USDA Hardiness Zone 10a
Frost Dates for 94403 — San Mateo, California
NOAA 1991–2020 normals from Redwood City, CA (31 ft), 6 miles southeast.
Updated
A freeze (32°F) reaches San Mateo in 88% of winters. When it does, the first fall frost comes around December 12 and the last spring frost around January 17; after February 16 the chance of another is under 9%.
- Last spring frost
- January 17
- First fall frost
- December 12
- Winters with a freeze
- 88%
- Hardiness zone
- 10a
50%, in freezing years
50%, in freezing years
32°F or below, 1991–2020
30 to 35°F (-1.1 to 1.7°C) coldest night
- First snow
- Rare
- Last snow
- Rare
- Winters with snow
- 5%
- Snow per year
- 0"
median, 1991–2020
median, 1991–2020
1 of 19 observed
NOAA normal
Frost risk by date in San Mateo
In the 88% of winters that bring a 32°F night, half of springs at Redwood City, CA see their last 32°F night by January 17; only one in ten is past it by December 20, and one in ten still has one after February 16. Counting the winters with no freeze at all, the risk of a 32°F night after February 16 is under 9% and after January 17 44%. The 30 days between the even-odds date and the one-in-ten date is narrower than at 48% of the 220 California stations behind this site's ZIP pages, so a fixed calendar date is a gamble here.
In fall the first 32°F night comes by December 12 in half of years, as early as November 24 one year in ten, and holds off past January 14 one year in ten. A hard freeze of 28°F is less common: it came in 38% of winters. The freeze-free season lasts 333 days in a typical year, longer than at 84% of the 219 California stations; it reaches 365 days one year in ten and falls short of 298 one year in ten. On average 6 nights a year drop to 32°F or below. Nights of 36°F, when frost can still settle on leaves in low, open spots, continue 39 days longer, to about February 25, and return around November 25. A 24°F night is unusual: 4% of winters had one.
| Low | 90% | 80% | 70% | 60% | 50% | 40% | 30% | 20% | 10% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Jan 25 | Feb 5 | Feb 13 | Feb 20 | Feb 25 | Mar 1 | Mar 7 | Mar 14 | Mar 31 |
| 32°F88% of yrs | Dec 20 | Dec 29 | Jan 3 | Jan 10 | Jan 17 | Jan 25 | Jan 31 | Feb 5 | Feb 16 |
| 28°F38% of yrs | too rare for a date | ||||||||
| 24°F4% of yrs | too rare for a date | ||||||||
| Low | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Nov 8 | Nov 13 | Nov 17 | Nov 21 | Nov 25 | Nov 28 | Dec 1 | Dec 5 | Dec 14 |
| 32°F88% of yrs | Nov 24 | Dec 1 | Dec 4 | Dec 8 | Dec 12 | Dec 17 | Dec 23 | Jan 1 | Jan 14 |
| 28°F38% of yrs | too rare for a date | ||||||||
| 24°F4% of yrs | too rare for a date | ||||||||
Freeze probabilities 1991–2020 at Redwood City, CA, from NOAA NCEI U.S. Climate Normals. Where a temperature is reached in fewer than 99.5% of years, the dates describe only the years that reach it. Not a forecast.
Frost risk on any date in 94403 →Compare San Mateo with another place →
Weather by month in San Mateo
July is the warmest month at Redwood City, CA, averaging 81°F by day and 57°F at night; December is the coldest at 59°F and 40°F. From March to October the chance of a 32°F night stays under 10% each month. In spring, February still brings one in 24% of years.
Warmth for growing adds up to 3,606 growing degree days a year (base 50°F), fewer than at 70% of the 283 California stations. 38% of them (1,375) have built up by the end of June, and July adds the most, 583. Precipitation totals 19.2 inches a year, most in December (3.9") and least in July (0.0"); May to September bring 4% of it.
| Month | High °F | Low °F | Precip. in | 32°F night | Nights ≤ 32°F | GDD 50°F |
|---|---|---|---|---|---|---|
| Jan | 59 | 41 | 3.8 | 55% | 3 | 50 |
| Feb | 62 | 43 | 3.7 | 24% | 1 | 88 |
| Mar | 65 | 45 | 3.0 | 0% | 0 | 163 |
| Apr | 69 | 47 | 1.2 | 0% | 0 | 227 |
| May | 73 | 50 | 0.5 | 0% | 0 | 361 |
| Jun | 79 | 54 | 0.2 | 0% | 0 | 486 |
| Jul | 81 | 57 | 0.0 | 0% | 0 | 583 |
| Aug | 81 | 57 | 0.0 | 0% | 0 | 578 |
| Sep | 79 | 54 | 0.1 | 0% | 0 | 504 |
| Oct | 75 | 50 | 0.9 | 0% | 0 | 378 |
| Nov | 65 | 44 | 1.9 | 17% | <1 | 144 |
| Dec | 59 | 40 | 3.9 | 64% | 3 | 44 |
Monthly normals 1991–2020 at Redwood City, CA (31 ft), NOAA NCEI. "32°F night": chance of at least one in the month. GDD: growing degree days, base 50°F.
Compared with the next station
The nearest other station, San Francisco International Airport, CA, 6 miles northwest of San Mateo, has a 32°F night in 12% of winters against 88% here, and a coolest-month low 5°F warmer (45°F).
When it snows in San Mateo, California
Measurable snow is rare here: only 1 of 19 winters from 1991 to 2020 recorded 0.1 inch or more at Redwood City, CA, 5.6 miles away, so there is no typical first-snow date to plan around.
Where these figures come from
ZIP 94403 is 6 miles from Redwood City, CA, which stands at 31 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 7 other ZIP codes share the station. The 2023 USDA Plant Hardiness Zone Map lists ZIP 94403 in zone 10a, the band where the coldest night of the year averaged 30 to 35°F (-1.1 to 1.7°C) over 1991–2020; a zone describes winter lows, not frost dates.
Frost dates across California →
Hardiness zone and temperature range: USDA Plant Hardiness Zone Map 2023 (USDA-ARS, Oregon State University), ZIP-code listing, unchanged. °C converted from °F by this site.
Thirty-year averages, not a forecast: check the forecast before planting out. How stations are matched and which pages are offered to search engines: methodology.