Casa de Tableta · Photo: W Nowicki · CC BY 3.0Summers in Portola Valley are cooler than in 94% of U.S. towns
The coldest hours of a January night bottom out at 41° on average. In January, nights here are milder than in 90% of U.S. towns. Since 1901–2000, the year has warmed by 1.8°, most of all in August. The outdoor season runs from April to November, with highs of 63–80°.
A year in Portola Valley at a glance
Each month’s typical high and low, and what it brings.
What stands out about Portola Valley’s climate
Compared with towns across the U.S., with Portola Valley’s place in California beside it.
What a year in Portola Valley feels like
Typical highs and lows by season and by month, from NOAA’s 30-year normals.
About 310 frost-free days a year
In half of years the last 32° night comes by February 2 and the first by December 6. Frost and day counts come from NOAA’s station at Los Gatos, CA, 18 miles from the town’s center, 380 ft lower.
The 2023 USDA map puts Portola Valley’s center in zone 10a: the coldest night of a typical winter, averaged over 1991–2020, reaches about 31°. Plant labels use the zone; frost dates tell you when to plant.
Freezing nights by month
Average nights at or below 32°
90° days by month
Average days at 90° or more
About 37 inches of rain a year
January is the wettest month and July the driest.
Rain + melted snow by month
Inches of water
Heating and cooling a home in Portola Valley
80% of the work is heating. Months averaging under 55° run from November through April.
Degree days add up how far each day’s average falls below (heating) or rises above (cooling) 65° over a year. NOAA normals at Los Gatos, CA.
Living with Portola Valley’s climate
What the numbers mean for a garden, a free afternoon and a bad-weather day.
310 frost-free days in zone 10a. The last frost comes by Feb 2 in half of years, so many gardeners wait a week or two past that to set out tomatoes. The first fall frost comes by Dec 6.
Months with highs of 62–84° and lows of 40° or more: April, May, June, July, August, September, October and November. 54 miles of trails are waiting.
Parks & trails →Of the yearly building losses FEMA expects in Portola Valley from natural hazards, earthquakes are the largest share (68%). Normals are averages; the hazards page covers the extremes.
Natural hazards →Portola Valley and its neighbors
Ladera has the coldest January nights next door (40°, 2° colder) and Ladera the hottest July afternoons (81°, 1° warmer). Differences under a degree are within what the 5 km grid can tell apart. Tap a town to see its climate.
Temperatures and rain come from NOAA’s 5 km grid, so each town gets its own value. Snow and frost-free days come from the nearest weather station, so neighbors often share them.
Warmer than it used to be
The last 30 years in Portola Valley averaged 1.8° warmer than the 1900s. The biggest change is in August (+2.3°).
A bigger shift than in 82% of U.S. towns.
Change by month
The two periods overlap (1991–2000 is in both), which makes the gap a little smaller than it would be between separate periods.
How Portola Valley compares
Against 12,954 U.S. towns of 2,500 or more (one per 5 km climate cell), with Portola Valley’s place among California’s.
Portola Valley weather by month
Highs, lows and precipitation from NOAA’s grid at Portola Valley’s center. 1991–2020 normals.
| Month | High | Low | Rain + melt | 30-yr range |
|---|---|---|---|---|
| January | 57° | 41° | 7.4" | 46° – 54° |
| February | 58° | 41° | 7.2" | 45° – 55° |
| March | 61° | 41° | 5.5" | 47° – 56° |
| April | 64° | 42° | 2.7" | 49° – 56° |
| May | 69° | 46° | 1.1" | 53° – 62° |
| June | 75° | 49° | 0.3" | 59° – 65° |
| July | 80° | 53° | 0.0" | 64° – 70° |
| August | 80° | 54° | 0.1" | 65° – 71° |
| September | 79° | 54° | 0.1" | 63° – 71° |
| October | 73° | 50° | 1.6" | 59° – 68° |
| November | 63° | 44° | 3.8" | 47° – 57° |
| December | 57° | 40° | 7.4" | 45° – 53° |
| Year | 68° | 46° | 37.0" |
How much a month can swing
A normal is an average. March swings the most: from 1991 to 2020 its average ranged from 47° to 56°. August is the most reliable, staying within 6°.
Where Portola Valley’s numbers come from
Normals are 30-year averages, not a forecast. How TownRatings reads climate data →
- Temperatures and precipitation: NOAA’s gridded 1991–2020 normals for the 5 km cell at Portola Valley’s center.
- Frost, freezing nights, 90° and 100° days, heating and cooling: Los Gatos, CA, 18 miles from the town’s center, 380 ft lower; the nearest station that passes our checks for distance, height and agreement with the grid.
- Growing zone: USDA Plant Hardiness Zone Map, 2023. USDA Agricultural Research Service and PRISM Group, Oregon State University, prism.oregonstate.edu/phzm, accessed 8 Oct 2026.
- Then and now: NOAA’s gridded normals for 1901–2000 at the same cell.
- Storms and extremes: FEMA National Risk Index (expected annual losses to buildings).
Common questions about Portola Valley’s climate
What is the weather like in Portola Valley?
A typical July afternoon reaches 80°, a January night drops to 41°. Milder January nights than 90% of U.S. towns; cooler July afternoons than 94% of U.S. towns.
How cold does it get in Portola Valley in winter?
December nights average 40°. About 13 nights a year reach 32° or below. In the coldest December of the last 30 years the month averaged 45°.
How hot does it get in Portola Valley?
July, with a typical high of 80° and a low of 53°. About 27 days a year reach 90° and 2 reach 100° (Los Gatos, CA).
When is the first and last frost in Portola Valley?
In half of years the last spring frost (32°) comes by February 2 and the first fall frost by December 6, about 310 frost-free days, at Los Gatos, CA.
What growing zone is Portola Valley in?
Zone 10a on the 2023 USDA Plant Hardiness Zone Map: the coldest night of a typical winter reaches about 31°.
What is the best time of year to visit Portola Valley?
By one simple rule (highs of 62–84° and lows of 40° or more), April, May, June, July, August, September, October and November. The rule is ours, not NOAA’s; the month table has the numbers to judge by.
Is Portola Valley getting warmer?
Yes: 1991–2020 averaged 1.8° warmer than 1901–2000 on NOAA’s grid. A bigger shift than 82% of U.S. towns.
Neighbors talk weather
Gardens, snow, and the best days of the year in Portola Valley.
Source: NOAA NCEI U.S. Climate Normals, 1991–2020 and 1901–2000; USDA/PRISM 2023 hardiness zones; FEMA National Risk Index. Today’s forecast: National Weather Service.