September is one of the warmer months of the year in Williamsport, with average highs near 22.5°C and lows around 10.4°C (mean 16.5°C). It is a relatively wet time of year, averaging 102 mm of rainfall and 74% relative humidity across the period.
September is one of the warmer months of the year in Williamsport. Across 45 years of records, the warmest September in Williamsport was 2015 — when the month averaged 19.1°C — while the coldest was 1984 at 13.8°C. Typical September days reach 22.5°C with nights near 10.4°C. The long-term trend is warming, about 0.5°C per decade.
September is a relatively wet time of year in Williamsport, averaging 102 mm of rainfall. The wettest September on record was 2011 with 318 mm, while the driest was 1984 at 35 mm.
Relative humidity in Williamsport during September averages 74%. The most humid September was 2011 at 88%, and the least humid was 1983 at 61%.
September receives about 4 kWh/m²/day of solar energy in Williamsport. The sunniest Septembers were 2005 and 2007 at 4.8 kWh/m²/day, and the cloudiest were 2011 and 2018 at 3.1.
Average wind speed in Williamsport during September is 0.1 m/s. The windiest Septembers were 2023, 2024 and 2025, among others at 0.1 m/s, while the calmest were 2009, 2011 and 2021, among others at 0 m/s.
Ranked against the other 11 months using 1981–2025 averages.
| Metric | 1980s | 1990s | 2000s | 2010s | 2020s | Change |
|---|---|---|---|---|---|---|
| Avg Temp | 15.8°C | 15.6°C | 16.4°C | 17.4°C | 17.1°C | +1.3°C |
| Avg High | 22.0°C | 21.7°C | 22.4°C | 23.3°C | 23.0°C | +1°C |
| Avg Low | 9.5°C | 9.6°C | 10.4°C | 11.4°C | 11.3°C | +1.8°C |
| Avg Rain | 77 mm | 105 mm | 111 mm | 117 mm | 95 mm | +17 mm |
September temperatures in Williamsport have risen by about 1.4°C between 1981–1985 and 2021–2025.
September has ranged from a record low of -2.2°C (1991) to a record high of 34.2°C (2002).
Rainfall in September is a relatively wet time of year, averaging 102 mm, with a wettest year of 318 mm.
One of the best times to visit Williamsport
September is one of the best times to visit Williamsport. Expect days averaging 22.5°C and nights around 10.4°C (mean 16.5°C) — warmer than Williamsport's yearly average — with higher rainfall (102 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 | 25.2°C | 18.5°C | 11.8°C | 29.2°C | 5.9°C | 54 mm | 66% | 0.1 m/s | 4.5 |
| 2024 | 23.3°C | 17.7°C | 12.0°C | 28.2°C | 6.3°C | 57 mm | 76% | 0.1 m/s | 4.2 |
| 2023 | 22.7°C | 16.9°C | 11.2°C | 32.0°C | 5.4°C | 93 mm | 79% | 0.1 m/s | 4.2 |
| 2022 | 21.6°C | 16.3°C | 11.1°C | 27.3°C | 2.6°C | 136 mm | 80% | 0.1 m/s | 4 |
| 2021 | 21.9°C | 16.5°C | 11.1°C | 26.5°C | 5.0°C | 173 mm | 83% | 0 m/s | 3.9 |
| 2020 | 23.3°C | 16.8°C | 10.3°C | 30.9°C | 1.0°C | 54 mm | 68% | 0.1 m/s | 4.1 |
| 2019 | 23.8°C | 17.6°C | 11.4°C | 29.6°C | 6.3°C | 35 mm | 75% | 0.1 m/s | 4.2 |
| 2018 | 22.5°C | 17.9°C | 13.4°C | 29.2°C | 4.7°C | 226 mm | 87% | 0.1 m/s | 3.1 |
| 2017 | 23.9°C | 17.8°C | 11.7°C | 32.5°C | 5.3°C | 41 mm | 74% | 0.1 m/s | 4.3 |
| 2016 | 25.3°C | 18.7°C | 12.2°C | 32.6°C | 4.7°C | 71 mm | 67% | 0.1 m/s | 4.7 |
| 2015 🔥 | 25.5°C | 19.1°C | 12.7°C | 32.7°C | 7.9°C | 99 mm | 70% | 0.1 m/s | 4.4 |
| 2014 | 23.3°C | 16.9°C | 10.4°C | 30.4°C | 4.8°C | 42 mm | 72% | 0.1 m/s | 4.4 |
| 2013 | 22.4°C | 15.5°C | 8.6°C | 31.4°C | 1.7°C | 57 mm | 70% | 0.1 m/s | 4.4 |
| 2012 | 21.8°C | 15.9°C | 10.1°C | 30.2°C | 3.3°C | 128 mm | 75% | 0.1 m/s | 3.9 |
| 2011 🌧️ | 21.5°C | 17.0°C | 12.4°C | 30.7°C | 2.6°C | 318 mm | 88% | 0 m/s | 3.1 |
| 2010 | 23.3°C | 17.1°C | 10.8°C | 32.7°C | 4.7°C | 154 mm | 68% | 0.1 m/s | 4.1 |
| 2009 | 21.1°C | 15.7°C | 10.2°C | 26.3°C | 4.0°C | 61 mm | 76% | 0 m/s | 3.9 |
| 2008 | 23.5°C | 17.1°C | 10.8°C | 32.0°C | 4.0°C | 111 mm | 74% | 0.1 m/s | 4.1 |
| 2007 | 24.3°C | 17.5°C | 10.8°C | 31.3°C | 2.6°C | 63 mm | 71% | 0.1 m/s | 4.8 |
| 2006 | 19.1°C | 14.3°C | 9.5°C | 24.4°C | 1.5°C | 106 mm | 84% | 0.1 m/s | 3.2 |
| 2005 | 25.1°C | 18.2°C | 11.4°C | 30.8°C | 4.1°C | 37 mm | 66% | 0.1 m/s | 4.8 |
| 2004 | 22.1°C | 16.8°C | 11.6°C | 26.0°C | 4.3°C | 233 mm | 84% | 0.1 m/s | 4.1 |
| 2003 | 20.2°C | 15.4°C | 10.5°C | 24.7°C | 4.2°C | 189 mm | 85% | 0 m/s | 3.6 |
| 2002 | 25.0°C | 18.3°C | 11.5°C | 34.2°C | 4.1°C | 124 mm | 65% | 0.1 m/s | 4.2 |
| 2001 | 21.7°C | 15.1°C | 8.5°C | 28.4°C | 2.9°C | 117 mm | 69% | 0.1 m/s | 4.5 |
| 2000 | 22.1°C | 15.5°C | 8.9°C | 30.8°C | -2.2°C | 66 mm | 70% | 0 m/s | 4 |
| 1999 | 22.5°C | 16.6°C | 10.7°C | 31.9°C | 2.9°C | 196 mm | 75% | 0.1 m/s | 4 |
| 1998 | 24.8°C | 17.8°C | 10.8°C | 31.6°C | 2.2°C | 56 mm | 67% | 0.1 m/s | 4.2 |
| 1997 | 20.9°C | 15.1°C | 9.2°C | 27.8°C | 1.3°C | 89 mm | 74% | 0 m/s | 4 |
| 1996 | 20.9°C | 15.8°C | 10.8°C | 28.9°C | 4.2°C | 162 mm | 82% | 0.1 m/s | 3.5 |
| 1995 | 22.5°C | 15.6°C | 8.6°C | 29.9°C | 0.6°C | 62 mm | 64% | 0 m/s | 4.1 |
| 1994 | 19.7°C | 14.3°C | 8.8°C | 26.0°C | 4.3°C | 91 mm | 81% | 0.1 m/s | 4 |
| 1993 | 20.7°C | 15.2°C | 9.7°C | 28.6°C | -1.1°C | 129 mm | 78% | 0 m/s | 3.2 |
| 1992 | 21.1°C | 15.2°C | 9.3°C | 27.4°C | -0.6°C | 99 mm | 77% | 0 m/s | 3.8 |
| 1991 | 22.3°C | 15.4°C | 8.6°C | 31.6°C | -2.2°C | 83 mm | 65% | 0.1 m/s | 4.1 |
| 1990 | 21.2°C | 15.2°C | 9.2°C | 27.8°C | -0.7°C | 79 mm | 77% | 0.1 m/s | 3.9 |
| 1989 | 22.6°C | 16.5°C | 10.5°C | 31.5°C | -0.4°C | 101 mm | 73% | 0 m/s | 3.6 |
| 1988 | 21.2°C | 14.6°C | 7.9°C | 26.0°C | 1.6°C | 63 mm | 74% | 0.1 m/s | 4.2 |
| 1987 | 20.6°C | 15.5°C | 10.5°C | 26.2°C | 2.5°C | 141 mm | 83% | 0.1 m/s | 3.5 |
| 1986 | 22.2°C | 16.1°C | 9.9°C | 29.0°C | 2.7°C | 69 mm | 74% | 0.1 m/s | 3.6 |
| 1985 | 23.4°C | 16.7°C | 10.1°C | 30.7°C | 2.7°C | 100 mm | 70% | 0.1 m/s | 4.7 |
| 1984 ❄️ | 20.0°C | 13.8°C | 7.5°C | 25.5°C | 1.8°C | 35 mm | 77% | 0.1 m/s | 4.5 |
| 1983 | 24.5°C | 17.1°C | 9.8°C | 32.4°C | 1.7°C | 44 mm | 61% | 0.1 m/s | — |
| 1982 | 22.9°C | 16.5°C | 10.1°C | 29.0°C | 2.7°C | 64 mm | 70% | 0.1 m/s | — |
| 1981 | 20.9°C | 15.1°C | 9.3°C | 28.1°C | 1.8°C | 78 mm | 75% | 0 m/s | — |
Actual recorded September data for each year, 1981–2025 — sourced from NASA POWER.
The average high in September is around 22.5°C with lows near 10.4°C (mean 16.5°C), based on 45 years of NASA POWER data.
September averages 102 mm of rainfall in Williamsport, a relatively wet time of year. The wettest on record was 318 mm in 2011.
The hottest September reached 34.2°C in 2002. The coldest dipped to -2.2°C in 1991.
Comparing the earliest years on record (1981–1985) with the most recent (2021–2025), average September temperatures have risen by about 1.4°C. Daytime highs moved from 22.3°C to 22.9°C and overnight lows from 9.4°C to 11.4°C over the same span.
September is one of the warmer months of the year in Williamsport and a relatively wet time of year, so plan for around 22.5°C daytime highs and 10.4°C at night.
Not quite — September is the 9th coolest month in Williamsport, averaging 16.5°C.
For September in Williamsport, pack light layers you can adjust through the day, with typical highs of 22.5°C and lows of 10.4°C. Bring an umbrella or light rain jacket too.
Relative humidity in September averages 74% — moderately humid.
Average wind speed in September is 0.1 m/s. The windiest September on record averaged 0.1 m/s (1982).
The wettest September on record was 2011 with 318 mm. On average, September sees 102 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 Williamsport 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.
