September is one of the warmer months of the year in Fort Collins, with average highs near 25.7°C and lows around 9.8°C (mean 17.8°C). It is about average for rainfall, averaging 37 mm of rainfall and 44% relative humidity across the period.
September is one of the warmer months of the year in Fort Collins. Across 45 years of records, the warmest September in Fort Collins was 2024 — when the month averaged 20.7°C — while the coldest was 1985 at 14.3°C. Typical September days reach 25.7°C with nights near 9.8°C. The long-term trend is warming, about 0.7°C per decade.
September is about average for rainfall in Fort Collins, averaging 37 mm of rainfall. The wettest September on record was 2013 with 170 mm, while the driest was 2010 at 5 mm.
Relative humidity in Fort Collins during September averages 44%. The most humid September was 2013 at 59%, and the least humid was 2010 at 35%.
September receives about 5 kWh/m²/day of solar energy in Fort Collins. The sunniest September was 2010 at 5.8 kWh/m²/day, and the cloudiest was 2013 at 4.3.
Average wind speed in Fort Collins during September is 2.1 m/s. The windiest Septembers were 1985, 1988 and 2009 at 2.5 m/s, while the calmest were 1999, 2015 and 2017, among others at 1.9 m/s.
Ranked against the other 11 months using 1981–2025 averages.
| Metric | 1980s | 1990s | 2000s | 2010s | 2020s | Change |
|---|---|---|---|---|---|---|
| Avg Temp | 16.5°C | 17.2°C | 17.4°C | 18.7°C | 19.6°C | +3.1°C |
| Avg High | 24.4°C | 25.1°C | 25.5°C | 26.6°C | 27.7°C | +3.3°C |
| Avg Low | 8.6°C | 9.4°C | 9.3°C | 10.9°C | 11.5°C | +2.9°C |
| Avg Rain | 34 mm | 41 mm | 36 mm | 40 mm | 30 mm | -5 mm |
September temperatures in Fort Collins have risen by about 3.3°C between 1981–1985 and 2021–2025.
September has ranged from a record low of -8.8°C (1985) to a record high of 36.9°C (2022).
Rainfall in September is about average for rainfall, averaging 37 mm, with a wettest year of 170 mm.
One of the best times to visit Fort Collins
September is one of the best times to visit Fort Collins. Expect days averaging 25.7°C and nights around 9.8°C (mean 17.8°C) — warmer than Fort Collins's yearly average — with moderate rainfall (37 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 | 26.7°C | 19.0°C | 11.3°C | 32.8°C | 6.8°C | 58 mm | 45% | 2.1 m/s | 4.6 |
| 2024 🔥 | 29.1°C | 20.7°C | 12.4°C | 33.6°C | 5.9°C | 25 mm | 35% | 2.1 m/s | 5.3 |
| 2023 | 27.4°C | 19.3°C | 11.1°C | 35.3°C | 6.6°C | 23 mm | 40% | 2 m/s | 5.4 |
| 2022 | 28.5°C | 20.4°C | 12.3°C | 36.9°C | 6.0°C | 28 mm | 39% | 2 m/s | 5 |
| 2021 | 28.5°C | 20.3°C | 12.0°C | 36.1°C | 3.9°C | 18 mm | 36% | 2.2 m/s | 5.3 |
| 2020 | 26.3°C | 18.0°C | 9.7°C | 36.8°C | -2.5°C | 25 mm | 37% | 2.3 m/s | 5.1 |
| 2019 | 28.6°C | 20.3°C | 12.0°C | 36.8°C | 5.0°C | 27 mm | 41% | 2.3 m/s | 5.4 |
| 2018 | 27.7°C | 19.4°C | 11.2°C | 33.8°C | 4.3°C | 11 mm | 38% | 2 m/s | 5.3 |
| 2017 | 24.8°C | 17.4°C | 9.9°C | 35.2°C | 0.6°C | 42 mm | 50% | 1.9 m/s | 4.5 |
| 2016 | 26.7°C | 18.4°C | 10.2°C | 31.8°C | 4.3°C | 11 mm | 42% | 2.1 m/s | 5.1 |
| 2015 | 28.3°C | 20.3°C | 12.3°C | 32.8°C | 5.1°C | 8 mm | 40% | 1.9 m/s | 5.2 |
| 2014 | 25.7°C | 17.9°C | 10.2°C | 33.2°C | -2.2°C | 48 mm | 51% | 2 m/s | 4.9 |
| 2013 🌧️ | 24.6°C | 17.9°C | 11.3°C | 34.5°C | 1.5°C | 170 mm | 59% | 2.2 m/s | 4.3 |
| 2012 | 26.3°C | 18.8°C | 11.3°C | 33.4°C | 7.0°C | 43 mm | 41% | 2.1 m/s | 5 |
| 2011 | 25.3°C | 17.6°C | 10.0°C | 34.9°C | 4.6°C | 35 mm | 45% | 2.1 m/s | 5.1 |
| 2010 | 28.2°C | 19.3°C | 10.4°C | 32.7°C | 5.3°C | 5 mm | 35% | 2.2 m/s | 5.8 |
| 2009 | 24.8°C | 17.1°C | 9.3°C | 32.4°C | 2.3°C | 33 mm | 47% | 2.5 m/s | 4.8 |
| 2008 | 24.9°C | 16.7°C | 8.5°C | 29.8°C | 3.7°C | 44 mm | 47% | 2 m/s | 5.1 |
| 2007 | 26.3°C | 18.3°C | 10.3°C | 32.0°C | 1.6°C | 39 mm | 45% | 2.1 m/s | 5 |
| 2006 | 22.8°C | 15.0°C | 7.2°C | 29.7°C | 1.7°C | 30 mm | 47% | 2.3 m/s | 4.8 |
| 2005 | 27.3°C | 19.1°C | 10.9°C | 32.3°C | 5.6°C | 11 mm | 38% | 2 m/s | 5.2 |
| 2004 | 25.0°C | 17.4°C | 9.7°C | 32.6°C | 2.5°C | 65 mm | 47% | 2.1 m/s | 4.8 |
| 2003 | 24.7°C | 16.1°C | 7.6°C | 30.8°C | -0.1°C | 15 mm | 43% | 2.1 m/s | 5.4 |
| 2002 | 25.9°C | 17.9°C | 9.9°C | 34.6°C | 1.8°C | 40 mm | 43% | 2.1 m/s | 4.9 |
| 2001 | 27.4°C | 19.0°C | 10.6°C | 32.7°C | 1.8°C | 27 mm | 40% | 2.1 m/s | 5.1 |
| 2000 | 26.3°C | 17.7°C | 9.2°C | 34.8°C | -2.9°C | 52 mm | 40% | 2.2 m/s | 4.9 |
| 1999 | 23.0°C | 14.8°C | 6.7°C | 30.5°C | -3.2°C | 47 mm | 47% | 1.9 m/s | 5.2 |
| 1998 | 28.1°C | 20.1°C | 12.2°C | 34.6°C | 5.4°C | 22 mm | 42% | 2.2 m/s | 5.2 |
| 1997 | 25.6°C | 18.1°C | 10.7°C | 29.9°C | 3.8°C | 56 mm | 53% | 1.9 m/s | 4.8 |
| 1996 | 23.3°C | 15.7°C | 8.0°C | 33.3°C | -4.7°C | 65 mm | 50% | 2.2 m/s | 4.6 |
| 1995 | 23.8°C | 16.4°C | 9.0°C | 35.7°C | -4.5°C | 65 mm | 49% | 1.9 m/s | 4.5 |
| 1994 | 26.6°C | 18.5°C | 10.4°C | 32.1°C | -0.9°C | 18 mm | 38% | 2.1 m/s | 4.8 |
| 1993 | 23.0°C | 14.9°C | 6.8°C | 30.8°C | -2.0°C | 59 mm | 48% | 2.4 m/s | 4.9 |
| 1992 | 26.4°C | 17.9°C | 9.4°C | 31.9°C | 3.1°C | 7 mm | 37% | 2.3 m/s | 5.4 |
| 1991 | 24.3°C | 16.7°C | 9.1°C | 29.3°C | 2.4°C | 30 mm | 48% | 2.1 m/s | 5.1 |
| 1990 | 26.5°C | 18.9°C | 11.3°C | 34.0°C | 4.2°C | 42 mm | 47% | 2.1 m/s | 5 |
| 1989 | 24.3°C | 16.3°C | 8.3°C | 32.0°C | -0.5°C | 53 mm | 47% | 2.3 m/s | 5.3 |
| 1988 | 24.9°C | 16.7°C | 8.6°C | 34.0°C | 2.7°C | 33 mm | 38% | 2.5 m/s | 5 |
| 1987 | 25.0°C | 16.9°C | 8.8°C | 31.3°C | 2.6°C | 19 mm | 42% | 1.9 m/s | 5.2 |
| 1986 | 23.1°C | 15.6°C | 8.1°C | 30.0°C | 1.1°C | 31 mm | 50% | 2 m/s | 4.4 |
| 1985 ❄️ | 22.1°C | 14.3°C | 6.5°C | 30.9°C | -8.8°C | 48 mm | 48% | 2.5 m/s | 4.5 |
| 1984 | 24.2°C | 16.2°C | 8.1°C | 32.8°C | -4.7°C | 21 mm | 46% | 2.1 m/s | 4.8 |
| 1983 | 26.5°C | 18.1°C | 9.7°C | 33.4°C | -2.7°C | 11 mm | 42% | 2.2 m/s | — |
| 1982 | 23.3°C | 16.0°C | 8.7°C | 31.9°C | 0.7°C | 71 mm | 56% | 2.1 m/s | — |
| 1981 | 26.2°C | 18.6°C | 10.9°C | 32.5°C | 5.6°C | 23 mm | 43% | 2.1 m/s | — |
Actual recorded September data for each year, 1981–2025 — sourced from NASA POWER.
The average high in September is around 25.7°C with lows near 9.8°C (mean 17.8°C), based on 45 years of NASA POWER data.
September averages 37 mm of rainfall in Fort Collins, about average for rainfall. The wettest on record was 170 mm in 2013.
The hottest September reached 36.9°C in 2022. The coldest dipped to -8.8°C in 1985.
Comparing the earliest years on record (1981–1985) with the most recent (2021–2025), average September temperatures have risen by about 3.3°C. Daytime highs moved from 24.5°C to 28°C and overnight lows from 8.8°C to 11.8°C over the same span.
September is one of the warmer months of the year in Fort Collins and about average for rainfall, so plan for around 25.7°C daytime highs and 9.8°C at night.
Not quite — September is the 9th coolest month in Fort Collins, averaging 17.8°C.
For September in Fort Collins, pack light layers you can adjust through the day, with typical highs of 25.7°C and lows of 9.8°C.
Relative humidity in September averages 44% — relatively dry.
Average wind speed in September is 2.1 m/s. The windiest September on record averaged 2.5 m/s (1985).
The wettest September on record was 2013 with 170 mm. On average, September sees 37 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 Fort Collins 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.
