Mount Diablo Junction, California climate: Climate normals at a NOAA weather station
Mount Diablo Junction's normal year runs from 41°F December nights to 83°F July afternoons, with 26.1 inches of precipitation and less than an inch of snow (0.7 in) a year. (NOAA 1991-2020 normals)
In metric: a yearly mean of 14.7°C, December lows of 5°C, July highs of 28°C, 662 mm of precipitation a year.
- NOAA stationMount Diablo Junction
- Station IDUSC00045915
- Location37.88°N 121.93°W
- Elevation2,170 ft (661 m)
On October 23, the normal high in Mount Diablo Junction is 69°F and the normal low 52°F, and the chance of measurable rain or snow is 14%.
Normals describe a typical year at the station, not a forecast. The National Weather Service forecast for Mount Diablo Junction.
- Jan 54° 42°
- Feb 55° 42°
- Mar 58° 43°
- Apr 62° 44°
- May 68° 49°
- Jun 76° 55°
- Jul 83° 62°
- Aug 83° 61°
- Sep 80° 60°
- Oct 72° 54°
- Nov 60° 46°
- Dec 53° 41°
Each month: normal high (large) and low, °F. Fill: the month's normal mean, colder blue to hotter red.
What is the weather like at Mount Diablo Junction each month?
July is the warmest month, with normal highs of 83°F; December the coldest, with normal lows of 41°F. The most precipitation falls in December (5.08 in), the least in July (0.01 in).
| Month | High | Low | Mean | Precip. | Wet days | 90°F days | Freezing nights | Daylight |
|---|---|---|---|---|---|---|---|---|
| January | 54°F | 42°F | 47.7°F | 5.06 | 12 | <1 | 4 | 9 h 50 min |
| February | 55°F | 42°F | 48.2°F | 4.90 | 11 | <1 | 4 | 10 h 50 min |
| March | 58°F | 43°F | 50.4°F | 3.69 | 10 | <1 | 3 | 11 h 50 min |
| April | 62°F | 44°F | 53.2°F | 1.92 | 7 | <1 | 2 | 13 h 10 min |
| May | 68°F | 49°F | 58.6°F | 1.13 | 4 | 1 | <1 | 14 h 10 min |
| June | 76°F | 55°F | 65.6°F | 0.27 | 1 | 4 | <1 | 14 h 50 min |
| July | 83°F | 62°F | 72.0°F | 0.01 | <1 | 8 | <1 | 14 h 30 min |
| August | 83°F | 61°F | 71.9°F | 0.03 | <1 | 8 | <1 | 13 h 40 min |
| September | 80°F | 60°F | 69.9°F | 0.12 | 1 | 6 | <1 | 12 h 30 min |
| October | 72°F | 54°F | 62.9°F | 1.19 | 3 | 1 | <1 | 11 h 20 min |
| November | 60°F | 46°F | 53.3°F | 2.65 | 7 | <1 | 2 | 10 h 10 min |
| December | 53°F | 41°F | 47.4°F | 5.08 | 11 | <1 | 5 | 9 h 30 min |
| Year (annual) | 67°F | 50°F | 58.4°F | 26.05 | 68 | 27 | 19 |
The same normals in °C and millimetres
| Month | High | Low | Mean | Precip. |
|---|---|---|---|---|
| January | 12°C | 5°C | 8.7°C | 129 mm |
| February | 13°C | 5°C | 9.0°C | 124 mm |
| March | 14°C | 6°C | 10.2°C | 94 mm |
| April | 17°C | 7°C | 11.8°C | 49 mm |
| May | 20°C | 10°C | 14.8°C | 29 mm |
| June | 24°C | 13°C | 18.7°C | 7 mm |
| July | 28°C | 16°C | 22.2°C | 0 mm |
| August | 28°C | 16°C | 22.2°C | 1 mm |
| September | 27°C | 15°C | 21.1°C | 3 mm |
| October | 22°C | 12°C | 17.2°C | 30 mm |
| November | 16°C | 8°C | 11.8°C | 67 mm |
| December | 12°C | 5°C | 8.6°C | 129 mm |
Source: NOAA NCEI 1991-2020 monthly normals, station USC00045915; annual normals. Wet days: 0.01 inch or more; 90°F days and freezing nights: NOAA's day counts. Daylight: sunrise to sunset on the 15th, computed from the latitude.
Which months are mildest at Mount Diablo Junction?
Normal highs sit between 65°F and 85°F in May, June, July, August, September and October. Of those, July has the fewest days with rain (0).
That is a measurement, not a recommendation: it counts the months whose normal afternoon high sits between 65°F and 85°F, where a light layer is enough by day.
Source: NOAA NCEI 1991-2020 monthly normals, station USC00045915.
When is the last frost at Mount Diablo Junction?
In Mount Diablo Junction, the last spring frost (32°F) comes after March 8 in half of years and after April 15 in one year in ten; the first autumn frost comes before December 22 in half of years and before November 23 in one year in ten.
Frost-free in half of yearsFrost in 1 to 9 years out of 10Frost most years
- Last spring frost, half of years after
- March 81 year in 10 after April 15; 9 in 10 after January 12
- First autumn frost, half of years before
- December 221 year in 10 before November 23; 9 in 10 before February 11
NOAA puts the median frost-free season at 282 days, and the season without a hard freeze (28°F) at 364 days.
A hard freeze (28°F) comes after February 8 in half of springs and before January 16 in half of autumns.
The USDA's 2023 Plant Hardiness Zone Map puts Mount Diablo Junction in zone 9b in ZIP code 94517, the nearest ZIP area. By the map's own method, the average of each year's coldest night over 1991-2020, the station is in zone 9b: the coldest night of a typical year reaches 26°F (28 complete years).
Frost dates for every NOAA station in California · Frost dates by ZIP code
Source: NOAA NCEI 1991-2020 annual normals (frost probabilities), station USC00045915; USDA 2023 Plant Hardiness Zone Map, ZIP listings: PRISM Group, Oregon State University, https://prism.oregonstate.edu, accessed October 8, 2026; the station zone: ACIS daily lows, 1991-2020. A 10% date means one year in ten has a frost after it in spring or before it in autumn.
How hot and how cold does it get at Mount Diablo Junction?
In a normal year in Mount Diablo Junction, 27 days a year reach 90°F (1 of them 100°F or more), 19 nights fall to 32°F or colder and 68 days bring measurable precipitation.
- Days at 90°F or more
- 27a normal year
- Days at 100°F or more
- 1a normal year
- Nights at 32°F or colder
- 19a normal year
- Days that stay below freezing
- fewer than 1high of 32°F or less
- Days with 0.01 inch of precipitation
- 68a normal year
The highest temperature in this station's record (since 1952) is 111°F, on July 15, 1972; the lowest is 14°F, on February 6, 1989.
The wettest calendar year on that record is 2017, with 46.79 in; the snowiest season is 1974–75, with 25.0 inches.
Source: NOAA NCEI 1991-2020 annual normals, station USC00045915; records: ACIS (NOAA Regional Climate Centers), station USC00045915. Records use complete months only; a month missing more than three days is left out.
How sunny is it at Mount Diablo Junction?
Over a year 89% of the sunlight that would reach the ground under clear skies gets through the clouds at Mount Diablo Junction, 5.3 kWh per square metre a day. July is the sunniest month (97%) and January the cloudiest (74%).
| Month | Share of clear-sky sunshine | Sunshine, kWh/m² a day |
|---|---|---|
| January | 74% | 2.4 |
| February | 78% | 3.5 |
| March | 81% | 4.8 |
| April | 85% | 6.2 |
| May | 90% | 7.5 |
| June | 95% | 8.3 |
| July | 97% | 8.2 |
| August | 97% | 7.4 |
| September | 95% | 6.0 |
| October | 91% | 4.4 |
| November | 83% | 2.9 |
| December | 74% | 2.2 |
Source: NASA POWER solar climatology, 2001-2020 (CERES SYN1deg). Satellite-based, for the 1-degree cell (about 100 km across) around the station, so it describes the area rather than the airport.
How many heating and cooling degree days does Mount Diablo Junction record?
A normal year at Mount Diablo Junction brings 3,309 heating degree days, 38% fewer than the median U.S. place (5,373), and 925 cooling degree days, 2% more than the median (908). Six months need 300 or more heating degree days, a monthly mean of about 55°F or colder. For gardens and crops, NOAA counts 3,645 growing degree days a year above 50°F.
| Month | Heating | Cooling |
|---|---|---|
| January | 536 | 0 |
| February | 472 | 0 |
| March | 454 | 1 |
| April | 363 | 9 |
| May | 244 | 44 |
| June | 100 | 120 |
| July | 24 | 241 |
| August | 21 | 235 |
| September | 45 | 190 |
| October | 145 | 78 |
| November | 358 | 8 |
| December | 547 | 0 |
| Year (annual) | 3,309 | 925 |
Source: NOAA NCEI 1991-2020 monthly normals, station USC00045915. A degree day is one degree of a day's mean temperature below 65°F (heating) or above it (cooling); utilities use them to estimate energy use.
Have recent years at Mount Diablo Junction been warmer than normal?
2025 averaged 59.5°F, 1.1°F above the 1991-2020 normal of 58.4°F. 4 of the 5 complete years since 2021 ran warmer than the normal.
| Year | Mean | Against normal | Precip. |
|---|---|---|---|
| 2021 | 59.6°F | +1.2°F | 24.71 |
| 2022 | 60.0°F | +1.6°F | 17.22 |
| 2023 | 56.8°F | −1.6°F | 35.00 |
| 2024 | 59.6°F | +1.2°F | 19.65 |
| 2025 | 59.5°F | +1.1°F | 18.20 |
Over 58 complete years from 1953 to 2025, the station's annual mean temperature rose at a fitted 4.8°F per century (+0.27°C per decade). A fitted rate summarizes the whole record; single years swing well above and below it.
Source: observed years: ACIS, station USC00045915; trend: NOAA GHCN-Monthly v4. A year counts only with all twelve months complete.
How does Mount Diablo Junction's climate compare?
Mount Diablo Junction is warmer than 67 of the 219 places with their own NOAA record in California, and wetter than 162. Across the 7,247 U.S. stations with normals, its annual mean of 58.4°F is warmer than 70%.
In the Köppen classification computed from these normals, Mount Diablo Junction is Csa: temperate, dry-summer, hot summer. What the classes mean.
Places with the most similar year elsewhere
Measured month by month on the normal high, low and precipitation, among places of 20,000 or more people in other states:
Source: NOAA NCEI 1991-2020 normals for every station PlainClimate covers. Similarity: the root-mean-square difference of the 36 monthly figures, precipitation weighted so an inch counts like 6°F.
Which places near Mount Diablo Junction share this climate record?
10 places have no NOAA station with 1991-2020 normals of their own, and this station is the nearest full record. A place's own climate can differ from these figures, by several degrees where it sits much higher or lower than the station, nearer the coast, or inside a large city's heat; for the larger places the next nearest records are named with their yearly mean, so the spread around them is visible, and places of 50,000 or more have their own page with NOAA's gridded normals.
- San Ramon, CAalso near: Livermore (8 mi, 59.8°F a year); Hayward (13 mi, 59.6°F a year)85,924 people · 8 mi S
- Danville, CA43,410 people · 5 mi SW
- Alamo, CA14,441 people · 5 mi W
- Clayton, CA10,933 people · 4 mi N
- Blackhawk, CA10,404 people · 5 mi S
- Camino Tassajara, CA6,395 people · 7 mi S
- San Miguel, CA3,991 people · 6 mi W
- Castle Hill, CA1,303 people · 7 mi W
- Diablo, CA1,072 people · 3 mi SW
- North Gate, CA719 people · 4 mi NW
Source: Census Bureau 2024 Gazetteer (place centers) and Vintage 2024 population estimates. Distance and direction from the station to each place's center.
Questions about Mount Diablo Junction's climate
What is the coldest month at Mount Diablo Junction?
December, with a normal high of 53°F and a normal low of 41°F. The warmest is July (83°F by day, 62°F at night).
Does it snow at Mount Diablo Junction?
Rarely: NOAA's normal is 0.7 inch a year.
What is the rainiest month at Mount Diablo Junction?
December, with 5.08 in of precipitation in a normal year; the driest is July with 0.01 in.
What plant hardiness zone is Mount Diablo Junction in?
The USDA's 2023 map puts Mount Diablo Junction in zone 9b in ZIP code 94517, the nearest ZIP area. By the same method at the station, the coldest night of a typical year reaches 26°F (28 years, 1991-2020): zone 9b.
How late can frost come at Mount Diablo Junction?
In half of years the last 32°F night comes after March 8; in one year in ten it comes after April 15. Waiting until April 15 leaves about a one-in-ten chance of a later frost.
Nearby and next
- California climateThe statewide normals and every place in the state
- Concord, CA11 miles from this station, 62.0°F a year
- Concord Wastewater Treatment Plant, CA11 miles from this station, 62.4°F a year
- Antioch, CA12 miles from this station, 62.5°F a year
- Martinez, CA14 miles from this station, 58.2°F a year
- Compare two placesMonth by month, side by side
Download these normals (CSV) · JSON · How these figures are made