September is one of the warmer months of the year in Watertown, with average highs near 20.7°C and lows around 10.6°C (mean 15.7°C). It is about average for rainfall, averaging 97 mm of rainfall and 78% relative humidity across the period.
September is one of the warmer months of the year in Watertown. Across 45 years of records, the warmest September in Watertown was 2015 — when the month averaged 18.6°C — while the coldest was 1995 at 13.3°C. Typical September days reach 20.7°C with nights near 10.6°C. The long-term trend is warming, about 0.7°C per decade.
September is about average for rainfall in Watertown, averaging 97 mm of rainfall. The wettest September on record was 1981 with 174 mm, while the driest was 2023 at 34 mm.
Relative humidity in Watertown during September averages 78%. The most humid September was 1981 at 85%, and the least humid was 2016 at 71%.
September receives about 4 kWh/m²/day of solar energy in Watertown. The sunniest September was 2016 at 4.5 kWh/m²/day, and the cloudiest were 1990 and 2006 at 3.3.
Average wind speed in Watertown during September is 0.3 m/s. The windiest September was 1991 at 0.4 m/s, while the calmest were 2023, 2024 and 2025, among others at 0.3 m/s.
Ranked against the other 11 months using 1981–2025 averages.
| Metric | 1980s | 1990s | 2000s | 2010s | 2020s | Change |
|---|---|---|---|---|---|---|
| Avg Temp | 14.6°C | 14.6°C | 16.0°C | 16.7°C | 16.7°C | +2.1°C |
| Avg High | 19.5°C | 19.5°C | 21.1°C | 21.8°C | 21.9°C | +2.4°C |
| Avg Low | 9.7°C | 9.7°C | 10.8°C | 11.5°C | 11.5°C | +1.8°C |
| Avg Rain | 116 mm | 105 mm | 96 mm | 91 mm | 64 mm | -51 mm |
September temperatures in Watertown have risen by about 2.1°C between 1981–1985 and 2021–2025.
September has ranged from a record low of -2.7°C (1991) to a record high of 32.1°C (1999).
Rainfall in September is about average for rainfall, averaging 97 mm, with a wettest year of 174 mm.
Daytime highs and overnight lows have shifted differently — highs rose about 0.8°C per decade while lows rose 0.6°C per decade, widening the gap between day and night.
One of the best times to visit Watertown
September is one of the best times to visit Watertown. Expect days averaging 20.7°C and nights around 10.6°C (mean 15.7°C) — warmer than Watertown's yearly average — with higher rainfall (97 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 | 22.7°C | 16.9°C | 11.2°C | 26.4°C | 4.8°C | 60 mm | 73% | 0.3 m/s | 4.4 |
| 2024 | 22.8°C | 17.5°C | 12.2°C | 27.6°C | 7.2°C | 59 mm | 79% | 0.3 m/s | 4.4 |
| 2023 | 22.8°C | 17.3°C | 11.9°C | 30.1°C | 7.1°C | 34 mm | 77% | 0.3 m/s | 4.3 |
| 2022 | 20.6°C | 15.8°C | 11.1°C | 27.6°C | 3.8°C | 110 mm | 80% | 0.3 m/s | 3.6 |
| 2021 | 21.4°C | 16.7°C | 12.0°C | 26.1°C | 6.4°C | 73 mm | 78% | 0.3 m/s | 3.7 |
| 2020 | 21.0°C | 15.9°C | 10.8°C | 27.6°C | 1.2°C | 50 mm | 74% | 0.3 m/s | 3.9 |
| 2019 | 21.4°C | 16.1°C | 10.8°C | 26.7°C | 6.9°C | 79 mm | 77% | 0.3 m/s | 4.1 |
| 2018 | 23.2°C | 18.0°C | 12.7°C | 31.5°C | 5.9°C | 77 mm | 76% | 0.3 m/s | 4 |
| 2017 | 22.9°C | 17.5°C | 12.1°C | 31.8°C | 5.6°C | 61 mm | 77% | 0.3 m/s | 4.4 |
| 2016 | 23.5°C | 17.7°C | 11.9°C | 30.8°C | 3.9°C | 53 mm | 71% | 0.3 m/s | 4.5 |
| 2015 🔥 | 23.8°C | 18.6°C | 13.4°C | 29.5°C | 7.3°C | 98 mm | 75% | 0.3 m/s | 4.3 |
| 2014 | 20.8°C | 15.6°C | 10.3°C | 27.0°C | 0.4°C | 62 mm | 79% | 0.3 m/s | 4 |
| 2013 | 19.7°C | 14.5°C | 9.3°C | 28.7°C | 2.0°C | 82 mm | 77% | 0.3 m/s | 4.1 |
| 2012 | 20.6°C | 15.7°C | 10.8°C | 28.1°C | 5.8°C | 122 mm | 75% | 0.3 m/s | 3.9 |
| 2011 | 21.6°C | 17.0°C | 12.3°C | 28.6°C | 3.3°C | 119 mm | 82% | 0.3 m/s | 3.9 |
| 2010 | 20.3°C | 15.9°C | 11.5°C | 29.5°C | 5.2°C | 155 mm | 79% | 0.3 m/s | 3.4 |
| 2009 | 19.9°C | 14.8°C | 9.8°C | 24.5°C | 2.9°C | 71 mm | 79% | 0.3 m/s | 4.3 |
| 2008 | 20.9°C | 15.9°C | 10.9°C | 27.8°C | 3.1°C | 82 mm | 82% | 0.3 m/s | 3.9 |
| 2007 | 22.3°C | 16.7°C | 11.1°C | 30.2°C | 4.0°C | 81 mm | 75% | 0.3 m/s | 4.4 |
| 2006 | 19.2°C | 14.5°C | 9.8°C | 24.8°C | 4.0°C | 128 mm | 83% | 0.3 m/s | 3.3 |
| 2005 | 21.9°C | 16.9°C | 11.8°C | 27.2°C | 4.4°C | 124 mm | 78% | 0.3 m/s | 4.3 |
| 2004 | 21.9°C | 16.8°C | 11.6°C | 27.1°C | 4.7°C | 86 mm | 81% | 0.3 m/s | 4.4 |
| 2003 | 21.4°C | 16.5°C | 11.5°C | 27.3°C | 6.2°C | 86 mm | 78% | 0.3 m/s | 4 |
| 2002 | 23.6°C | 18.0°C | 12.3°C | 31.8°C | 5.1°C | 104 mm | 71% | 0.3 m/s | 4.3 |
| 2001 | 20.9°C | 15.7°C | 10.5°C | 29.6°C | 2.7°C | 101 mm | 74% | 0.3 m/s | 4.2 |
| 2000 | 19.2°C | 14.1°C | 9.0°C | 28.3°C | -1.4°C | 94 mm | 78% | 0.3 m/s | 3.9 |
| 1999 | 22.4°C | 17.2°C | 12.1°C | 32.1°C | 4.9°C | 153 mm | 75% | 0.3 m/s | 3.7 |
| 1998 | 20.5°C | 15.7°C | 10.9°C | 25.3°C | 3.8°C | 79 mm | 81% | 0.3 m/s | 4 |
| 1997 | 18.9°C | 14.0°C | 9.2°C | 25.2°C | 0.7°C | 101 mm | 79% | 0.3 m/s | 3.6 |
| 1996 | 19.8°C | 15.5°C | 11.2°C | 26.4°C | 4.0°C | 113 mm | 84% | 0.3 m/s | 3.6 |
| 1995 ❄️ | 18.7°C | 13.3°C | 7.9°C | 24.5°C | 1.2°C | 87 mm | 73% | 0.3 m/s | 3.9 |
| 1994 | 19.1°C | 14.3°C | 9.5°C | 25.2°C | 4.7°C | 100 mm | 80% | 0.3 m/s | 3.9 |
| 1993 | 18.6°C | 13.9°C | 9.2°C | 26.8°C | 0.3°C | 127 mm | 79% | 0.3 m/s | 3.4 |
| 1992 | 19.3°C | 14.3°C | 9.4°C | 25.9°C | -0.5°C | 107 mm | 80% | 0.3 m/s | 3.8 |
| 1991 | 18.8°C | 13.6°C | 8.4°C | 27.1°C | -2.7°C | 100 mm | 74% | 0.4 m/s | 3.9 |
| 1990 | 18.9°C | 13.9°C | 9.0°C | 25.2°C | 1.4°C | 83 mm | 79% | 0.3 m/s | 3.3 |
| 1989 | 20.3°C | 15.1°C | 9.9°C | 28.0°C | -0.6°C | 121 mm | 78% | 0.3 m/s | 3.7 |
| 1988 | 19.9°C | 14.6°C | 9.3°C | 26.0°C | 1.3°C | 73 mm | 74% | 0.3 m/s | 4 |
| 1987 | 19.3°C | 14.8°C | 10.4°C | 23.9°C | 3.0°C | 146 mm | 81% | 0.3 m/s | 3.6 |
| 1986 | 18.3°C | 13.8°C | 9.3°C | 23.3°C | 2.7°C | 142 mm | 82% | 0.3 m/s | 3.4 |
| 1985 | 20.9°C | 15.7°C | 10.4°C | 28.0°C | 3.2°C | 131 mm | 79% | 0.3 m/s | 4 |
| 1984 | 18.5°C | 13.4°C | 8.3°C | 24.8°C | 1.6°C | 78 mm | 78% | 0.3 m/s | 4.3 |
| 1983 | 21.2°C | 16.0°C | 10.9°C | 28.9°C | 5.1°C | 72 mm | 73% | 0.3 m/s | — |
| 1982 | 20.0°C | 14.8°C | 9.7°C | 27.0°C | 5.2°C | 104 mm | 79% | 0.3 m/s | — |
| 1981 🌧️ | 17.5°C | 13.4°C | 9.3°C | 25.0°C | -1.4°C | 174 mm | 85% | 0.3 m/s | — |
Actual recorded September data for each year, 1981–2025 — sourced from NASA POWER.
The average high in September is around 20.7°C with lows near 10.6°C (mean 15.7°C), based on 45 years of NASA POWER data.
September averages 97 mm of rainfall in Watertown, about average for rainfall. The wettest on record was 174 mm in 1981.
The hottest September reached 32.1°C in 1999. The coldest dipped to -2.7°C in 1991.
Comparing the earliest years on record (1981–1985) with the most recent (2021–2025), average September temperatures have risen by about 2.1°C. Daytime highs moved from 19.6°C to 22.1°C and overnight lows from 9.7°C to 11.7°C over the same span.
September is one of the warmer months of the year in Watertown and about average for rainfall, so plan for around 20.7°C daytime highs and 10.6°C at night.
Not quite — September is the 9th coolest month in Watertown, averaging 15.7°C.
For September in Watertown, pack light layers you can adjust through the day, with typical highs of 20.7°C and lows of 10.6°C. Bring an umbrella or light rain jacket too.
Relative humidity in September averages 78% — quite humid.
Average wind speed in September is 0.3 m/s. The windiest September on record averaged 0.4 m/s (1991).
The wettest September on record was 1981 with 174 mm. On average, September sees 97 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 Watertown 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.
