September is one of the warmer months of the year in Sammamish, with average highs near 19.9°C and lows around 9.5°C (mean 14.7°C). It is one of the drier months, averaging 76 mm of rainfall and 76% relative humidity across the period.
September is one of the warmer months of the year in Sammamish. Across 45 years of records, the warmest September in Sammamish was 1990 — when the month averaged 17°C — while the coldest was 1983 at 11.7°C. Typical September days reach 19.9°C with nights near 9.5°C. The long-term trend is warming, about 0.2°C per decade.
September is one of the drier months in Sammamish, averaging 76 mm of rainfall. The wettest September on record was 2013 with 229 mm, while the driest was 2022 at 8 mm.
Relative humidity in Sammamish during September averages 76%. The most humid September was 2010 at 87%, and the least humid was 1989 at 66%.
September receives about 4 kWh/m²/day of solar energy in Sammamish. The sunniest September was 1998 at 4.8 kWh/m²/day, and the cloudiest were 2010, 2013 and 2019 at 3.2.
Average wind speed in Sammamish during September is 0.3 m/s. The windiest Septembers were 1993, 1998 and 2012, among others at 0.4 m/s, while the calmest were 2024 and 2025 at 0.2 m/s.
Ranked against the other 11 months using 1981–2025 averages.
| Metric | 1980s | 1990s | 2000s | 2010s | 2020s | Change |
|---|---|---|---|---|---|---|
| Avg Temp | 13.8°C | 15.4°C | 14.5°C | 14.7°C | 15.3°C | +1.5°C |
| Avg High | 19.2°C | 20.8°C | 19.6°C | 19.6°C | 20.5°C | +1.3°C |
| Avg Low | 8.4°C | 9.9°C | 9.3°C | 9.8°C | 10.1°C | +1.7°C |
| Avg Rain | 86 mm | 57 mm | 73 mm | 88 mm | 76 mm | -9 mm |
September temperatures in Sammamish have risen by about 2°C between 1981–1985 and 2021–2025.
September has ranged from a record low of -0.4°C (1983) to a record high of 37.9°C (1988).
Rainfall in September is one of the drier months, averaging 76 mm, with a wettest year of 229 mm.
One of the best times to visit Sammamish
September is one of the best times to visit Sammamish. Expect days averaging 19.9°C and nights around 9.5°C (mean 14.7°C) — warmer than Sammamish's yearly average — with moderate rainfall (76 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 | 19.1°C | 14.3°C | 9.4°C | 30.8°C | 3.8°C | 72 mm | 84% | 0.2 m/s | 4 |
| 2024 | 19.4°C | 14.2°C | 9.0°C | 34.4°C | 2.5°C | 57 mm | 83% | 0.2 m/s | 3.7 |
| 2023 | 19.9°C | 14.8°C | 9.6°C | 30.5°C | 4.6°C | 89 mm | 76% | 0.3 m/s | 3.9 |
| 2022 | 22.6°C | 16.9°C | 11.2°C | 33.2°C | 8.3°C | 8 mm | 72% | 0.3 m/s | 4.3 |
| 2021 | 20.0°C | 14.9°C | 9.8°C | 30.7°C | 6.2°C | 122 mm | 75% | 0.3 m/s | 3.8 |
| 2020 | 22.1°C | 16.8°C | 11.5°C | 33.0°C | 8.4°C | 107 mm | 76% | 0.3 m/s | 3.7 |
| 2019 | 18.3°C | 14.0°C | 9.8°C | 32.0°C | 0.6°C | 109 mm | 83% | 0.3 m/s | 3.2 |
| 2018 | 19.3°C | 14.2°C | 9.1°C | 28.7°C | 5.9°C | 60 mm | 76% | 0.3 m/s | 3.9 |
| 2017 | 20.9°C | 15.9°C | 11.0°C | 30.7°C | 6.0°C | 56 mm | 73% | 0.3 m/s | 3.8 |
| 2016 | 18.0°C | 13.5°C | 8.9°C | 28.0°C | 5.6°C | 70 mm | 79% | 0.3 m/s | 4 |
| 2015 | 18.7°C | 13.6°C | 8.6°C | 32.3°C | 4.6°C | 56 mm | 77% | 0.3 m/s | 3.8 |
| 2014 | 21.0°C | 15.7°C | 10.4°C | 31.8°C | 7.1°C | 78 mm | 76% | 0.3 m/s | 4.1 |
| 2013 🌧️ | 19.1°C | 14.8°C | 10.5°C | 33.0°C | 5.1°C | 229 mm | 85% | 0.3 m/s | 3.2 |
| 2012 | 21.3°C | 15.5°C | 9.7°C | 29.9°C | 3.2°C | 11 mm | 71% | 0.4 m/s | 4.6 |
| 2011 | 21.6°C | 16.1°C | 10.7°C | 30.0°C | 6.7°C | 60 mm | 73% | 0.3 m/s | 4 |
| 2010 | 18.2°C | 13.8°C | 9.5°C | 24.4°C | 6.1°C | 152 mm | 87% | 0.3 m/s | 3.2 |
| 2009 | 21.2°C | 15.6°C | 9.9°C | 32.4°C | 3.7°C | 77 mm | 73% | 0.3 m/s | 4.1 |
| 2008 | 20.8°C | 14.9°C | 9.0°C | 33.9°C | 4.1°C | 31 mm | 74% | 0.3 m/s | 4.2 |
| 2007 | 17.9°C | 13.2°C | 8.5°C | 29.7°C | 4.6°C | 64 mm | 80% | 0.3 m/s | 3.7 |
| 2006 | 20.7°C | 15.3°C | 10.0°C | 32.8°C | 4.7°C | 52 mm | 71% | 0.3 m/s | 4.4 |
| 2005 | 17.7°C | 13.1°C | 8.5°C | 23.4°C | 5.5°C | 105 mm | 78% | 0.3 m/s | 4.2 |
| 2004 | 17.1°C | 13.1°C | 9.0°C | 25.8°C | 5.4°C | 142 mm | 86% | 0.3 m/s | 3.6 |
| 2003 | 22.1°C | 16.4°C | 10.7°C | 33.0°C | 5.3°C | 62 mm | 70% | 0.3 m/s | 4.2 |
| 2002 | 19.8°C | 14.5°C | 9.3°C | 28.1°C | 3.9°C | 34 mm | 73% | 0.3 m/s | 4.3 |
| 2001 | 19.9°C | 14.8°C | 9.7°C | 29.5°C | 3.7°C | 28 mm | 76% | 0.3 m/s | 4 |
| 2000 | 18.8°C | 13.7°C | 8.7°C | 29.6°C | 1.1°C | 131 mm | 79% | 0.3 m/s | 3.9 |
| 1999 | 20.6°C | 14.6°C | 8.7°C | 29.7°C | 3.2°C | 22 mm | 71% | 0.3 m/s | 4.7 |
| 1998 | 21.4°C | 16.2°C | 10.9°C | 27.2°C | 6.8°C | 25 mm | 74% | 0.4 m/s | 4.8 |
| 1997 | 20.5°C | 15.4°C | 10.2°C | 32.8°C | 6.8°C | 142 mm | 82% | 0.3 m/s | 3.8 |
| 1996 | 17.1°C | 12.7°C | 8.4°C | 30.5°C | 2.5°C | 100 mm | 82% | 0.3 m/s | 3.6 |
| 1995 | 22.0°C | 16.5°C | 11.0°C | 31.8°C | 7.1°C | 72 mm | 74% | 0.3 m/s | 3.9 |
| 1994 | 21.3°C | 16.1°C | 10.8°C | 30.7°C | 5.8°C | 61 mm | 74% | 0.3 m/s | 4.1 |
| 1993 | 21.3°C | 15.6°C | 9.8°C | 31.3°C | 4.1°C | 12 mm | 71% | 0.4 m/s | 4.7 |
| 1992 | 18.5°C | 13.4°C | 8.3°C | 27.0°C | 2.5°C | 114 mm | 75% | 0.3 m/s | 4.1 |
| 1991 | 21.7°C | 16.0°C | 10.4°C | 30.7°C | 5.4°C | 10 mm | 73% | 0.3 m/s | 4.4 |
| 1990 🔥 | 23.3°C | 17.0°C | 10.8°C | 30.9°C | 6.7°C | 13 mm | 71% | 0.4 m/s | 4 |
| 1989 | 22.4°C | 15.9°C | 9.3°C | 30.4°C | 4.4°C | 20 mm | 66% | 0.3 m/s | 4.5 |
| 1988 | 20.5°C | 14.6°C | 8.7°C | 37.9°C | 2.8°C | 115 mm | 72% | 0.3 m/s | 4 |
| 1987 | 22.8°C | 16.6°C | 10.4°C | 31.2°C | 4.0°C | 30 mm | 67% | 0.3 m/s | 4.2 |
| 1986 | 16.8°C | 12.3°C | 7.8°C | 26.2°C | 4.3°C | 117 mm | 78% | 0.3 m/s | 3.5 |
| 1985 | 17.4°C | 12.1°C | 6.8°C | 24.6°C | 3.4°C | 101 mm | 74% | 0.3 m/s | 3.8 |
| 1984 | 17.6°C | 12.6°C | 7.7°C | 24.3°C | 1.4°C | 85 mm | 77% | 0.3 m/s | 3.7 |
| 1983 ❄️ | 16.5°C | 11.7°C | 7.0°C | 23.9°C | -0.4°C | 96 mm | 80% | 0.3 m/s | — |
| 1982 | 19.4°C | 14.2°C | 9.0°C | 30.8°C | 4.0°C | 102 mm | 78% | 0.3 m/s | — |
| 1981 | 19.6°C | 14.4°C | 9.1°C | 34.2°C | 4.7°C | 104 mm | 77% | 0.3 m/s | — |
Actual recorded September data for each year, 1981–2025 — sourced from NASA POWER.
The average high in September is around 19.9°C with lows near 9.5°C (mean 14.7°C), based on 45 years of NASA POWER data.
September averages 76 mm of rainfall in Sammamish, one of the drier months. The wettest on record was 229 mm in 2013.
The hottest September reached 37.9°C in 1988. The coldest dipped to -0.4°C in 1983.
Comparing the earliest years on record (1981–1985) with the most recent (2021–2025), average September temperatures have risen by about 2°C. Daytime highs moved from 18.1°C to 20.2°C and overnight lows from 7.9°C to 9.8°C over the same span.
September is one of the warmer months of the year in Sammamish and one of the drier months, so plan for around 19.9°C daytime highs and 9.5°C at night.
Not quite — September is the 10th coolest month in Sammamish, averaging 14.7°C.
For September in Sammamish, pack light layers you can adjust through the day, with typical highs of 19.9°C and lows of 9.5°C. Bring an umbrella or light rain jacket too.
Relative humidity in September averages 76% — quite humid.
Average wind speed in September is 0.3 m/s. The windiest September on record averaged 0.4 m/s (1990).
The wettest September on record was 2013 with 229 mm. On average, September sees 76 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 Sammamish 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.
