Diablo has milder winter nights than 91% of U.S. towns
On a normal July day the thermometer tops out near 84°. From frosty January to warm July, readings climb 43°. Precipitation is light here, less than falls on 87% of U.S. towns. The outdoor season runs from April to October, with highs of 64–84°.
A year in Diablo at a glance
Each month’s typical high and low, and what it brings.
What stands out about Diablo’s climate
Compared with towns across the U.S., with Diablo’s place in California beside it.
What a year in Diablo feels like
Typical highs and lows by season and by month, from NOAA’s 30-year normals.
A freeze most winters
Frost and day counts come from NOAA’s station at Vallejo, CA, 22 miles from the town’s center, 340 ft lower.
The 2023 USDA map puts Diablo’s center in zone 9b: the coldest night of a typical winter, averaged over 1991–2020, reaches about 27°. 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 26 inches of rain a year
January is the wettest month and July the driest. Rain falls on about 68 days a year. Snow is rare: about 0.7" in a normal year at Mt Diablo Junction, CA, so most winters see none.
Rain + melted snow by month
Inches of water
Heating and cooling a home in Diablo
71% 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 Vallejo, CA.
Living with Diablo’s climate
What the numbers mean for a garden, a free afternoon and a bad-weather day.
Zone 9b. Frost comes about 7 winters in 10, so tender plants can often stay out through winter.
Months with highs of 62–84° and lows of 40° or more: April, May, June, July, August, September and October. 1.1 miles of trails are waiting.
Parks & trails →Of the yearly building losses FEMA expects in Diablo from natural hazards, earthquakes are the largest share (48%); heat waves, cold waves, winter storms and ice storms together are 2%. Normals are averages; the hazards page covers the extremes.
Natural hazards →Diablo and its neighbors
Alamo has the coldest January nights next door (39°, 3° colder) and Danville the hottest July afternoons (86°, 2° warmer). Differences under a degree are within what the 5 km grid can tell apart. Tap a town to see its climate.
Climates most like Diablo’s, farther away
TownRatings’ own comparison of NOAA 1991–2020 normals, not a NOAA product: places of 5,000+ at least 150 miles away, one per state, closest on January low, July high, precipitation and snow.
| Town | Jan. low | July high | Precip. | Snow |
|---|---|---|---|---|
| Diablo, CA | 42° | 84° | 26" | 0.7" |
| Winston, OR | 37° | 86° | 33" | 0.0" |
| Aransas Pass, TX | 49° | 91° | 34" | 0.3" |
| Felida, WA | 35° | 81° | 42" | 0.8" |
| Sierra Vista Southeast, AZ | 32° | 91° | 15" | 0.7" |
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 Diablo averaged 1.6° warmer than the 1900s. The biggest change is in January (+2.2°).
A bigger shift than in 65% 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 Diablo compares
Against 12,954 U.S. towns of 2,500 or more (one per 5 km climate cell), with Diablo’s place among California’s.
Diablo weather by month
Highs, lows and precipitation from NOAA’s grid at Diablo’s center; snow, wet days and snow on the ground from Mt Diablo Junction, CA. 1991–2020 normals.
| Month | High | Low | Rain + melt | Snow | Wet days | Snow on ground | 30-yr range |
|---|---|---|---|---|---|---|---|
| January | 54° | 42° | 5.1" | 0.1" | 12 | 0 | 44° – 55° |
| February | 56° | 42° | 4.7" | 0.5" | 11 | 0 | 43° – 55° |
| March | 60° | 43° | 3.9" | 0.1" | 10 | 0 | 45° – 59° |
| April | 64° | 45° | 1.8" | 0.0" | 7 | 0 | 49° – 59° |
| May | 71° | 49° | 0.9" | 0.0" | 4 | 0 | 53° – 68° |
| June | 78° | 55° | 0.2" | 0.0" | 1 | 0 | 61° – 70° |
| July | 84° | 60° | 0.0" | 0.0" | 0 | 0 | 68° – 77° |
| August | 84° | 60° | 0.0" | 0.0" | 0 | 0 | 68° – 76° |
| September | 82° | 59° | 0.1" | 0.0" | 1 | 0 | 66° – 74° |
| October | 73° | 54° | 1.2" | 0.0" | 3 | 0 | 60° – 71° |
| November | 61° | 46° | 2.6" | 0.0" | 7 | 0 | 47° – 58° |
| December | 54° | 41° | 5.1" | 0.0" | 11 | 0 | 44° – 51° |
| Year | 68° | 50° | 25.5" | 0.7" | 68 | 0 |
How much a month can swing
A normal is an average. May swings the most: from 1991 to 2020 its average ranged from 53° to 68°. December is the most reliable, staying within 7°.
Where Diablo’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 Diablo’s center.
- Snow: Mt Diablo Junction, CA.
- Frost, freezing nights, 90° and 100° days, heating and cooling: Vallejo, CA, 22 miles from the town’s center, 340 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 Diablo’s climate
What is the weather like in Diablo?
A typical July afternoon reaches 84°, a January night drops to 42°, and snow is almost unheard of. Milder January nights than 91% of U.S. towns; cooler July afternoons than 62% of U.S. towns.
How cold does it get in Diablo in winter?
December nights average 41°. About 5 nights a year reach 32° or below. In the coldest December of the last 30 years the month averaged 44°.
How much snow does Diablo get?
About 0.7 inches in a normal winter at Mt Diablo Junction, CA, so most winters see none.
How hot does it get in Diablo?
July, with a typical high of 84° and a low of 60°. About 36 days a year reach 90° and 7 reach 100° (Vallejo, CA).
When is the first and last frost in Diablo?
There is no reliable frost date: a 32° night comes in about 75% of winters at Vallejo, CA, so most years bring a freeze but some pass without one.
What growing zone is Diablo in?
Zone 9b on the 2023 USDA Plant Hardiness Zone Map: the coldest night of a typical winter reaches about 27°.
What is the best time of year to visit Diablo?
By one simple rule (highs of 62–84° and lows of 40° or more), April, May, June, July, August, September and October. The rule is ours, not NOAA’s; the month table has the numbers to judge by.
Is Diablo getting warmer?
Yes: 1991–2020 averaged 1.6° warmer than 1901–2000 on NOAA’s grid. A bigger shift than 65% of U.S. towns.
Neighbors talk weather
Gardens, snow, and the best days of the year in Diablo.
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.