September is one of the warmer months of the year in Great Falls, with average highs near 21.3°C and lows around 7.5°C (mean 14.4°C). It is about average for rainfall, averaging 40 mm of rainfall and 55% relative humidity across the period.
September is one of the warmer months of the year in Great Falls. Across 45 years of records, the warmest September in Great Falls was 2025 — when the month averaged 18.7°C — while the coldest was 1985 at 9°C. Typical September days reach 21.3°C with nights near 7.5°C. The long-term trend is warming, about 0.9°C per decade.
September is about average for rainfall in Great Falls, averaging 40 mm of rainfall. The wettest September on record was 1985 with 107 mm, while the driest was 2012 at 2 mm.
Relative humidity in Great Falls during September averages 55%. The most humid September was 1986 at 69%, and the least humid was 2012 at 45%.
September receives about 4.3 kWh/m²/day of solar energy in Great Falls. The sunniest September was 1990 at 5.2 kWh/m²/day, and the cloudiest was 1986 at 3.4.
Average wind speed in Great Falls during September is 2.3 m/s. The windiest September was 1992 at 3.1 m/s, while the calmest was 2025 at 1.8 m/s.
Ranked against the other 11 months using 1981–2025 averages.
| Metric | 1980s | 1990s | 2000s | 2010s | 2020s | Change |
|---|---|---|---|---|---|---|
| Avg Temp | 12.4°C | 14.4°C | 14.3°C | 14.6°C | 17.1°C | +4.7°C |
| Avg High | 19.1°C | 21.5°C | 21.4°C | 21.3°C | 24.0°C | +4.9°C |
| Avg Low | 5.7°C | 7.3°C | 7.3°C | 8.0°C | 10.0°C | +4.3°C |
| Avg Rain | 51 mm | 27 mm | 40 mm | 44 mm | 41 mm | -10 mm |
September temperatures in Great Falls have risen by about 5.6°C between 1981–1985 and 2021–2025.
September has ranged from a record low of -7.6°C (2019) to a record high of 36.5°C (2022).
Rainfall in September is about average for rainfall, averaging 40 mm, with a wettest year of 107 mm.
One of the best times to visit Great Falls
September is one of the best times to visit Great Falls. Expect days averaging 21.3°C and nights around 7.5°C (mean 14.4°C) — warmer than Great Falls's yearly average — with moderate rainfall (40 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 🔥 | 25.7°C | 18.7°C | 11.7°C | 31.6°C | 6.4°C | 21 mm | 52% | 1.8 m/s | 4.6 |
| 2024 | 24.6°C | 17.7°C | 10.7°C | 32.7°C | 2.8°C | 75 mm | 57% | 2.4 m/s | 4.3 |
| 2023 | 22.6°C | 16.2°C | 9.8°C | 31.8°C | 4.0°C | 67 mm | 58% | 2.1 m/s | 4.3 |
| 2022 | 24.9°C | 17.8°C | 10.6°C | 36.5°C | 3.5°C | 52 mm | 51% | 2.1 m/s | 4.3 |
| 2021 | 23.7°C | 16.5°C | 9.2°C | 31.7°C | 3.5°C | 8 mm | 47% | 2.5 m/s | 4.5 |
| 2020 | 22.8°C | 15.5°C | 8.2°C | 31.7°C | -0.9°C | 22 mm | 54% | 2.6 m/s | 4.1 |
| 2019 | 19.6°C | 13.8°C | 8.0°C | 30.9°C | -7.6°C | 69 mm | 64% | 2.4 m/s | 3.6 |
| 2018 | 19.0°C | 12.9°C | 6.8°C | 31.3°C | -1.7°C | 37 mm | 62% | 2 m/s | 3.8 |
| 2017 | 20.7°C | 14.2°C | 7.6°C | 32.3°C | -0.5°C | 56 mm | 54% | 2.2 m/s | 4.2 |
| 2016 | 19.6°C | 13.4°C | 7.2°C | 31.1°C | -1.0°C | 72 mm | 60% | 2.5 m/s | 3.9 |
| 2015 | 22.0°C | 15.1°C | 8.1°C | 30.2°C | 2.1°C | 56 mm | 54% | 2.3 m/s | 4.4 |
| 2014 | 21.1°C | 14.2°C | 7.4°C | 30.8°C | -0.6°C | 36 mm | 59% | 2.3 m/s | 4.3 |
| 2013 | 22.6°C | 16.3°C | 10.0°C | 32.5°C | -0.6°C | 41 mm | 57% | 2.6 m/s | 4.1 |
| 2012 | 23.8°C | 16.2°C | 8.7°C | 29.2°C | 2.8°C | 2 mm | 45% | 2 m/s | 4.8 |
| 2011 | 24.6°C | 16.7°C | 8.8°C | 31.3°C | 2.9°C | 14 mm | 49% | 2.3 m/s | 5 |
| 2010 | 19.8°C | 13.4°C | 7.0°C | 27.6°C | 2.2°C | 55 mm | 65% | 2.3 m/s | 4 |
| 2009 | 25.5°C | 17.5°C | 9.5°C | 33.5°C | 1.2°C | 26 mm | 47% | 2.3 m/s | 4.9 |
| 2008 | 20.0°C | 13.0°C | 6.1°C | 28.4°C | 2.6°C | 39 mm | 55% | 2 m/s | 4.2 |
| 2007 | 20.7°C | 13.7°C | 6.8°C | 32.6°C | 0.5°C | 45 mm | 53% | 2.5 m/s | 4.1 |
| 2006 | 21.3°C | 14.3°C | 7.3°C | 30.1°C | 0.2°C | 46 mm | 54% | 2.3 m/s | 4.4 |
| 2005 | 20.7°C | 13.7°C | 6.8°C | 30.9°C | -0.4°C | 41 mm | 55% | 2.6 m/s | 4.2 |
| 2004 | 19.8°C | 13.3°C | 6.7°C | 31.2°C | 0.7°C | 43 mm | 62% | 2.4 m/s | 4.2 |
| 2003 | 20.5°C | 13.9°C | 7.4°C | 33.7°C | -0.5°C | 42 mm | 53% | 2.5 m/s | 4.1 |
| 2002 | 21.2°C | 14.2°C | 7.3°C | 32.6°C | 0.2°C | 38 mm | 55% | 2.4 m/s | 4.4 |
| 2001 | 24.1°C | 16.3°C | 8.5°C | 33.4°C | 3.9°C | 38 mm | 48% | 2 m/s | 4.7 |
| 2000 | 20.3°C | 13.3°C | 6.2°C | 33.8°C | -6.2°C | 40 mm | 52% | 2.8 m/s | 4 |
| 1999 | 18.7°C | 12.0°C | 5.4°C | 28.4°C | -3.1°C | 38 mm | 54% | 2.4 m/s | 4.5 |
| 1998 | 24.9°C | 17.6°C | 10.3°C | 33.6°C | 3.1°C | 19 mm | 51% | 2 m/s | 4.7 |
| 1997 | 23.1°C | 15.7°C | 8.3°C | 30.1°C | 0.7°C | 13 mm | 53% | 2.6 m/s | 4.7 |
| 1996 | 19.1°C | 12.7°C | 6.3°C | 29.3°C | -0.6°C | 44 mm | 58% | 2.4 m/s | 4.1 |
| 1995 | 20.1°C | 13.3°C | 6.6°C | 31.5°C | -4.7°C | 42 mm | 59% | 2.3 m/s | 4.5 |
| 1994 | 24.2°C | 16.5°C | 8.8°C | 31.1°C | 2.2°C | 12 mm | 47% | 2 m/s | 4.7 |
| 1993 | 17.8°C | 11.1°C | 4.5°C | 26.7°C | -1.5°C | 44 mm | 66% | 2.2 m/s | 4.4 |
| 1992 | 20.4°C | 13.4°C | 6.3°C | 29.9°C | 0.4°C | 16 mm | 48% | 3.1 m/s | 4.3 |
| 1991 | 21.6°C | 14.5°C | 7.3°C | 30.7°C | -0.9°C | 33 mm | 53% | 2.1 m/s | 4.3 |
| 1990 | 25.6°C | 17.6°C | 9.6°C | 31.1°C | 2.6°C | 5 mm | 46% | 2.1 m/s | 5.2 |
| 1989 | 20.7°C | 13.3°C | 6.0°C | 27.9°C | -2.4°C | 36 mm | 57% | 2.1 m/s | 4.7 |
| 1988 | 20.6°C | 13.6°C | 6.6°C | 33.6°C | 0.0°C | 66 mm | 52% | 2.5 m/s | 4 |
| 1987 | 23.4°C | 15.7°C | 8.0°C | 30.6°C | 2.0°C | 19 mm | 48% | 2 m/s | 4.7 |
| 1986 | 15.5°C | 10.0°C | 4.6°C | 24.6°C | 0.4°C | 75 mm | 69% | 2.6 m/s | 3.4 |
| 1985 ❄️ 🌧️ | 15.1°C | 9.0°C | 2.9°C | 25.0°C | -5.8°C | 107 mm | 66% | 2.7 m/s | 3.5 |
| 1984 | 17.0°C | 10.6°C | 4.3°C | 30.6°C | -6.6°C | 33 mm | 55% | 2.6 m/s | 4 |
| 1983 | 18.1°C | 11.7°C | 5.3°C | 33.0°C | -5.1°C | 48 mm | 57% | 2.8 m/s | — |
| 1982 | 19.5°C | 12.8°C | 6.1°C | 30.4°C | -2.2°C | 60 mm | 57% | 2.4 m/s | — |
| 1981 | 22.4°C | 15.0°C | 7.7°C | 32.7°C | 0.8°C | 15 mm | 48% | 2.1 m/s | — |
Actual recorded September data for each year, 1981–2025 — sourced from NASA POWER.
The average high in September is around 21.3°C with lows near 7.5°C (mean 14.4°C), based on 45 years of NASA POWER data.
September averages 40 mm of rainfall in Great Falls, about average for rainfall. The wettest on record was 107 mm in 1985.
The hottest September reached 36.5°C in 2022. The coldest dipped to -7.6°C in 2019.
Comparing the earliest years on record (1981–1985) with the most recent (2021–2025), average September temperatures have risen by about 5.6°C. Daytime highs moved from 18.4°C to 24.3°C and overnight lows from 5.3°C to 10.4°C over the same span.
September is one of the warmer months of the year in Great Falls and about average for rainfall, so plan for around 21.3°C daytime highs and 7.5°C at night.
Not quite — September is the 9th coolest month in Great Falls, averaging 14.4°C.
For September in Great Falls, pack warm layers and a jacket for chilly mornings, with typical highs of 21.3°C and lows of 7.5°C.
Relative humidity in September averages 55% — relatively dry.
Average wind speed in September is 2.3 m/s. The windiest September on record averaged 3.1 m/s (1992).
The wettest September on record was 1985 with 107 mm. On average, September sees 40 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 Great Falls 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.
