September is one of the warmer months of the year in Washington, with average highs near 26.4°C and lows around 15.8°C (mean 21.1°C). It is a relatively wet time of year, averaging 104 mm of rainfall and 70% relative humidity across the period.
September is one of the warmer months of the year in Washington. Across 45 years of records, the warmest September in Washington was 2019 — when the month averaged 24.3°C — while the coldest was 2000 at 18.7°C. Typical September days reach 26.4°C with nights near 15.8°C. The long-term trend is warming, about 0.4°C per decade.
September is a relatively wet time of year in Washington, averaging 104 mm of rainfall. The wettest September on record was 1999 with 293 mm, while the driest was 2005 at 8 mm.
Relative humidity in Washington during September averages 70%. The most humid September was 2011 at 87%, and the least humid was 1983 at 58%.
September receives about 4.4 kWh/m²/day of solar energy in Washington. The sunniest September was 2007 at 5.3 kWh/m²/day, and the cloudiest were 2011 and 2018 at 3.4.
Average wind speed in Washington during September is 2.4 m/s. The windiest September was 1999 at 2.9 m/s, while the calmest was 2011 at 2 m/s.
Ranked against the other 11 months using 1981–2025 averages.
| Metric | 1980s | 1990s | 2000s | 2010s | 2020s | Change |
|---|---|---|---|---|---|---|
| Avg Temp | 20.7°C | 20.6°C | 20.9°C | 22.0°C | 21.6°C | +0.9°C |
| Avg High | 26.3°C | 26.0°C | 26.1°C | 27.2°C | 26.8°C | +0.5°C |
| Avg Low | 15.0°C | 15.2°C | 15.6°C | 16.8°C | 16.5°C | +1.5°C |
| Avg Rain | 88 mm | 113 mm | 106 mm | 111 mm | 99 mm | +12 mm |
September temperatures in Washington have risen by about 1.3°C between 1981–1985 and 2021–2025.
September has ranged from a record low of 5.5°C (1992) to a record high of 37.5°C (1983).
Rainfall in September is a relatively wet time of year, averaging 104 mm, with a wettest year of 293 mm.
Daytime highs and overnight lows have shifted differently — highs rose about 0.3°C per decade while lows rose 0.6°C per decade, narrowing the gap between day and night.
One of the best times to visit Washington
September is one of the best times to visit Washington. Expect days averaging 26.4°C and nights around 15.8°C (mean 21.1°C) — warmer than Washington's yearly average — with higher rainfall (104 mm), so conditions are generally comfortable for sightseeing and outdoor plans.
| Year | Avg High | Avg Temp | Avg Low | Hottest Day | Coldest Day | Rainfall | Humidity | Wind | Solar |
|---|---|---|---|---|---|---|---|---|---|
| 2025 | 27.9°C | 22.3°C | 16.7°C | 32.9°C | 12.7°C | 71 mm | 66% | 2.1 m/s | 4.2 |
| 2024 | 26.8°C | 21.9°C | 17.0°C | 30.4°C | 10.4°C | 77 mm | 72% | 2.4 m/s | 3.9 |
| 2023 | 27.6°C | 22.4°C | 17.2°C | 37.0°C | 12.0°C | 131 mm | 70% | 2.3 m/s | 4.4 |
| 2022 | 27.2°C | 21.7°C | 16.3°C | 32.9°C | 9.4°C | 86 mm | 67% | 2.4 m/s | 4.6 |
| 2021 | 26.7°C | 21.6°C | 16.5°C | 31.5°C | 10.8°C | 122 mm | 74% | 2.4 m/s | 4.7 |
| 2020 | 24.5°C | 19.8°C | 15.0°C | 30.6°C | 6.6°C | 107 mm | 79% | 2.1 m/s | 4.2 |
| 2019 🔥 | 30.3°C | 24.3°C | 18.2°C | 36.3°C | 11.4°C | 9 mm | 62% | 2.2 m/s | 4.7 |
| 2018 | 26.3°C | 22.6°C | 19.0°C | 32.4°C | 12.7°C | 217 mm | 86% | 2.4 m/s | 3.4 |
| 2017 | 26.5°C | 21.3°C | 16.1°C | 32.0°C | 10.3°C | 50 mm | 69% | 2.3 m/s | 4.7 |
| 2016 | 28.3°C | 23.2°C | 18.1°C | 35.5°C | 12.4°C | 120 mm | 68% | 2.6 m/s | 4 |
| 2015 | 28.3°C | 22.8°C | 17.2°C | 35.2°C | 11.1°C | 109 mm | 68% | 2.3 m/s | 4.4 |
| 2014 | 26.4°C | 21.2°C | 16.0°C | 32.5°C | 10.6°C | 56 mm | 70% | 2.2 m/s | 4.3 |
| 2013 | 26.5°C | 20.6°C | 14.7°C | 34.9°C | 7.5°C | 33 mm | 64% | 2.1 m/s | 5.1 |
| 2012 | 26.1°C | 20.9°C | 15.7°C | 32.0°C | 7.9°C | 86 mm | 71% | 2.2 m/s | 4.7 |
| 2011 | 24.4°C | 20.5°C | 16.6°C | 28.7°C | 7.9°C | 237 mm | 87% | 2 m/s | 3.4 |
| 2010 | 28.6°C | 22.7°C | 16.7°C | 35.4°C | 11.6°C | 191 mm | 63% | 2.6 m/s | 5 |
| 2009 | 24.4°C | 19.8°C | 15.3°C | 29.9°C | 10.4°C | 96 mm | 76% | 2.5 m/s | 4.2 |
| 2008 | 26.6°C | 21.5°C | 16.3°C | 34.5°C | 10.6°C | 153 mm | 72% | 2.5 m/s | 4.3 |
| 2007 | 28.8°C | 22.7°C | 16.5°C | 34.1°C | 10.5°C | 25 mm | 64% | 2.1 m/s | 5.3 |
| 2006 | 24.0°C | 19.3°C | 14.7°C | 28.9°C | 8.7°C | 151 mm | 77% | 2.5 m/s | 3.8 |
| 2005 | 29.7°C | 23.6°C | 17.5°C | 33.6°C | 9.4°C | 8 mm | 62% | 2.2 m/s | 5.1 |
| 2004 | 26.5°C | 21.6°C | 16.6°C | 29.9°C | 9.8°C | 131 mm | 77% | 2.4 m/s | 3.9 |
| 2003 | 24.1°C | 19.9°C | 15.7°C | 28.6°C | 6.9°C | 203 mm | 83% | 2.6 m/s | 4.2 |
| 2002 | 27.4°C | 21.9°C | 16.5°C | 31.4°C | 12.1°C | 97 mm | 69% | 2.5 m/s | 4.6 |
| 2001 | 25.5°C | 19.7°C | 13.9°C | 30.6°C | 7.1°C | 65 mm | 66% | 2.3 m/s | 4.8 |
| 2000 ❄️ | 24.1°C | 18.7°C | 13.3°C | 29.2°C | 6.5°C | 137 mm | 76% | 2.3 m/s | 4.3 |
| 1999 🌧️ | 24.6°C | 19.8°C | 14.9°C | 29.6°C | 6.9°C | 293 mm | 79% | 2.9 m/s | 4.3 |
| 1998 | 29.9°C | 23.6°C | 17.3°C | 35.2°C | 11.3°C | 47 mm | 59% | 2.4 m/s | 4.9 |
| 1997 | 25.9°C | 20.2°C | 14.6°C | 32.1°C | 8.7°C | 67 mm | 65% | 2.5 m/s | 4.5 |
| 1996 | 24.2°C | 19.7°C | 15.2°C | 29.7°C | 9.1°C | 168 mm | 80% | 2.7 m/s | 4 |
| 1995 | 26.3°C | 20.6°C | 14.9°C | 32.8°C | 9.1°C | 85 mm | 63% | 2.3 m/s | 4.2 |
| 1994 | 25.2°C | 19.7°C | 14.2°C | 31.7°C | 8.9°C | 81 mm | 69% | 2.4 m/s | 4.6 |
| 1993 | 26.4°C | 21.2°C | 16.1°C | 34.9°C | 7.1°C | 110 mm | 68% | 2.4 m/s | 3.9 |
| 1992 | 24.6°C | 19.7°C | 14.9°C | 30.3°C | 5.5°C | 140 mm | 75% | 2.5 m/s | 3.9 |
| 1991 | 26.4°C | 20.7°C | 15.1°C | 34.9°C | 5.9°C | 95 mm | 65% | 2.3 m/s | 4.5 |
| 1990 | 26.1°C | 20.3°C | 14.5°C | 34.4°C | 6.3°C | 46 mm | 67% | 2.4 m/s | 4.3 |
| 1989 | 25.3°C | 20.5°C | 15.8°C | 34.0°C | 6.6°C | 126 mm | 75% | 2.5 m/s | 3.7 |
| 1988 | 25.6°C | 19.8°C | 13.9°C | 31.3°C | 9.3°C | 63 mm | 68% | 2.2 m/s | 4.5 |
| 1987 | 25.6°C | 20.9°C | 16.2°C | 29.3°C | 10.4°C | 158 mm | 75% | 2.2 m/s | 4.2 |
| 1986 | 26.8°C | 21.3°C | 15.7°C | 33.0°C | 7.4°C | 25 mm | 68% | 2.3 m/s | 4.2 |
| 1985 | 27.1°C | 21.3°C | 15.4°C | 34.6°C | 9.2°C | 140 mm | 65% | 2.1 m/s | 5 |
| 1984 | 25.5°C | 19.6°C | 13.6°C | 33.0°C | 6.9°C | 48 mm | 66% | 2.3 m/s | 4.8 |
| 1983 | 28.1°C | 21.7°C | 15.3°C | 37.5°C | 6.2°C | 77 mm | 58% | 2.3 m/s | — |
| 1982 | 26.7°C | 20.9°C | 15.1°C | 31.9°C | 9.3°C | 74 mm | 66% | 2.2 m/s | — |
| 1981 | 26.1°C | 20.2°C | 14.4°C | 33.0°C | 7.1°C | 78 mm | 65% | 2.4 m/s | — |
Actual recorded September data for each year, 1981–2025 — sourced from NASA POWER.
The average high in September is around 26.4°C with lows near 15.8°C (mean 21.1°C), based on 45 years of NASA POWER data.
September averages 104 mm of rainfall in Washington, a relatively wet time of year. The wettest on record was 293 mm in 1999.
The hottest September reached 37.5°C in 1983. The coldest dipped to 5.5°C in 1992.
Comparing the earliest years on record (1981–1985) with the most recent (2021–2025), average September temperatures have risen by about 1.3°C. Daytime highs moved from 26.7°C to 27.2°C and overnight lows from 14.8°C to 16.7°C over the same span.
September is one of the warmer months of the year in Washington and a relatively wet time of year, so plan for around 26.4°C daytime highs and 15.8°C at night.
Not quite — September is the 9th coolest month in Washington, averaging 21.1°C.
For September in Washington, pack light layers you can adjust through the day, with typical highs of 26.4°C and lows of 15.8°C. Bring an umbrella or light rain jacket too.
Relative humidity in September averages 70% — moderately humid.
Average wind speed in September is 2.4 m/s. The windiest September on record averaged 2.9 m/s (1999).
The wettest September on record was 1999 with 293 mm. On average, September sees 104 mm.
All figures are derived from NASA POWER (Prediction Of Worldwide Energy Resources), based on MERRA-2 satellite reanalysis, covering 1981–2025.
Explore how the weather in Washington behaves each September, based on 45 years of NASA POWER climate data from 1981 to 2025. This page breaks down average temperatures, rainfall, humidity, wind and solar radiation for September specifically, along with year-by-year records and how the month has changed over the decades.
