Will Pipes Freeze in San Mateo County, CA? No Normal Risk Nights
Freeze nights in San Mateo County
A normal winter in San Mateo County, CA never drops to 20°F, the point where bare pipes in unheated space start to freeze.
Risk level: No normal risk nights.
It depends where the pipe is
| Pipe | Freezes near | Nights a year |
|---|---|---|
| Outdoor spigot, open crawlspace | 32°F | 18 nights |
| Bare pipe in garage, attic or outside wall | 20°F | none |
| Insulated pipe in unheated space | 10°F | none |
| Inside the heated house | 0°F | none |
Month by month
Nights at or below 20°F in a normal year.
First 32°F night is usually around 11/25, last around 02/25.
How cold San Mateo County nights get
Average winter (Dec-Feb) low: 41°F; the coldest month, December, averages 40°F overnight. 5 months normally bring at least one night below 32°F (Jan, Feb, Mar, Nov, Dec).
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 40°F | 42°F | 42°F | 44°F | 47°F | 49°F | 52°F | 52°F | 51°F | 47°F | 43°F | 40°F |
San Mateo County next to its neighbours
The six closest counties all sit within a night of San Mateo County.
| County | Distance | Below 32°F | Below 20°F | Jan low |
|---|---|---|---|---|
| San Mateo County, CA | - | 18 nights | none | 40°F |
| Alameda County, CA | 48 km northeast | 14 nights | none | 39°F |
| Santa Cruz County, CA | 55 km southeast | 20 nights | under 1 night | 40°F |
| Santa Clara County, CA | 64 km east | 11 nights | under 1 night | 40°F |
| Contra Costa County, CA | 67 km northeast | 11 nights | none | 42°F |
| San Francisco County, CA | 68 km northwest | 3 nights | none | 44°F |
| Marin County, CA | 78 km northwest | 7 nights | none | 42°F |
San Mateo County within CA
San Mateo County ranks 51 of 58 CA counties for nights below 20°F. The most is Mono County, CA at 130 nights, with a 13°F average winter low. The fewest is Orange County, CA: none, winter lows near 46°F. At 594 ft, San Mateo County sits 704 ft below the state's median county elevation of 1,298 ft.
What to do here
- Disconnect hoses in a cold snap; no routine prep needed.
Even here, a once-a-decade freeze catches houses built without insulation: why pipes burst in warm places.
Where this comes from
NOAA 1991-2020 climate normals, blended from 3 nearby stations and adjusted for elevation; closest is Woodside Fire Station 1, CA, 10 km away (close match). A normal year, not tonight's forecast: check the forecast low for the night itself. Why elevation is part of the blend: how county numbers are made.