San Mateo, CA weather in September
San Mateo is mild and dry in September: highs average 75°F and nights drop to about 56°F. Expect about 1 wet day. This is the peak of the warm season in San Mateo. A great month for outdoor plans.
- Normal high
- 75°F
- Normal low
- 56°F
- Wet days
- 1
- 0.07 in total
- Muggy days
- 0
- dew point 65°F+
How September changes from start to finish
| September 1 | September 30 | |
|---|---|---|
| Normal high | 75°F | 75°F |
| Normal low | 56°F | 55°F |
| Chance of a wet day | 2% | 5% |
| Sunrise / sunset | 6:39 am / 7:38 pm | 7:04 am / 6:54 pm |
| Daylight | 13.0 h | 11.8 h |
Chance of a wet day: average for the first and last 7 days of the month. Times are local clock time (Pacific Time).
A typical September day
- 6 am58°Fdew point 53°F
- 9 am64°Fdew point 55°F
- Noon70°Fdew point 55°F
- 3 pm71°Fdew point 53°F
- 6 pm64°Fdew point 53°F
- 9 pm61°Fdew point 53°F
NOAA hourly normals (local standard time) from San Francisco Intl Airport (5 mi away); average wind 11 mph; clear or few clouds in 64% of daytime hours, mostly cloudy or overcast in 20%.
What kind of days to expect
- Comfortable dayshigh 65-80°F and dew point under 60°F23 of 30
- Beach dayshigh 80-95°F, under 0.05 in of rain, afternoon wind under 15 mph4 of 30
- Hot dayshigh 90°F or more2 of 30
- Muggy daysdaily mean dew point 65°F or more0 of 30
- Wet daysat least 0.01 in of rain or melted snow1 of 30
- Notable rainat least 0.10 in0 of 30
- Freezing nightslow 32°F or below0 of 30
Average days per September, counted from daily records for each year 1991-2020.
Rain in September
Normal precipitation is 0.07 in; a dry September brings about 0.0 in and a wet one about 0.1 in (25th and 75th percentiles). Days with half an inch or more are rare.
How September varies from year to year
- Warmest September, 1991-20202017: average high 79°F
- Coolest September2012: average high 68°F
- Wettest September2014: 0.4 in
- Driest September1992: no measurable precipitation
- Highest reading104°F (September 2, 2017)
- Lowest reading49°F (September 24, 2018)
From the station's daily records with NOAA quality flags removed; most recent date shown when a reading repeats.
September compared with August and October
| August | September | October | |
|---|---|---|---|
| Normal high | 73°F | 75°F | 72°F |
| Normal low | 56°F | 56°F | 53°F |
| Wet days | 0 | 1 | 3 |
| Comfortable days | 27 | 23 | 22 |
| Daylight | 13.6 h | 12.4 h | 11.2 h |
Highs change by +2°F from August and -3°F into October.
Nearby alternatives in September
- Warmer: San Jose, CA28 mi · high 81°F · 1 wet day
How San Mateo ranks in September
- Normal high288th warmest of 378 cities; 52nd of 60 in California
- Wet days43rd driest of 378
- Comfortable days7th most of 342
Heating and cooling
September has 46 heating degree days and 56 cooling degree days (base 65°F), 4% of the year's total heating and cooling demand.
Questions
How hot is San Mateo in September?
The normal high is 75°F and the normal low is 56°F. From 1991 to 2020 the hottest September reading was 104°F (most recently on September 2, 2017).
Does it rain a lot in San Mateo in September?
About 1 days in September have at least 0.01 in of precipitation, and the normal total is 0.07 in. The wettest September in 1991-2020 was in 2014 (0.4 in); the driest was in 1992, with no measurable precipitation.
Is September a good time to visit San Mateo?
September ranks 4th of 12 months on our comfort score for San Mateo (83 out of 100). It has about 23 comfortable days and about 2 days at 90°F or hotter. The best months are July and August.
Where the numbers come from
- Temperature, rain, frost dates, humidity and wind by hour and cloud cover: San Francisco Intl Airport (USW00023234), 4.5 mi NNW of the city center, 8 ft . NOAA normals and daily records, 1991-2020.
- Snowfall and snow depth: Redwood City (USC00047339), 7.6 mi SE of the city center, 31 ft . NOAA normals and daily records, 1991-2020.
- Daily humidity and wind: San Francisco International Airport (724940-23234), 4.5 mi NNW of the city center, 10 ft . Hourly observations, 1991-2020.