September is one of the warmer months of the year in Boise, with average highs near 26°C and lows around 11.4°C (mean 18.7°C). It is one of the drier months, averaging 16 mm of rainfall and 35% relative humidity across the period.
September is one of the warmer months of the year in Boise. Across 45 years of records, the warmest September in Boise was 2022 — when the month averaged 22.1°C — while the coldest was 1985 at 13.2°C. Typical September days reach 26°C with nights near 11.4°C. The long-term trend is warming, about 0.7°C per decade.
September is one of the drier months in Boise, averaging 16 mm of rainfall. The wettest September on record was 1986 with 71 mm, while the driest was 1987 at 0 mm.
Relative humidity in Boise during September averages 35%. The most humid September was 1986 at 53%, and the least humid was 1987 at 23%.
September receives about 5.2 kWh/m²/day of solar energy in Boise. The sunniest Septembers were 1990, 1993 and 1999 at 5.7 kWh/m²/day, and the cloudiest was 1986 at 4.4.
Average wind speed in Boise during September is 2 m/s. The windiest Septembers were 2007 and 2013 at 2.3 m/s, while the calmest was 1998 at 1.6 m/s.
Ranked against the other 11 months using 1981–2025 averages.
| Metric | 1980s | 1990s | 2000s | 2010s | 2020s | Change |
|---|---|---|---|---|---|---|
| Avg Temp | 16.7°C | 18.9°C | 18.6°C | 19.2°C | 20.6°C | +3.9°C |
| Avg High | 23.9°C | 26.3°C | 26.1°C | 26.3°C | 28.0°C | +4.1°C |
| Avg Low | 9.4°C | 11.5°C | 11.2°C | 12.1°C | 13.2°C | +3.8°C |
| Avg Rain | 26 mm | 10 mm | 12 mm | 17 mm | 15 mm | -11 mm |
September temperatures in Boise have risen by about 4.3°C between 1981–1985 and 2021–2025.
September has ranged from a record low of -1.2°C (1985) to a record high of 40.2°C (2022).
Rainfall in September is one of the drier months, averaging 16 mm, with a wettest year of 71 mm.
One of the best times to visit Boise
September is one of the best times to visit Boise. Expect days averaging 26°C and nights around 11.4°C (mean 18.7°C) — warmer than Boise's yearly average — with low rainfall (16 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 | 28.6°C | 21.4°C | 14.3°C | 37.0°C | 8.7°C | 23 mm | 38% | 1.8 m/s | 5 |
| 2024 | 28.2°C | 20.8°C | 13.4°C | 36.8°C | 4.3°C | 17 mm | 36% | 1.9 m/s | 5 |
| 2023 | 26.0°C | 19.1°C | 12.1°C | 32.5°C | 5.4°C | 19 mm | 41% | 1.9 m/s | 5 |
| 2022 🔥 | 29.7°C | 22.1°C | 14.6°C | 40.2°C | 9.3°C | 10 mm | 30% | 1.9 m/s | 5.2 |
| 2021 | 27.1°C | 19.3°C | 11.5°C | 36.5°C | 5.0°C | 17 mm | 32% | 2 m/s | 5.2 |
| 2020 | 28.2°C | 20.6°C | 13.1°C | 37.5°C | 6.3°C | 6 mm | 29% | 1.9 m/s | 5.1 |
| 2019 | 24.4°C | 17.8°C | 11.3°C | 36.6°C | 0.9°C | 27 mm | 46% | 2 m/s | 4.8 |
| 2018 | 26.1°C | 18.8°C | 11.5°C | 35.5°C | 4.5°C | 1 mm | 28% | 2 m/s | 5.4 |
| 2017 | 25.2°C | 18.6°C | 12.0°C | 38.5°C | 2.2°C | 28 mm | 40% | 1.8 m/s | 4.7 |
| 2016 | 24.5°C | 17.6°C | 10.7°C | 31.5°C | 6.7°C | 16 mm | 39% | 2.2 m/s | 5 |
| 2015 | 26.7°C | 19.5°C | 12.3°C | 34.6°C | 6.2°C | 20 mm | 36% | 1.7 m/s | 5.1 |
| 2014 | 27.3°C | 20.1°C | 12.8°C | 34.4°C | 5.7°C | 27 mm | 36% | 2 m/s | 5.2 |
| 2013 | 24.5°C | 18.5°C | 12.4°C | 35.7°C | 1.6°C | 47 mm | 49% | 2.3 m/s | 4.5 |
| 2012 | 28.2°C | 20.6°C | 12.9°C | 33.0°C | 6.0°C | 1 mm | 26% | 1.9 m/s | 5.3 |
| 2011 | 29.4°C | 21.6°C | 13.9°C | 34.8°C | 8.1°C | 4 mm | 29% | 1.8 m/s | 5.6 |
| 2010 | 26.6°C | 19.1°C | 11.5°C | 33.4°C | 5.5°C | 3 mm | 33% | 1.9 m/s | 5.4 |
| 2009 | 28.4°C | 20.8°C | 13.2°C | 34.0°C | 2.9°C | 2 mm | 30% | 2.1 m/s | 5.5 |
| 2008 | 26.1°C | 18.6°C | 11.0°C | 32.9°C | 4.9°C | 20 mm | 31% | 1.9 m/s | 5.5 |
| 2007 | 25.0°C | 17.6°C | 10.3°C | 38.4°C | 1.8°C | 25 mm | 36% | 2.3 m/s | 5.2 |
| 2006 | 25.7°C | 18.4°C | 11.1°C | 37.1°C | 3.3°C | 9 mm | 32% | 2 m/s | 5.2 |
| 2005 | 25.1°C | 17.6°C | 10.1°C | 35.5°C | 3.7°C | 8 mm | 33% | 1.8 m/s | 5.5 |
| 2004 | 24.6°C | 17.4°C | 10.3°C | 33.3°C | 3.3°C | 12 mm | 42% | 2.1 m/s | 5.2 |
| 2003 | 26.7°C | 19.3°C | 12.0°C | 35.3°C | 4.9°C | 2 mm | 30% | 2 m/s | 5.3 |
| 2002 | 26.3°C | 18.8°C | 11.2°C | 35.0°C | 3.7°C | 8 mm | 31% | 2 m/s | 5.3 |
| 2001 | 28.6°C | 20.7°C | 12.9°C | 34.8°C | 7.0°C | 11 mm | 30% | 2 m/s | 5.4 |
| 2000 | 24.6°C | 17.2°C | 9.9°C | 37.3°C | 1.3°C | 18 mm | 39% | 2.2 m/s | 5 |
| 1999 | 25.8°C | 18.0°C | 10.2°C | 32.6°C | -0.4°C | 1 mm | 25% | 2.1 m/s | 5.7 |
| 1998 | 27.5°C | 20.6°C | 13.7°C | 38.2°C | 6.0°C | 30 mm | 42% | 1.6 m/s | 5.2 |
| 1997 | 26.6°C | 19.3°C | 12.1°C | 34.8°C | 4.5°C | 20 mm | 40% | 2 m/s | 5.1 |
| 1996 | 23.5°C | 16.5°C | 9.4°C | 34.2°C | 3.1°C | 14 mm | 38% | 2.1 m/s | 5.1 |
| 1995 | 26.3°C | 19.2°C | 12.0°C | 36.1°C | 4.1°C | 6 mm | 35% | 2 m/s | 5.4 |
| 1994 | 27.9°C | 20.2°C | 12.6°C | 35.0°C | 7.4°C | 6 mm | 27% | 1.9 m/s | 5.4 |
| 1993 | 25.2°C | 17.6°C | 9.9°C | 33.7°C | 2.0°C | 1 mm | 33% | 1.9 m/s | 5.7 |
| 1992 | 24.8°C | 17.4°C | 10.0°C | 33.9°C | 4.3°C | 6 mm | 33% | 2.2 m/s | 5 |
| 1991 | 25.9°C | 18.8°C | 11.7°C | 33.1°C | 3.3°C | 17 mm | 37% | 1.8 m/s | 5.4 |
| 1990 | 29.9°C | 21.7°C | 13.5°C | 35.9°C | 8.6°C | 3 mm | 30% | 1.8 m/s | 5.7 |
| 1989 | 25.3°C | 17.5°C | 9.8°C | 30.8°C | 4.6°C | 26 mm | 37% | 1.9 m/s | 5.6 |
| 1988 | 25.0°C | 17.4°C | 9.9°C | 37.4°C | 1.6°C | 8 mm | 30% | 2.1 m/s | 5.3 |
| 1987 | 27.6°C | 19.9°C | 12.2°C | 36.9°C | 5.0°C | 0 mm | 23% | 1.8 m/s | 5.6 |
| 1986 🌧️ | 19.8°C | 14.0°C | 8.2°C | 33.9°C | 1.7°C | 71 mm | 53% | 1.8 m/s | 4.4 |
| 1985 ❄️ | 19.9°C | 13.2°C | 6.4°C | 27.5°C | -1.2°C | 59 mm | 50% | 2 m/s | 4.8 |
| 1984 | 23.8°C | 16.2°C | 8.6°C | 34.0°C | -0.7°C | 14 mm | 39% | 2 m/s | 5.1 |
| 1983 | 23.9°C | 16.4°C | 8.9°C | 32.4°C | 0.3°C | 15 mm | 41% | 2.2 m/s | — |
| 1982 | 23.3°C | 16.4°C | 9.4°C | 34.6°C | 0.7°C | 36 mm | 47% | 2.2 m/s | — |
| 1981 | 26.2°C | 18.9°C | 11.5°C | 36.2°C | 3.3°C | 8 mm | 32% | 2 m/s | — |
Actual recorded September data for each year, 1981–2025 — sourced from NASA POWER.
The average high in September is around 26°C with lows near 11.4°C (mean 18.7°C), based on 45 years of NASA POWER data.
September averages 16 mm of rainfall in Boise, one of the drier months. The wettest on record was 71 mm in 1986.
The hottest September reached 40.2°C in 2022. The coldest dipped to -1.2°C in 1985.
Comparing the earliest years on record (1981–1985) with the most recent (2021–2025), average September temperatures have risen by about 4.3°C. Daytime highs moved from 23.4°C to 27.9°C and overnight lows from 9°C to 13.2°C over the same span.
September is one of the warmer months of the year in Boise and one of the drier months, so plan for around 26°C daytime highs and 11.4°C at night.
Not quite — September is the 9th coolest month in Boise, averaging 18.7°C.
For September in Boise, pack light layers you can adjust through the day, with typical highs of 26°C and lows of 11.4°C.
Relative humidity in September averages 35% — relatively dry.
Average wind speed in September is 2 m/s. The windiest September on record averaged 2.3 m/s (2007).
The wettest September on record was 1986 with 71 mm. On average, September sees 16 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 Boise 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.
