016, 1930
5. 75.5° - 2018, 2010
6. 75.4° - 1998 (warmer than June)
7. 75.0° - 2005, 1970
8. 74.9° - 2015
9. 74.8° - 1961 (warmer than June)
10. 74.4° - 1921 (warmer than June, August)

 

DC’s hottest summers (NOAA)

1. 81.3° - 2010
2. 81.1° - 2011 (triple-digit heat in all 3 months)
3. 80.9° - 2024 (all 3 months)
4. 80.5° - 2016
5. 80.4° - 2012
6. 80.2° - 2020 (no triple-digit heat)
7. 80.0° - 1980 (July, August, September)
8. 79.8° - 2026
9. 79.5° - 2021, 2019 (no triple-digit heat in either year)
10. 79.4° - 2002, 1991

Feeling more like summer

 

An early fall day at the National Cathedral

Although September is the first month of meteorological fall (the three-month period of September, October and November), this September has gotten off to a very summery start in the nation’s capital. The DMV (DC, Maryland and Virginia) will continue to experience July-like heat much of this week.

More typically, September features some of the best weather of the year in the nation’s capital. Average daily high/low temperatures drop from 85°/69° on September 1 to 75°/59° by the end of the month. The amount of daylight continues to decline throughout September. Sunset will occur at 6:54 p.m. on September 30. The combination of reduced daylight and a lower sun angle contributes to the cooler average temperatures as September wears on. 

Five of DC’s 10 warmest September’s have occurred since 2005. DC’s hottest September temperature on record is 104° on September 7, 1881. Triple-digit September heat is a rarity in the nation’s capital and has occurred only four times since 1871, with none since 1980. To add to that, yesterday’s high temperature of 98° was DC’s hottest September temperature since September 7, 2023 (also 98°). By comparison, DC’s coolest September temperature is 36° from September 23, 1904.

Although Washington, D.C. averages 3.93” of September rainfall, the amount of rain can fluctuate wildly year-by-year. September can be a very wet month if decaying tropical systems bring appreciable rainfall. Such was the case with Floyd in 1999, Isabel in 2003 and Florence in 2018. This month can also be very dry in the nation’s capital. The driest September on record was in 2005 (0.11”) and the fourth driest occurred in 2019 (0.25”).

Last September featured near average temperatures and rainfall in the nation’s capital. By comparison, NOAA’s Climate Prediction Center expects this September to finish with warmer than average temperatures and near average rainfall.

The end of summer?

 

A late August day at the Avenel Park, Potomac, MD

The second half of August 2026 has felt more like September in the nation’s capital. This month will finish warmer than average. It will also become DC’s third wetter than average August in the last four years.

Following such a hot and humid June and July; that included four days of triple-digit heat, the stretches of cooler than average weather this month were welcome by residents of the DMV (DC, Maryland and Virginia). DC’s warmest temperature this month was 94°. High temperatures today could reach the low 90s and that would give Washingtonians 12 days of 90-degree heat. That’s right at DC’s average of between 10 and 11 days of 90-degree August heat over the last 30 years, according to NOAA data .

There has been a lot of fluctuation in August temperatures over the last decade. For example, the nation’s capital saw only five days in the 90s last August which was DC’s coolest since 2000. Meanwhile, a total of 20 days of 90-degree heat occurred in August 2016 when DC experienced its second hottest August on record. This August will finish almost a full degree warmer than average, giving DC residents the first warmer than average August since 2022. 

Unlike last August that finished as DC’s driest on record with only 0.2” of rain, this month has been wetter than average with 4.27” as of August 30. That’s welcome news for the DMV that has been mired in drought conditions for much of the year.

Although drought conditions have improved for much of the DMV over the course of the month, widespread areas of “abnormally dry” conditions still exist in the immediate DC Area, along with other areas of “moderate drought” conditions. The near record-setting heat on tap with highs in the mid to upper 90s for much of this week will exacerbate these dry conditions.