September is one of the warmer months of the year in Helena, with average highs near 19.6°C and lows around 4.6°C (mean 12.1°C). It is about average for rainfall, averaging 40 mm of rainfall and 58% relative humidity across the period.
September is one of the warmer months of the year in Helena. Across 45 years of records, the warmest September in Helena was 2025 — when the month averaged 16°C — while the coldest was 1985 at 7.2°C. Typical September days reach 19.6°C with nights near 4.6°C. The long-term trend is warming, about 0.8°C per decade.
September is about average for rainfall in Helena, averaging 40 mm of rainfall. The wettest September on record was 1985 with 90 mm, while the driest was 2012 at 4 mm.
Relative humidity in Helena during September averages 58%. The most humid September was 1986 at 71%, and the least humid was 1994 at 46%.
September receives about 4.5 kWh/m²/day of solar energy in Helena. The sunniest September was 1990 at 5.3 kWh/m²/day, and the cloudiest was 1986 at 3.5.
Average wind speed in Helena during September is 1.2 m/s. The windiest September was 1992 at 1.5 m/s, while the calmest were 1998, 2018 and 2025, among others at 1 m/s.
Ranked against the other 11 months using 1981–2025 averages.
| Metric | 1980s | 1990s | 2000s | 2010s | 2020s | Change |
|---|---|---|---|---|---|---|
| Avg Temp | 10.3°C | 12.2°C | 11.9°C | 12.4°C | 14.5°C | +4.2°C |
| Avg High | 17.6°C | 19.9°C | 19.7°C | 19.5°C | 22.1°C | +4.5°C |
| Avg Low | 2.9°C | 4.6°C | 4.2°C | 5.3°C | 7.0°C | +4.1°C |
| Avg Rain | 47 mm | 26 mm | 39 mm | 45 mm | 43 mm | -3 mm |
September temperatures in Helena have risen by about 5.2°C between 1981–1985 and 2021–2025.
September has ranged from a record low of -9°C (1983) to a record high of 34.1°C (2022).
Rainfall in September is about average for rainfall, averaging 40 mm, with a wettest year of 90 mm.
One of the best times to visit Helena
September is one of the best times to visit Helena. Expect days averaging 19.6°C and nights around 4.6°C (mean 12.1°C) — warmer than Helena'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 🔥 | 23.4°C | 16.0°C | 8.7°C | 30.8°C | 3.2°C | 25 mm | 55% | 1 m/s | 4.6 |
| 2024 | 22.8°C | 15.2°C | 7.6°C | 31.4°C | -1.4°C | 65 mm | 58% | 1.1 m/s | 4.2 |
| 2023 | 20.3°C | 13.4°C | 6.5°C | 28.4°C | 1.3°C | 82 mm | 64% | 1.1 m/s | 4.3 |
| 2022 | 22.9°C | 15.4°C | 7.9°C | 34.1°C | 1.4°C | 53 mm | 54% | 1.1 m/s | 4.5 |
| 2021 | 21.8°C | 13.8°C | 5.7°C | 30.0°C | -0.8°C | 8 mm | 51% | 1.2 m/s | 4.9 |
| 2020 | 21.3°C | 13.3°C | 5.4°C | 30.9°C | -2.8°C | 28 mm | 57% | 1.3 m/s | 4.3 |
| 2019 | 17.9°C | 11.8°C | 5.7°C | 29.2°C | -6.3°C | 74 mm | 69% | 1.2 m/s | 3.8 |
| 2018 | 17.5°C | 10.9°C | 4.4°C | 27.8°C | -2.3°C | 25 mm | 66% | 1 m/s | 4.6 |
| 2017 | 18.7°C | 11.8°C | 5.0°C | 30.3°C | -2.9°C | 64 mm | 60% | 1.1 m/s | 3.8 |
| 2016 | 17.8°C | 10.9°C | 4.1°C | 29.5°C | -2.9°C | 51 mm | 65% | 1.3 m/s | 4 |
| 2015 | 20.5°C | 13.1°C | 5.6°C | 27.8°C | 1.4°C | 56 mm | 59% | 1.2 m/s | 4.5 |
| 2014 | 19.3°C | 11.9°C | 4.6°C | 27.4°C | -4.4°C | 39 mm | 63% | 1.2 m/s | 4.6 |
| 2013 | 19.9°C | 13.4°C | 7.0°C | 30.7°C | -5.6°C | 71 mm | 64% | 1.2 m/s | 3.8 |
| 2012 | 22.4°C | 14.0°C | 5.7°C | 27.9°C | 0.5°C | 4 mm | 47% | 1.1 m/s | 5 |
| 2011 | 22.4°C | 14.1°C | 5.8°C | 27.7°C | 1.1°C | 14 mm | 54% | 1.1 m/s | 5.1 |
| 2010 | 18.8°C | 11.7°C | 4.7°C | 25.2°C | -0.3°C | 50 mm | 67% | 1.2 m/s | 4.4 |
| 2009 | 23.0°C | 14.6°C | 6.3°C | 29.6°C | -0.8°C | 23 mm | 53% | 1.2 m/s | 5.1 |
| 2008 | 18.1°C | 10.7°C | 3.3°C | 26.3°C | -1.1°C | 34 mm | 57% | 1.1 m/s | 4.6 |
| 2007 | 18.9°C | 11.1°C | 3.3°C | 31.1°C | -4.6°C | 49 mm | 58% | 1.2 m/s | 4.4 |
| 2006 | 19.4°C | 11.9°C | 4.5°C | 29.2°C | -2.0°C | 44 mm | 57% | 1.2 m/s | 4.4 |
| 2005 | 19.3°C | 11.6°C | 3.8°C | 29.2°C | -2.2°C | 44 mm | 59% | 1.2 m/s | 4.4 |
| 2004 | 17.6°C | 10.7°C | 3.8°C | 29.4°C | -3.7°C | 56 mm | 67% | 1.2 m/s | 4.1 |
| 2003 | 19.7°C | 11.9°C | 4.1°C | 30.6°C | -3.4°C | 24 mm | 54% | 1.2 m/s | 4.7 |
| 2002 | 19.3°C | 11.8°C | 4.2°C | 29.8°C | -2.1°C | 37 mm | 58% | 1.2 m/s | 4.7 |
| 2001 | 23.0°C | 14.4°C | 5.9°C | 30.4°C | -0.9°C | 38 mm | 49% | 1.1 m/s | 5.1 |
| 2000 | 18.5°C | 10.7°C | 3.0°C | 31.7°C | -7.4°C | 44 mm | 58% | 1.4 m/s | 4.1 |
| 1999 | 18.0°C | 10.4°C | 2.8°C | 25.9°C | -4.6°C | 17 mm | 53% | 1.2 m/s | 4.9 |
| 1998 | 22.4°C | 14.9°C | 7.3°C | 31.4°C | 0.6°C | 21 mm | 56% | 1 m/s | 4.8 |
| 1997 | 20.5°C | 12.9°C | 5.3°C | 26.4°C | -0.6°C | 24 mm | 63% | 1.2 m/s | 4.6 |
| 1996 | 17.1°C | 10.0°C | 2.9°C | 26.2°C | -4.9°C | 31 mm | 62% | 1.2 m/s | 4.1 |
| 1995 | 18.6°C | 11.4°C | 4.2°C | 29.8°C | -5.8°C | 54 mm | 63% | 1.1 m/s | 4.4 |
| 1994 | 22.6°C | 14.5°C | 6.3°C | 29.2°C | 0.5°C | 5 mm | 46% | 1.1 m/s | 4.9 |
| 1993 | 16.9°C | 9.5°C | 2.1°C | 24.9°C | -3.8°C | 32 mm | 66% | 1.1 m/s | 4.9 |
| 1992 | 18.5°C | 10.7°C | 2.9°C | 27.0°C | -3.4°C | 20 mm | 52% | 1.5 m/s | 4.5 |
| 1991 | 20.1°C | 12.5°C | 5.0°C | 28.5°C | -1.4°C | 46 mm | 56% | 1.1 m/s | 4.6 |
| 1990 | 23.9°C | 15.3°C | 6.8°C | 29.1°C | 1.3°C | 7 mm | 49% | 1 m/s | 5.3 |
| 1989 | 19.6°C | 11.4°C | 3.3°C | 25.9°C | -3.2°C | 29 mm | 56% | 1.1 m/s | 4.9 |
| 1988 | 18.7°C | 11.2°C | 3.6°C | 30.8°C | -2.6°C | 52 mm | 54% | 1.2 m/s | 4.4 |
| 1987 | 22.0°C | 13.7°C | 5.3°C | 29.5°C | -1.3°C | 13 mm | 48% | 1.1 m/s | 4.9 |
| 1986 | 14.4°C | 8.2°C | 2.1°C | 24.4°C | -2.8°C | 63 mm | 71% | 1.2 m/s | 3.5 |
| 1985 ❄️ 🌧️ | 13.6°C | 7.2°C | 0.8°C | 23.6°C | -8.4°C | 90 mm | 69% | 1.3 m/s | 4 |
| 1984 | 15.7°C | 8.8°C | 1.8°C | 27.8°C | -6.7°C | 31 mm | 57% | 1.3 m/s | 4.1 |
| 1983 | 16.4°C | 9.1°C | 1.7°C | 29.6°C | -9.0°C | 53 mm | 64% | 1.3 m/s | — |
| 1982 | 17.2°C | 10.1°C | 2.9°C | 28.3°C | -4.4°C | 65 mm | 62% | 1.2 m/s | — |
| 1981 | 20.4°C | 12.7°C | 4.9°C | 30.3°C | -3.4°C | 26 mm | 52% | 1.1 m/s | — |
Actual recorded September data for each year, 1981–2025 — sourced from NASA POWER.
The average high in September is around 19.6°C with lows near 4.6°C (mean 12.1°C), based on 45 years of NASA POWER data.
September averages 40 mm of rainfall in Helena, about average for rainfall. The wettest on record was 90 mm in 1985.
The hottest September reached 34.1°C in 2022. The coldest dipped to -9°C in 1983.
Comparing the earliest years on record (1981–1985) with the most recent (2021–2025), average September temperatures have risen by about 5.2°C. Daytime highs moved from 16.7°C to 22.2°C and overnight lows from 2.4°C to 7.3°C over the same span.
September is one of the warmer months of the year in Helena and about average for rainfall, so plan for around 19.6°C daytime highs and 4.6°C at night.
Not quite — September is the 9th coolest month in Helena, averaging 12.1°C.
For September in Helena, pack warm layers and a jacket for chilly mornings, with typical highs of 19.6°C and lows of 4.6°C.
Relative humidity in September averages 58% — moderately humid.
Average wind speed in September is 1.2 m/s. The windiest September on record averaged 1.5 m/s (1992).
The wettest September on record was 1985 with 90 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 Helena 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.
