September is one of the warmer months of the year in Washington, with average highs near 25.3°C and lows around 12.3°C (mean 18.8°C). It is about average for rainfall, averaging 80 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 22.2°C — while the coldest was 1994 at 16.6°C. Typical September days reach 25.3°C with nights near 12.3°C. The long-term trend is warming, about 0.6°C per decade.
September is about average for rainfall in Washington, averaging 80 mm of rainfall. The wettest September on record was 2018 with 208 mm, while the driest was 1985 at 22 mm.
Relative humidity in Washington during September averages 70%. The most humid September was 2018 at 84%, and the least humid was 1985 at 59%.
September receives about 4.2 kWh/m²/day of solar energy in Washington. The sunniest Septembers were 2005 and 2007 at 4.9 kWh/m²/day, and the cloudiest was 1993 at 3.4.
Average wind speed in Washington during September is 0.1 m/s. The windiest Septembers were 2022, 2023 and 2025, among others at 0.1 m/s, while the calmest were 1989, 2011 and 2024 at 0 m/s.
Ranked against the other 11 months using 1981–2025 averages.
| Metric | 1980s | 1990s | 2000s | 2010s | 2020s | Change |
|---|---|---|---|---|---|---|
| Avg Temp | 18.2°C | 18.0°C | 18.8°C | 19.8°C | 19.5°C | +1.3°C |
| Avg High | 24.8°C | 24.5°C | 25.4°C | 26.3°C | 26.0°C | +1.2°C |
| Avg Low | 11.6°C | 11.5°C | 12.2°C | 13.3°C | 13.0°C | +1.4°C |
| Avg Rain | 78 mm | 83 mm | 81 mm | 90 mm | 62 mm | -16 mm |
September temperatures in Washington have risen by about 1.6°C between 1981–1985 and 2021–2025.
September has ranged from a record low of 1°C (1989) to a record high of 35.9°C (2011).
Rainfall in September is about average for rainfall, averaging 80 mm, with a wettest year of 208 mm.
One of the best times to visit Washington
September is one of the best times to visit Washington. Expect days averaging 25.3°C and nights around 12.3°C (mean 18.8°C) — warmer than Washington's yearly average — with higher rainfall (80 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 | 27.4°C | 20.5°C | 13.5°C | 31.1°C | 6.8°C | 66 mm | 63% | 0.1 m/s | 4.6 |
| 2024 | 27.3°C | 20.5°C | 13.7°C | 32.0°C | 5.8°C | 58 mm | 66% | 0 m/s | 4.4 |
| 2023 | 25.7°C | 19.4°C | 13.1°C | 33.9°C | 7.6°C | 41 mm | 70% | 0.1 m/s | 4.3 |
| 2022 | 24.5°C | 18.6°C | 12.7°C | 30.9°C | 4.2°C | 80 mm | 75% | 0.1 m/s | 3.9 |
| 2021 | 25.4°C | 19.2°C | 13.0°C | 30.5°C | 7.3°C | 90 mm | 74% | 0.1 m/s | 4.4 |
| 2020 | 25.5°C | 18.8°C | 12.2°C | 32.1°C | 3.4°C | 38 mm | 69% | 0.1 m/s | 4.4 |
| 2019 🔥 | 29.5°C | 22.2°C | 15.0°C | 34.1°C | 10.5°C | 53 mm | 65% | 0.1 m/s | 4.7 |
| 2018 🌧️ | 25.8°C | 20.9°C | 16.0°C | 34.8°C | 9.2°C | 208 mm | 84% | 0.1 m/s | 3.7 |
| 2017 | 26.0°C | 19.3°C | 12.7°C | 32.6°C | 5.0°C | 29 mm | 68% | 0.1 m/s | 4.4 |
| 2016 | 29.1°C | 21.9°C | 14.8°C | 33.7°C | 9.3°C | 69 mm | 65% | 0.1 m/s | 4.8 |
| 2015 | 27.8°C | 21.1°C | 14.5°C | 34.7°C | 8.5°C | 84 mm | 64% | 0.1 m/s | 4.5 |
| 2014 | 26.1°C | 19.0°C | 11.8°C | 33.2°C | 4.6°C | 27 mm | 67% | 0.1 m/s | 4.6 |
| 2013 | 24.6°C | 17.8°C | 11.0°C | 32.4°C | 2.9°C | 46 mm | 71% | 0.1 m/s | 4.3 |
| 2012 | 24.4°C | 18.0°C | 11.6°C | 32.9°C | 4.5°C | 121 mm | 71% | 0.1 m/s | 4.2 |
| 2011 | 23.3°C | 18.2°C | 13.2°C | 35.9°C | 3.4°C | 156 mm | 82% | 0 m/s | 3.5 |
| 2010 | 25.9°C | 19.1°C | 12.3°C | 34.8°C | 7.6°C | 109 mm | 65% | 0.1 m/s | 4.4 |
| 2009 | 24.3°C | 18.4°C | 12.5°C | 28.8°C | 5.4°C | 64 mm | 71% | 0.1 m/s | 4 |
| 2008 | 26.2°C | 19.4°C | 12.6°C | 33.3°C | 8.6°C | 70 mm | 68% | 0.1 m/s | 4.4 |
| 2007 | 27.2°C | 19.9°C | 12.6°C | 33.8°C | 4.4°C | 51 mm | 67% | 0.1 m/s | 4.9 |
| 2006 | 22.4°C | 17.0°C | 11.6°C | 28.6°C | 4.3°C | 107 mm | 76% | 0.1 m/s | 3.5 |
| 2005 | 28.4°C | 21.0°C | 13.6°C | 32.7°C | 4.6°C | 29 mm | 60% | 0.1 m/s | 4.9 |
| 2004 | 24.6°C | 18.8°C | 12.9°C | 30.1°C | 5.2°C | 177 mm | 78% | 0.1 m/s | 4.6 |
| 2003 | 23.1°C | 17.4°C | 11.7°C | 27.7°C | 5.4°C | 113 mm | 80% | 0.1 m/s | 3.9 |
| 2002 | 28.2°C | 20.9°C | 13.6°C | 34.6°C | 7.4°C | 81 mm | 62% | 0.1 m/s | 4.7 |
| 2001 | 24.8°C | 17.4°C | 9.9°C | 32.8°C | 3.4°C | 50 mm | 64% | 0.1 m/s | 4.6 |
| 2000 | 24.7°C | 17.8°C | 10.8°C | 33.2°C | 1.9°C | 70 mm | 68% | 0.1 m/s | 4.1 |
| 1999 | 26.5°C | 18.9°C | 11.3°C | 33.6°C | 1.3°C | 53 mm | 60% | 0.1 m/s | 4.5 |
| 1998 | 27.6°C | 20.4°C | 13.2°C | 35.8°C | 5.2°C | 53 mm | 63% | 0.1 m/s | 4.7 |
| 1997 | 23.8°C | 17.3°C | 10.8°C | 30.0°C | 4.6°C | 81 mm | 70% | 0.1 m/s | 4.2 |
| 1996 | 22.8°C | 17.5°C | 12.2°C | 29.4°C | 5.8°C | 146 mm | 79% | 0.1 m/s | 3.7 |
| 1995 | 24.8°C | 17.8°C | 10.8°C | 31.8°C | 1.1°C | 47 mm | 63% | 0.1 m/s | 4.3 |
| 1994 ❄️ | 22.9°C | 16.6°C | 10.2°C | 29.6°C | 4.2°C | 50 mm | 73% | 0.1 m/s | 4.4 |
| 1993 | 23.9°C | 17.9°C | 11.9°C | 34.0°C | 1.6°C | 119 mm | 71% | 0.1 m/s | 3.4 |
| 1992 | 24.4°C | 17.8°C | 11.2°C | 29.7°C | 1.5°C | 68 mm | 73% | 0.1 m/s | 4 |
| 1991 | 25.5°C | 18.5°C | 11.5°C | 33.4°C | 1.4°C | 66 mm | 63% | 0.1 m/s | 4.4 |
| 1990 | 22.9°C | 17.4°C | 11.8°C | 30.0°C | 3.8°C | 151 mm | 79% | 0.1 m/s | 3.8 |
| 1989 | 23.9°C | 17.7°C | 11.6°C | 30.0°C | 1.0°C | 104 mm | 78% | 0 m/s | 4 |
| 1988 | 24.5°C | 17.7°C | 10.9°C | 30.4°C | 6.0°C | 89 mm | 72% | 0.1 m/s | 3.9 |
| 1987 | 23.8°C | 18.2°C | 12.5°C | 27.7°C | 4.4°C | 106 mm | 77% | 0.1 m/s | 3.7 |
| 1986 | 26.1°C | 19.7°C | 13.4°C | 30.4°C | 3.5°C | 84 mm | 70% | 0.1 m/s | 3.9 |
| 1985 | 26.8°C | 19.2°C | 11.6°C | 32.1°C | 3.2°C | 22 mm | 59% | 0.1 m/s | 4.6 |
| 1984 | 24.1°C | 17.1°C | 10.1°C | 32.2°C | 3.2°C | 47 mm | 67% | 0.1 m/s | 4 |
| 1983 | 26.2°C | 19.0°C | 11.8°C | 34.0°C | 2.9°C | 69 mm | 61% | 0.1 m/s | — |
| 1982 | 24.6°C | 17.8°C | 10.9°C | 32.2°C | 5.1°C | 69 mm | 68% | 0.1 m/s | — |
| 1981 | 22.9°C | 17.1°C | 11.2°C | 29.2°C | 2.2°C | 107 mm | 74% | 0.1 m/s | — |
Actual recorded September data for each year, 1981–2025 — sourced from NASA POWER.
The average high in September is around 25.3°C with lows near 12.3°C (mean 18.8°C), based on 45 years of NASA POWER data.
September averages 80 mm of rainfall in Washington, about average for rainfall. The wettest on record was 208 mm in 2018.
The hottest September reached 35.9°C in 2011. The coldest dipped to 1°C in 1989.
Comparing the earliest years on record (1981–1985) with the most recent (2021–2025), average September temperatures have risen by about 1.6°C. Daytime highs moved from 24.9°C to 26.1°C and overnight lows from 11.1°C to 13.2°C over the same span.
September is one of the warmer months of the year in Washington and about average for rainfall, so plan for around 25.3°C daytime highs and 12.3°C at night.
Not quite — September is the 9th coolest month in Washington, averaging 18.8°C.
For September in Washington, pack light layers you can adjust through the day, with typical highs of 25.3°C and lows of 12.3°C. Bring an umbrella or light rain jacket too.
Relative humidity in September averages 70% — moderately humid.
Average wind speed in September is 0.1 m/s. The windiest September on record averaged 0.1 m/s (1981).
The wettest September on record was 2018 with 208 mm. On average, September sees 80 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.
