September is one of the warmer months of the year in Simi Valley, with average highs near 31.8°C and lows around 15.5°C (mean 23.7°C). It is one of the drier months, averaging 6 mm of rainfall and 38% relative humidity across the period.
September is one of the warmer months of the year in Simi Valley. Across 45 years of records, the warmest September in Simi Valley was 2020 — when the month averaged 26.2°C — while the coldest was 1986 at 18.9°C. Typical September days reach 31.8°C with nights near 15.5°C. The long-term trend is warming, about 0.4°C per decade.
September is one of the drier months in Simi Valley, averaging 6 mm of rainfall. The wettest September on record was 1983 with 53 mm, while the driest were 2009, 2018 and 2020, among others at 0 mm.
Relative humidity in Simi Valley during September averages 38%. The most humid September was 2023 at 60%, and the least humid was 2020 at 29%.
September receives about 6.1 kWh/m²/day of solar energy in Simi Valley. The sunniest Septembers were 2003 and 2018 at 6.5 kWh/m²/day, and the cloudiest was 1984 at 5.4.
Average wind speed in Simi Valley during September is 1.9 m/s. The windiest Septembers were 1982 and 1989 at 2.1 m/s, while the calmest was 1991 at 1.6 m/s.
Ranked against the other 11 months using 1981–2025 averages.
| Metric | 1980s | 1990s | 2000s | 2010s | 2020s | Change |
|---|---|---|---|---|---|---|
| Avg Temp | 22.7°C | 23.5°C | 23.8°C | 24.0°C | 24.6°C | +1.9°C |
| Avg High | 30.7°C | 31.6°C | 32.3°C | 32.2°C | 32.4°C | +1.7°C |
| Avg Low | 14.8°C | 15.4°C | 15.3°C | 15.8°C | 16.8°C | +2°C |
| Avg Rain | 16 mm | 4 mm | 3 mm | 3 mm | 5 mm | -11 mm |
September temperatures in Simi Valley have risen by about 1.2°C between 1981–1985 and 2021–2025.
September has ranged from a record low of 5.4°C (1986) to a record high of 44.5°C (2020).
Rainfall in September is one of the drier months, averaging 6 mm, with a wettest year of 53 mm.
One of the best times to visit Simi Valley
September is one of the best times to visit Simi Valley. Expect days averaging 31.8°C and nights around 15.5°C (mean 23.7°C) — warmer than Simi Valley's yearly average — with low rainfall (6 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 | 30.9°C | 24.0°C | 17.1°C | 37.5°C | 13.1°C | 10 mm | 51% | 1.7 m/s | 5.8 |
| 2024 | 33.6°C | 25.1°C | 16.6°C | 42.9°C | 8.9°C | 0 mm | 37% | 1.8 m/s | 6.1 |
| 2023 | 28.9°C | 21.8°C | 14.6°C | 34.9°C | 10.5°C | 4 mm | 60% | 1.8 m/s | 5.6 |
| 2022 | 33.3°C | 26.1°C | 18.9°C | 42.1°C | 12.0°C | 14 mm | 43% | 1.8 m/s | 5.9 |
| 2021 | 32.8°C | 24.6°C | 16.5°C | 38.9°C | 10.3°C | 1 mm | 35% | 1.8 m/s | 6.2 |
| 2020 🔥 | 35.1°C | 26.2°C | 17.4°C | 44.5°C | 13.9°C | 0 mm | 29% | 1.9 m/s | 6.1 |
| 2019 | 31.6°C | 23.5°C | 15.5°C | 38.7°C | 6.0°C | 1 mm | 39% | 1.9 m/s | 6.2 |
| 2018 | 32.8°C | 23.9°C | 15.0°C | 39.0°C | 10.6°C | 0 mm | 35% | 1.7 m/s | 6.5 |
| 2017 | 30.4°C | 22.6°C | 14.8°C | 40.3°C | 6.8°C | 5 mm | 45% | 1.9 m/s | 6.1 |
| 2016 | 31.2°C | 23.0°C | 14.7°C | 37.0°C | 9.0°C | 0 mm | 37% | 2 m/s | 6.3 |
| 2015 | 32.2°C | 24.7°C | 17.2°C | 38.1°C | 10.9°C | 20 mm | 45% | 1.8 m/s | 6.1 |
| 2014 | 32.9°C | 25.0°C | 17.0°C | 39.0°C | 10.3°C | 1 mm | 37% | 1.7 m/s | 6.3 |
| 2013 | 32.0°C | 23.7°C | 15.4°C | 37.6°C | 7.4°C | 0 mm | 36% | 2 m/s | 6.4 |
| 2012 | 33.9°C | 25.8°C | 17.8°C | 37.6°C | 13.3°C | 1 mm | 31% | 1.8 m/s | 6.1 |
| 2011 | 32.6°C | 24.2°C | 15.7°C | 37.9°C | 11.3°C | 4 mm | 37% | 1.8 m/s | 6.1 |
| 2010 | 32.8°C | 23.8°C | 14.8°C | 39.5°C | 9.1°C | 1 mm | 32% | 1.8 m/s | 6.4 |
| 2009 | 34.0°C | 25.5°C | 17.0°C | 38.3°C | 8.0°C | 0 mm | 32% | 1.9 m/s | 6.3 |
| 2008 | 33.4°C | 24.8°C | 16.2°C | 38.0°C | 10.7°C | 0 mm | 34% | 1.7 m/s | 6.3 |
| 2007 | 29.5°C | 21.7°C | 13.9°C | 40.8°C | 8.0°C | 13 mm | 45% | 2 m/s | 6.3 |
| 2006 | 32.0°C | 23.5°C | 15.1°C | 38.0°C | 8.3°C | 1 mm | 32% | 1.9 m/s | 6.3 |
| 2005 | 29.6°C | 21.3°C | 13.0°C | 34.3°C | 9.0°C | 7 mm | 39% | 1.8 m/s | 6.3 |
| 2004 | 32.0°C | 23.6°C | 15.1°C | 39.1°C | 8.4°C | 0 mm | 32% | 1.8 m/s | 6.4 |
| 2003 | 34.5°C | 25.7°C | 16.8°C | 38.1°C | 12.4°C | 1 mm | 33% | 1.8 m/s | 6.5 |
| 2002 | 32.6°C | 24.3°C | 16.0°C | 39.3°C | 9.8°C | 2 mm | 33% | 1.9 m/s | 6.1 |
| 2001 | 33.1°C | 24.2°C | 15.4°C | 37.0°C | 12.7°C | 0 mm | 37% | 1.8 m/s | 6.4 |
| 2000 | 31.8°C | 23.3°C | 14.9°C | 39.2°C | 10.5°C | 4 mm | 38% | 1.9 m/s | 6.1 |
| 1999 | 31.5°C | 23.3°C | 15.0°C | 36.5°C | 10.9°C | 2 mm | 39% | 1.9 m/s | 6.3 |
| 1998 | 29.0°C | 21.8°C | 14.6°C | 38.4°C | 9.1°C | 15 mm | 53% | 1.8 m/s | 5.6 |
| 1997 | 32.4°C | 24.7°C | 16.9°C | 37.4°C | 12.3°C | 9 mm | 38% | 1.9 m/s | 6 |
| 1996 | 30.9°C | 22.7°C | 14.6°C | 36.4°C | 10.1°C | 0 mm | 37% | 1.8 m/s | 6.3 |
| 1995 | 33.3°C | 24.8°C | 16.3°C | 37.7°C | 8.9°C | 0 mm | 32% | 1.9 m/s | 6.3 |
| 1994 | 31.0°C | 23.1°C | 15.2°C | 36.6°C | 9.0°C | 4 mm | 36% | 1.8 m/s | 5.9 |
| 1993 | 31.8°C | 23.1°C | 14.4°C | 39.5°C | 8.3°C | 0 mm | 33% | 1.9 m/s | 6.3 |
| 1992 | 32.6°C | 24.2°C | 15.9°C | 36.6°C | 10.2°C | 0 mm | 32% | 1.9 m/s | 6.2 |
| 1991 | 32.1°C | 24.2°C | 16.2°C | 36.6°C | 8.0°C | 2 mm | 38% | 1.6 m/s | 6.1 |
| 1990 | 31.7°C | 23.3°C | 15.0°C | 39.3°C | 11.1°C | 2 mm | 36% | 1.9 m/s | 6.2 |
| 1989 | 31.4°C | 22.8°C | 14.2°C | 37.7°C | 9.2°C | 11 mm | 34% | 2.1 m/s | 6.1 |
| 1988 | 32.0°C | 23.6°C | 15.3°C | 40.9°C | 9.6°C | 2 mm | 34% | 1.9 m/s | 5.9 |
| 1987 | 32.3°C | 23.7°C | 15.1°C | 38.6°C | 10.7°C | 4 mm | 33% | 1.9 m/s | 5.8 |
| 1986 ❄️ | 26.9°C | 18.9°C | 11.0°C | 38.2°C | 5.4°C | 20 mm | 44% | 2 m/s | 5.8 |
| 1985 | 27.9°C | 20.2°C | 12.6°C | 35.3°C | 7.3°C | 7 mm | 45% | 2 m/s | 5.6 |
| 1984 | 33.0°C | 25.6°C | 18.2°C | 38.9°C | 11.2°C | 8 mm | 40% | 2 m/s | 5.4 |
| 1983 🌧️ | 31.7°C | 24.2°C | 16.7°C | 38.3°C | 9.9°C | 53 mm | 39% | 1.9 m/s | — |
| 1982 | 29.3°C | 21.9°C | 14.4°C | 39.4°C | 5.7°C | 27 mm | 43% | 2.1 m/s | — |
| 1981 | 32.0°C | 23.7°C | 15.4°C | 36.2°C | 11.1°C | 10 mm | 30% | 1.9 m/s | — |
Actual recorded September data for each year, 1981–2025 — sourced from NASA POWER.
The average high in September is around 31.8°C with lows near 15.5°C (mean 23.7°C), based on 45 years of NASA POWER data.
September averages 6 mm of rainfall in Simi Valley, one of the drier months. The wettest on record was 53 mm in 1983.
The hottest September reached 44.5°C in 2020. The coldest dipped to 5.4°C in 1986.
Comparing the earliest years on record (1981–1985) with the most recent (2021–2025), average September temperatures have risen by about 1.2°C. Daytime highs moved from 30.8°C to 31.9°C and overnight lows from 15.5°C to 16.7°C over the same span.
September is one of the warmer months of the year in Simi Valley and one of the drier months, so plan for around 31.8°C daytime highs and 15.5°C at night.
Not quite — September is the 10th coolest month in Simi Valley, averaging 23.7°C.
For September in Simi Valley, pack light, breathable clothing and sun protection, with typical highs of 31.8°C and lows of 15.5°C.
Relative humidity in September averages 38% — relatively dry.
Average wind speed in September is 1.9 m/s. The windiest September on record averaged 2.1 m/s (1982).
The wettest September on record was 1983 with 53 mm. On average, September sees 6 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 Simi Valley 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.
