September is one of the warmer months of the year in Thunder Bay, with average highs near 18°C and lows around 7.9°C (mean 13°C). It is a relatively wet time of year, averaging 69 mm of rainfall and 77% relative humidity across the period.
September is one of the warmer months of the year in Thunder Bay. Across 45 years of records, the warmest September in Thunder Bay was 2024 — when the month averaged 17.7°C — while the coldest was 1993 at 9.3°C. Typical September days reach 18°C with nights near 7.9°C. The long-term trend is warming, about 0.9°C per decade.
September is a relatively wet time of year in Thunder Bay, averaging 69 mm of rainfall. The wettest September on record was 1985 with 140 mm, while the driest was 2012 at 15 mm.
Relative humidity in Thunder Bay during September averages 77%. The most humid September was 2019 at 83%, and the least humid was 2012 at 67%.
September receives about 3.5 kWh/m²/day of solar energy in Thunder Bay. The sunniest September was 2024 at 4.2 kWh/m²/day, and the cloudiest was 1986 at 2.7.
Average wind speed in Thunder Bay during September is 0.7 m/s. The windiest Septembers were 2000, 2006 and 2010, among others at 0.8 m/s, while the calmest was 1994 at 0.5 m/s.
Ranked against the other 11 months using 1981–2025 averages.
| Metric | 1980s | 1990s | 2000s | 2010s | 2020s | Change |
|---|---|---|---|---|---|---|
| Avg Temp | 11.6°C | 12.2°C | 13.4°C | 13.4°C | 14.9°C | +3.3°C |
| Avg High | 16.4°C | 17.3°C | 18.6°C | 18.4°C | 20.2°C | +3.8°C |
| Avg Low | 6.8°C | 7.1°C | 8.2°C | 8.4°C | 9.7°C | +2.9°C |
| Avg Rain | 76 mm | 70 mm | 66 mm | 74 mm | 51 mm | -25 mm |
September temperatures in Thunder Bay have risen by about 4.2°C between 1981–1985 and 2021–2025.
September has ranged from a record low of -4.2°C (1991) to a record high of 32.2°C (2023).
Rainfall in September is a relatively wet time of year, averaging 69 mm, with a wettest year of 140 mm.
One of the best times to visit Thunder Bay
September is one of the best times to visit Thunder Bay. Expect days averaging 18°C and nights around 7.9°C (mean 13°C) — warmer than Thunder Bay's yearly average — with moderate rainfall (69 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 | 20.8°C | 15.5°C | 10.2°C | 26.5°C | 3.1°C | 33 mm | 76% | 0.6 m/s | 3.5 |
| 2024 🔥 | 23.6°C | 17.7°C | 11.8°C | 29.1°C | 3.7°C | 16 mm | 72% | 0.6 m/s | 4.2 |
| 2023 | 20.9°C | 15.9°C | 10.9°C | 32.2°C | 4.9°C | 52 mm | 78% | 0.6 m/s | 3.6 |
| 2022 | 18.7°C | 13.4°C | 8.1°C | 28.6°C | -0.7°C | 93 mm | 77% | 0.7 m/s | 3.6 |
| 2021 | 20.3°C | 15.1°C | 9.8°C | 25.9°C | 5.4°C | 62 mm | 75% | 0.7 m/s | 3.9 |
| 2020 | 16.9°C | 12.0°C | 7.1°C | 23.0°C | -0.6°C | 52 mm | 75% | 0.7 m/s | 3.6 |
| 2019 | 17.4°C | 13.1°C | 8.8°C | 24.4°C | 3.4°C | 128 mm | 83% | 0.7 m/s | 2.9 |
| 2018 | 18.2°C | 13.1°C | 8.1°C | 28.7°C | -1.2°C | 103 mm | 77% | 0.7 m/s | 3.4 |
| 2017 | 18.8°C | 14.0°C | 9.1°C | 26.7°C | 4.4°C | 85 mm | 82% | 0.7 m/s | 3.3 |
| 2016 | 20.2°C | 15.5°C | 10.8°C | 25.1°C | 5.8°C | 59 mm | 78% | 0.7 m/s | 3.4 |
| 2015 | 20.9°C | 15.8°C | 10.6°C | 27.4°C | 3.7°C | 59 mm | 76% | 0.6 m/s | 3.9 |
| 2014 | 16.6°C | 11.7°C | 6.8°C | 23.9°C | 0.1°C | 65 mm | 81% | 0.7 m/s | 3.4 |
| 2013 | 18.9°C | 13.5°C | 8.0°C | 25.9°C | 2.0°C | 88 mm | 78% | 0.7 m/s | 3.8 |
| 2012 | 18.2°C | 12.9°C | 7.6°C | 28.2°C | 1.1°C | 15 mm | 67% | 0.7 m/s | 3.8 |
| 2011 | 19.1°C | 13.4°C | 7.6°C | 28.9°C | 1.1°C | 37 mm | 74% | 0.7 m/s | 3.9 |
| 2010 | 15.3°C | 10.8°C | 6.3°C | 23.3°C | 2.5°C | 106 mm | 76% | 0.8 m/s | 3.3 |
| 2009 | 21.3°C | 15.8°C | 10.3°C | 25.5°C | -1.3°C | 27 mm | 80% | 0.6 m/s | 4.1 |
| 2008 | 17.5°C | 12.6°C | 7.7°C | 25.8°C | 2.7°C | 95 mm | 80% | 0.7 m/s | 3.5 |
| 2007 | 17.9°C | 13.2°C | 8.5°C | 28.5°C | 0.1°C | 123 mm | 79% | 0.7 m/s | 3.2 |
| 2006 | 18.5°C | 12.9°C | 7.3°C | 27.9°C | 1.4°C | 71 mm | 75% | 0.8 m/s | 3.7 |
| 2005 | 20.5°C | 15.0°C | 9.4°C | 31.4°C | 2.1°C | 48 mm | 74% | 0.7 m/s | 3.7 |
| 2004 | 19.0°C | 13.8°C | 8.6°C | 27.4°C | 3.8°C | 81 mm | 81% | 0.6 m/s | 3.7 |
| 2003 | 17.6°C | 13.0°C | 8.4°C | 24.2°C | -0.6°C | 57 mm | 80% | 0.7 m/s | 3.3 |
| 2002 | 19.0°C | 13.9°C | 8.7°C | 29.2°C | 0.9°C | 74 mm | 79% | 0.6 m/s | 3.7 |
| 2001 | 17.9°C | 12.6°C | 7.3°C | 24.4°C | 0.7°C | 29 mm | 78% | 0.7 m/s | 3.9 |
| 2000 | 16.7°C | 11.3°C | 5.9°C | 25.3°C | -2.6°C | 51 mm | 72% | 0.8 m/s | 3.8 |
| 1999 | 17.0°C | 12.3°C | 7.5°C | 26.5°C | -0.1°C | 83 mm | 81% | 0.7 m/s | 3.3 |
| 1998 | 20.1°C | 14.8°C | 9.4°C | 31.2°C | 2.4°C | 85 mm | 72% | 0.7 m/s | 3.8 |
| 1997 | 19.0°C | 13.8°C | 8.6°C | 24.8°C | 0.5°C | 40 mm | 72% | 0.7 m/s | 3.7 |
| 1996 | 17.7°C | 13.0°C | 8.3°C | 26.8°C | 0.3°C | 72 mm | 77% | 0.6 m/s | 3.6 |
| 1995 | 17.3°C | 11.7°C | 6.1°C | 29.9°C | -3.0°C | 69 mm | 72% | 0.7 m/s | 3.7 |
| 1994 | 19.4°C | 13.9°C | 8.4°C | 26.7°C | 2.0°C | 59 mm | 77% | 0.5 m/s | 3.4 |
| 1993 ❄️ | 14.4°C | 9.3°C | 4.3°C | 21.2°C | -1.4°C | 47 mm | 74% | 0.7 m/s | 3.1 |
| 1992 | 16.0°C | 11.1°C | 6.1°C | 23.8°C | -1.1°C | 77 mm | 76% | 0.7 m/s | 3.2 |
| 1991 | 15.6°C | 10.8°C | 6.1°C | 24.2°C | -4.2°C | 95 mm | 75% | 0.8 m/s | 3.4 |
| 1990 | 16.7°C | 11.7°C | 6.6°C | 26.3°C | -0.1°C | 73 mm | 77% | 0.6 m/s | 3.6 |
| 1989 | 17.8°C | 12.7°C | 7.6°C | 24.2°C | 0.2°C | 54 mm | 71% | 0.7 m/s | 4 |
| 1988 | 16.6°C | 11.9°C | 7.2°C | 23.4°C | 1.2°C | 54 mm | 77% | 0.7 m/s | 3.1 |
| 1987 | 17.9°C | 13.2°C | 8.4°C | 23.5°C | 1.7°C | 84 mm | 77% | 0.6 m/s | 3.5 |
| 1986 | 15.0°C | 10.5°C | 6.0°C | 21.5°C | 0.5°C | 97 mm | 80% | 0.6 m/s | 2.7 |
| 1985 🌧️ | 15.7°C | 10.9°C | 6.2°C | 25.0°C | -0.9°C | 140 mm | 79% | 0.7 m/s | 3.4 |
| 1984 | 14.5°C | 10.0°C | 5.5°C | 25.5°C | -1.0°C | 61 mm | 77% | 0.7 m/s | 3.2 |
| 1983 | 17.8°C | 13.3°C | 8.7°C | 31.1°C | -1.2°C | 83 mm | 75% | 0.6 m/s | — |
| 1982 | 16.0°C | 11.0°C | 6.0°C | 25.1°C | -1.0°C | 65 mm | 81% | 0.6 m/s | — |
| 1981 | 16.3°C | 11.2°C | 6.0°C | 25.9°C | -1.1°C | 49 mm | 72% | 0.8 m/s | — |
Actual recorded September data for each year, 1981–2025 — sourced from NASA POWER.
The average high in September is around 18°C with lows near 7.9°C (mean 13°C), based on 45 years of NASA POWER data.
September averages 69 mm of rainfall in Thunder Bay, a relatively wet time of year. The wettest on record was 140 mm in 1985.
The hottest September reached 32.2°C in 2023. The coldest dipped to -4.2°C in 1991.
Comparing the earliest years on record (1981–1985) with the most recent (2021–2025), average September temperatures have risen by about 4.2°C. Daytime highs moved from 16.1°C to 20.9°C and overnight lows from 6.5°C to 10.2°C over the same span.
September is one of the warmer months of the year in Thunder Bay and a relatively wet time of year, so plan for around 18°C daytime highs and 7.9°C at night.
Not quite — September is the 9th coolest month in Thunder Bay, averaging 13°C.
For September in Thunder Bay, pack warm layers and a jacket for chilly mornings, with typical highs of 18°C and lows of 7.9°C. Bring an umbrella or light rain jacket too.
Relative humidity in September averages 77% — quite humid.
Average wind speed in September is 0.7 m/s. The windiest September on record averaged 0.8 m/s (1981).
The wettest September on record was 1985 with 140 mm. On average, September sees 69 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 Thunder Bay 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.
