September is one of the warmer months of the year in Flagstaff, with average highs near 26.2°C and lows around 11.4°C (mean 18.8°C). It is about average for rainfall, averaging 43 mm of rainfall and 47% relative humidity across the period.
September is one of the warmer months of the year in Flagstaff. Across 45 years of records, the warmest September in Flagstaff was 2024 — when the month averaged 21.1°C — while the coldest was 1985 at 15.2°C. Typical September days reach 26.2°C with nights near 11.4°C. The long-term trend is warming, about 0.6°C per decade.
September is about average for rainfall in Flagstaff, averaging 43 mm of rainfall. The wettest September on record was 1983 with 134 mm, while the driest was 2020 at 2 mm.
Relative humidity in Flagstaff during September averages 47%. The most humid September was 1983 at 58%, and the least humid was 2020 at 30%.
September receives about 5.9 kWh/m²/day of solar energy in Flagstaff. The sunniest September was 1993 at 6.5 kWh/m²/day, and the cloudiest were 1990 and 1997 at 5.4.
Average wind speed in Flagstaff during September is 2.1 m/s. The windiest September was 1986 at 2.9 m/s, while the calmest was 2011 at 1.5 m/s.
Ranked against the other 11 months using 1981–2025 averages.
| Metric | 1980s | 1990s | 2000s | 2010s | 2020s | Change |
|---|---|---|---|---|---|---|
| Avg Temp | 17.6°C | 18.5°C | 18.8°C | 19.4°C | 20.3°C | +2.7°C |
| Avg High | 24.9°C | 25.8°C | 26.4°C | 26.7°C | 27.9°C | +3°C |
| Avg Low | 10.2°C | 11.2°C | 11.3°C | 12.1°C | 12.8°C | +2.6°C |
| Avg Rain | 53 mm | 57 mm | 35 mm | 34 mm | 31 mm | -22 mm |
September temperatures in Flagstaff have risen by about 2.3°C between 1981–1985 and 2021–2025.
September has ranged from a record low of -0.9°C (1999) to a record high of 35.8°C (2020).
Rainfall in September is about average for rainfall, averaging 43 mm, with a wettest year of 134 mm.
One of the best times to visit Flagstaff
September is one of the best times to visit Flagstaff. Expect days averaging 26.2°C and nights around 11.4°C (mean 18.8°C) — warmer than Flagstaff's yearly average — with moderate rainfall (43 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 | 27.3°C | 20.0°C | 12.7°C | 32.8°C | 7.6°C | 56 mm | 51% | 2 m/s | 5.6 |
| 2024 🔥 | 28.9°C | 21.1°C | 13.2°C | 33.3°C | 6.9°C | 7 mm | 37% | 2.1 m/s | 6.2 |
| 2023 | 26.9°C | 19.4°C | 11.9°C | 31.8°C | 7.9°C | 26 mm | 45% | 2.2 m/s | 5.8 |
| 2022 | 27.9°C | 20.7°C | 13.6°C | 34.9°C | 9.6°C | 50 mm | 51% | 1.8 m/s | 5.6 |
| 2021 | 27.5°C | 20.3°C | 13.1°C | 34.0°C | 5.7°C | 46 mm | 49% | 1.9 m/s | 5.8 |
| 2020 | 29.0°C | 20.5°C | 12.0°C | 35.8°C | 3.1°C | 2 mm | 30% | 1.9 m/s | 6.3 |
| 2019 | 26.6°C | 19.4°C | 12.3°C | 34.5°C | 6.0°C | 27 mm | 47% | 2.6 m/s | 6 |
| 2018 | 28.4°C | 21.0°C | 13.5°C | 31.8°C | 10.4°C | 14 mm | 39% | 1.8 m/s | 6 |
| 2017 | 26.1°C | 18.5°C | 11.0°C | 33.8°C | 1.3°C | 11 mm | 44% | 2.3 m/s | 6 |
| 2016 | 25.0°C | 17.8°C | 10.6°C | 31.0°C | 4.5°C | 30 mm | 49% | 2.3 m/s | 5.7 |
| 2015 | 27.0°C | 19.7°C | 12.5°C | 31.2°C | 8.8°C | 24 mm | 54% | 1.9 m/s | 5.8 |
| 2014 | 26.6°C | 19.6°C | 12.6°C | 31.5°C | 3.9°C | 61 mm | 54% | 1.8 m/s | 5.8 |
| 2013 | 25.3°C | 18.3°C | 11.2°C | 31.5°C | 0.5°C | 56 mm | 53% | 2.1 m/s | 5.8 |
| 2012 | 26.5°C | 19.5°C | 12.6°C | 29.9°C | 7.9°C | 23 mm | 46% | 1.7 m/s | 5.7 |
| 2011 | 26.7°C | 19.7°C | 12.7°C | 32.5°C | 8.4°C | 67 mm | 45% | 1.5 m/s | 5.6 |
| 2010 | 28.5°C | 20.3°C | 12.1°C | 32.2°C | 6.4°C | 25 mm | 38% | 2 m/s | 6.4 |
| 2009 | 26.0°C | 18.6°C | 11.1°C | 30.7°C | 3.5°C | 30 mm | 44% | 2 m/s | 5.8 |
| 2008 | 26.5°C | 19.1°C | 11.8°C | 30.6°C | 9.0°C | 16 mm | 44% | 1.7 m/s | 6.1 |
| 2007 | 26.1°C | 18.7°C | 11.3°C | 30.9°C | 1.6°C | 43 mm | 46% | 2.3 m/s | 6 |
| 2006 | 24.2°C | 16.6°C | 9.0°C | 28.8°C | 0.4°C | 39 mm | 48% | 2 m/s | 5.9 |
| 2005 | 25.7°C | 18.4°C | 11.2°C | 29.9°C | 5.7°C | 14 mm | 45% | 2.2 m/s | 5.9 |
| 2004 | 25.5°C | 17.9°C | 10.3°C | 31.9°C | 2.0°C | 62 mm | 40% | 2.4 m/s | 6.1 |
| 2003 | 27.5°C | 19.8°C | 12.1°C | 31.4°C | 5.5°C | 49 mm | 43% | 2 m/s | 6 |
| 2002 | 26.1°C | 19.0°C | 11.9°C | 32.6°C | 5.5°C | 73 mm | 44% | 2.3 m/s | 5.6 |
| 2001 | 28.7°C | 20.5°C | 12.3°C | 32.1°C | 5.7°C | 15 mm | 37% | 2 m/s | 6.3 |
| 2000 | 28.0°C | 19.9°C | 11.7°C | 32.6°C | 1.1°C | 4 mm | 37% | 2.5 m/s | 6 |
| 1999 | 25.6°C | 18.1°C | 10.7°C | 29.9°C | -0.9°C | 97 mm | 52% | 1.7 m/s | 5.7 |
| 1998 | 26.2°C | 18.8°C | 11.3°C | 30.1°C | 6.0°C | 59 mm | 50% | 2.2 m/s | 5.6 |
| 1997 | 26.5°C | 19.6°C | 12.7°C | 30.2°C | 6.8°C | 73 mm | 55% | 2 m/s | 5.4 |
| 1996 | 23.7°C | 16.7°C | 9.7°C | 30.2°C | 1.4°C | 105 mm | 56% | 2.1 m/s | 5.5 |
| 1995 | 26.1°C | 18.9°C | 11.7°C | 32.4°C | 1.0°C | 55 mm | 50% | 1.9 m/s | 5.9 |
| 1994 | 25.9°C | 19.0°C | 12.1°C | 30.9°C | 5.8°C | 48 mm | 48% | 1.9 m/s | 5.6 |
| 1993 | 25.9°C | 17.6°C | 9.3°C | 30.4°C | 3.2°C | 20 mm | 43% | 2.1 m/s | 6.5 |
| 1992 | 26.4°C | 18.6°C | 10.9°C | 29.1°C | 6.3°C | 8 mm | 46% | 2 m/s | 6.3 |
| 1991 | 25.5°C | 18.4°C | 11.2°C | 30.6°C | 7.0°C | 32 mm | 50% | 1.9 m/s | 5.8 |
| 1990 | 25.7°C | 19.0°C | 12.3°C | 33.3°C | 6.3°C | 70 mm | 56% | 1.7 m/s | 5.4 |
| 1989 | 26.5°C | 18.4°C | 10.4°C | 30.3°C | 3.5°C | 8 mm | 38% | 2.3 m/s | 6.3 |
| 1988 | 24.8°C | 16.7°C | 8.5°C | 29.6°C | 3.5°C | 29 mm | 45% | 2.2 m/s | 6.2 |
| 1987 | 25.0°C | 17.5°C | 10.0°C | 29.6°C | 4.9°C | 22 mm | 43% | 1.8 m/s | 5.7 |
| 1986 | 22.9°C | 15.7°C | 8.4°C | 30.7°C | 1.8°C | 64 mm | 50% | 2.9 m/s | 5.7 |
| 1985 ❄️ | 22.7°C | 15.2°C | 7.8°C | 27.2°C | 1.2°C | 67 mm | 52% | 2.5 m/s | 5.7 |
| 1984 | 25.8°C | 18.6°C | 11.3°C | 30.3°C | 4.1°C | 59 mm | 53% | 2 m/s | 5.6 |
| 1983 🌧️ | 26.2°C | 19.4°C | 12.6°C | 32.0°C | 5.6°C | 134 mm | 58% | 2.1 m/s | — |
| 1982 | 24.6°C | 17.7°C | 10.7°C | 31.7°C | 1.8°C | 53 mm | 55% | 2.6 m/s | — |
| 1981 | 26.0°C | 18.9°C | 11.8°C | 29.6°C | 8.5°C | 43 mm | 50% | 1.7 m/s | — |
Actual recorded September data for each year, 1981–2025 — sourced from NASA POWER.
The average high in September is around 26.2°C with lows near 11.4°C (mean 18.8°C), based on 45 years of NASA POWER data.
September averages 43 mm of rainfall in Flagstaff, about average for rainfall. The wettest on record was 134 mm in 1983.
The hottest September reached 35.8°C in 2020. The coldest dipped to -0.9°C in 1999.
Comparing the earliest years on record (1981–1985) with the most recent (2021–2025), average September temperatures have risen by about 2.3°C. Daytime highs moved from 25.1°C to 27.7°C and overnight lows from 10.8°C to 12.9°C over the same span.
September is one of the warmer months of the year in Flagstaff and about average for rainfall, so plan for around 26.2°C daytime highs and 11.4°C at night.
Not quite — September is the 9th coolest month in Flagstaff, averaging 18.8°C.
For September in Flagstaff, pack light layers you can adjust through the day, with typical highs of 26.2°C and lows of 11.4°C.
Relative humidity in September averages 47% — relatively dry.
Average wind speed in September is 2.1 m/s. The windiest September on record averaged 2.9 m/s (1986).
The wettest September on record was 1983 with 134 mm. On average, September sees 43 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 Flagstaff 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.
