September weather in Portola Valley, CA
The average temperature in Portola Valley in September is 68.0°F (57.1°F–78.9°F). Average precipitation: 0.18 inches.
September is a fall month in Portola Valley, and is warmer than the annual average of 57.1°F. Daily temperatures swing about 21.8°F between overnight low and afternoon high. Expect about 2 days with precipitation. Daylight averages about 12h 14m. For travel, September is one of the best months of the year to visit.
Avg High
78.9°F
Avg Low
57.1°F
Precipitation
0.18 in
Daylight
12h 14m
September records in Portola Valley
| Date | Record High | Record Low | Record Precip |
|---|---|---|---|
| September 1 | 100.9°F (2017) | 45.0°F (2008) | 0.01 in (2000) |
| September 2 | 104.0°F (2017) | 43.0°F (2023) | 0.47 in (2000) |
| September 3 | 100.0°F (2017) | 46.0°F (2000) | 0.11 in (2023) |
| September 4 | 98.1°F (2022) | 44.1°F (2015) | 0.00 in (1995) |
| September 5 | 100.9°F (2022) | 46.0°F (2000) | 0.02 in (2023) |
| September 6 | 102.0°F (2020) | 46.9°F (2002) | 0.01 in (2025) |
| September 7 | 102.9°F (2020) | 44.1°F (2002) | 0.00 in (1995) |
| September 8 | 99.0°F (2022) | 46.9°F (2005) | 0.07 in (2010) |
| September 9 | 97.0°F (2022) | 45.0°F (2001) | 0.18 in (1999) |
| September 10 | 97.0°F (2015) | 45.0°F (2005) | 0.29 in (2025) |
| September 11 | 93.9°F (2014) | 42.1°F (2005) | 0.04 in (2017) |
| September 12 | 91.9°F (2014) | 46.0°F (2005) | 0.01 in (2025) |
| September 13 | 91.0°F (2019) | 42.1°F (2005) | 0.00 in (1995) |
| September 14 | 90.0°F (2006) | 42.1°F (2005) | 0.46 in (2009) |
| September 15 | 86.0°F (2012) | 43.0°F (2005) | 0.02 in (1997) |
| September 16 | 87.1°F (2025) | 44.1°F (2005) | 0.14 in (2024) |
| September 17 | 89.1°F (2016) | 44.1°F (2008) | 0.01 in (2005) |
| September 18 | 91.0°F (2016) | 43.0°F (2008) | 0.89 in (2022) |
| September 19 | 93.0°F (2016) | 44.1°F (2008) | 0.15 in (2014) |
| September 20 | 93.0°F (2015) | 39.9°F (2007) | 0.04 in (2010) |
| September 21 | 93.0°F (2015) | 43.0°F (2007) | 0.78 in (2013) |
| September 22 | 91.0°F (2003) | 43.0°F (2007) | 0.18 in (2007) |
| September 23 | 93.0°F (2024) | 44.1°F (2010) | 0.15 in (2007) |
| September 24 | 91.9°F (2011) | 44.1°F (2005) | 0.06 in (2014) |
| September 25 | 91.0°F (2016) | 45.0°F (2013) | 0.63 in (2014) |
| September 26 | 93.9°F (2016) | 45.0°F (2013) | 0.16 in (2001) |
| September 27 | 91.0°F (2009) | 45.0°F (1998) | 0.16 in (1998) |
| September 28 | 95.0°F (2009) | 44.1°F (2019) | 0.05 in (1998) |
| September 29 | 96.1°F (2010) | 42.1°F (2019) | 0.13 in (2007) |
| September 30 | 93.0°F (2010) | 41.0°F (2019) | 0.38 in (2023) |