San Mateo County Weather in September
Average highs run from 66°F on September 1 to 67°F on September 30, and lows from 53°F to 50°F. Normal precipitation for the month is 0.21 in over about 6 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
- 66°F
- Average low
- 51°F
- Precipitation
- 0.21 in
- Record high
- 110°FSep 6, 2022
- Record low
- 38°FSep 26, 1980
- Chance of a freeze
- 0%
- Daylight
- 12 h 58 min to 11 h 50 min
- Cloud cover
- 51%
- Humidity
- 83%
- Dew point
- 55°F
- Wind
- 8.3 mph
Every day of the month
A wet day has at least 0.01 inch of precipitation.
| Date | Normal high | Normal low | Record high | Record low | Chance wet |
|---|---|---|---|---|---|
| Sep 1 | 66°F | 53°F | 108°F 2017 | 46°F 1975 | 20% |
| Sep 2 | 66°F | 53°F | 108°F 2017 | 45°F 1975 | 20% |
| Sep 3 | 66°F | 53°F | 105°F 1950 | 47°F 1933 | 20% |
| Sep 4 | 66°F | 53°F | 103°F 1950 | 46°F 1965 | 20% |
| Sep 5 | 66°F | 53°F | 108°F 2022 | 47°F 1932 | 20% |
| Sep 6 | 66°F | 52°F | 110°F 2022 | 46°F 1964 | 20% |
| Sep 7 | 66°F | 52°F | 107°F 2020 | 45°F 1933 | 20% |
| Sep 8 | 66°F | 52°F | 103°F 1944 | 46°F 1964 | 20% |
| Sep 9 | 66°F | 52°F | 103°F 1932 | 44°F 1931 | 21% |
| Sep 10 | 66°F | 52°F | 98°F 1944 | 45°F 1931 | 21% |
| Sep 11 | 66°F | 52°F | 103°F 1983 | 42°F 1931 | 21% |
| Sep 12 | 66°F | 52°F | 103°F 1979 | 44°F 1934 | 21% |
| Sep 13 | 66°F | 52°F | 106°F 1971 | 44°F 1970 | 21% |
| Sep 14 | 66°F | 52°F | 106°F 1971 | 42°F 1970 | 21% |
| Sep 15 | 66°F | 51°F | 105°F 1971 | 44°F 1936 | 21% |
| Sep 16 | 66°F | 51°F | 99°F 1971 | 45°F 1931 | 21% |
| Sep 17 | 66°F | 51°F | 97°F 1935 | 44°F 2006 | 21% |
| Sep 18 | 67°F | 51°F | 97°F 2009 | 44°F 1965 | 21% |
| Sep 19 | 67°F | 51°F | 97°F 1943 | 44°F 1965 | 22% |
| Sep 20 | 67°F | 51°F | 103°F 1939 | 42°F 1965 | 22% |
| Sep 21 | 67°F | 51°F | 102°F 1939 | 42°F 1965 | 22% |
| Sep 22 | 67°F | 51°F | 102°F 1939 | 39°F 1980 | 22% |
| Sep 23 | 67°F | 50°F | 101°F 1939 | 43°F 1972 | 22% |
| Sep 24 | 67°F | 50°F | 99°F 1964 | 40°F 1980 | 22% |
| Sep 25 | 67°F | 50°F | 99°F 2019 | 41°F 1948 | 22% |
| Sep 26 | 67°F | 50°F | 99°F 1963 | 38°F 1980 | 22% |
| Sep 27 | 67°F | 50°F | 97°F 2010 | 42°F 1934 | 22% |
| Sep 28 | 67°F | 50°F | 98°F 2010 | 45°F 1934 | 22% |
| Sep 29 | 67°F | 50°F | 96°F 1966 | 43°F 1955 | 22% |
| Sep 30 | 67°F | 50°F | 95°F 2001 | 41°F 1950 | 23% |
Chance of rain
- Chance of a wet day
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 6:54 PM
Other months
Sources
Normals: NOAA NCEI U.S. Climate Normals, 1991-2020, station USC00043714 (Half Moon Bay), public domain. Records: NOAA GHCN-Daily, station USC00047339, 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.