Portola Valley Weather in September
Average highs run from 81°F on September 1 to 78°F on September 30, and lows from 56°F to 53°F. Normal precipitation for the month is 0.07 in over about 1 wet days.
September day by day
The normal high and low for each date, and the record, in °F.
- Normal low
- Normal high
- Record high
- Record low
September at a glance
- Average high
- 79°F
- Average low
- 54°F
- Precipitation
- 0.07 in
- Record high
- 105°FSep 2, 2017
- Record low
- 37°FSep 13, 2012
- Chance of a freeze
- 0%
- Days of 90°F or more
- 4.2
- Daylight
- 12 h 58 min to 11 h 49 min
- Cloud cover
- 16%
- Humidity
- 60%
- Dew point
- 48°F
- Wind
- 7.2 mph
Every day of the month
A wet day has at least 0.01 inch of precipitation; snow means at least 0.1 inch of snowfall.
| Date | Normal high | Normal low | Record high | Record low | Chance wet | Chance of snow |
|---|---|---|---|---|---|---|
| Sep 1 | 81°F | 56°F | 94°F 2005 | 48°F 1964 | 1% | 0% |
| Sep 2 | 81°F | 55°F | 105°F 2017 | 46°F 1964 | 1% | 0% |
| Sep 3 | 80°F | 55°F | 104°F 2017 | 45°F 1964 | 1% | 0% |
| Sep 4 | 80°F | 55°F | 95°F 2008 | 43°F 1954 | 1% | 0% |
| Sep 5 | 80°F | 55°F | 97°F 2008 | 45°F 1954 | 1% | 0% |
| Sep 6 | 80°F | 55°F | 98°F 2008 | 47°F 1960 | 1% | 0% |
| Sep 7 | 80°F | 55°F | 100°F 1958 | 47°F 1960 | 1% | 0% |
| Sep 8 | 80°F | 55°F | 99°F 1957 | 46°F 1978 | 1% | 0% |
| Sep 9 | 80°F | 55°F | 98°F 1976 | 39°F 2011 | 2% | 0% |
| Sep 10 | 80°F | 55°F | 97°F 2015 | 46°F 1964 | 2% | 0% |
| Sep 11 | 80°F | 55°F | 95°F 1984 | 40°F 2012 | 2% | 0% |
| Sep 12 | 80°F | 55°F | 103°F 1979 | 45°F 1993 | 2% | 0% |
| Sep 13 | 80°F | 55°F | 101°F 1979 | 37°F 2012 | 2% | 0% |
| Sep 14 | 80°F | 55°F | 103°F 1971 | 42°F 1970 | 2% | 0% |
| Sep 15 | 79°F | 55°F | 105°F 1971 | 44°F 1955 | 2% | 0% |
| Sep 16 | 79°F | 54°F | 102°F 1971 | 45°F 2006 | 2% | 0% |
| Sep 17 | 79°F | 54°F | 94°F 1968 | 45°F 1959 | 2% | 0% |
| Sep 18 | 79°F | 54°F | 95°F 1968 | 42°F 1954 | 2% | 0% |
| Sep 19 | 79°F | 54°F | 93°F 2002 | 42°F 1993 | 2% | 0% |
| Sep 20 | 79°F | 54°F | 95°F 1958 | 41°F 1965 | 2% | 0% |
| Sep 21 | 79°F | 54°F | 97°F 2003 | 43°F 1982 | 3% | 0% |
| Sep 22 | 79°F | 54°F | 97°F 2015 | 44°F 1968 | 3% | 0% |
| Sep 23 | 79°F | 54°F | 95°F 1956 | 37°F 2010 | 3% | 0% |
| Sep 24 | 79°F | 54°F | 94°F 1964 | 43°F 1955 | 3% | 0% |
| Sep 25 | 78°F | 53°F | 99°F 1954 | 43°F 1993 | 3% | 0% |
| Sep 26 | 78°F | 53°F | 95°F 1978 | 44°F 1993 | 3% | 0% |
| Sep 27 | 78°F | 53°F | 95°F 1963 | 43°F 1982 | 3% | 0% |
| Sep 28 | 78°F | 53°F | 95°F 1958 | 45°F 1965 | 4% | 0% |
| Sep 29 | 78°F | 53°F | 94°F 2010 | 41°F 1981 | 4% | 0% |
| Sep 30 | 78°F | 53°F | 95°F 1966 | 42°F 1954 | 4% | 0% |
Chance of rain
- Chance of a wet day
- Chance of snowfall
Sunrise and sunset
Times for 2026, local time (PDT).
- September 1
- sunrise 6:40 AM, sunset 7:38 PM
- September 30
- sunrise 7:04 AM, sunset 6:53 PM
Other months
Sources
Normals: NOAA NCEI U.S. Climate Normals, 1991-2020, station USC00047339 (Redwood City), public domain. Records: NOAA GHCN-Daily, station USC00046646, every year on file, quality-flagged values removed. Cloud, humidity, wind and sunshine: NASA POWER (MERRA-2, CERES and GEWEX SRB), 1991-2020. The data was obtained from the National Aeronautics and Space Administration (NASA) Langley Research Center's Prediction Of Worldwide Energy Resources (POWER) project funded through the NASA Earth Science Division: POWER Daily API v2, accessed 2026-09-24. Sun and moon: computed with the NOAA solar equations and Meeus.