St. Helena, California climate: Normal weather by month, frost dates and records
St. Helena's normal year runs from 40°F December nights to 89°F July afternoons, with 34.0 inches of precipitation a year. (NOAA 1991-2020 normals)
In metric: a yearly mean of 16.1°C, December lows of 4°C, July highs of 32°C, 863 mm of precipitation a year.
- NOAA stationSaint Helena
- Station IDUSC00047643
- Location38.51°N 122.47°W
- Elevation225 ft (69 m)
- Population5,257
On October 14, the normal high in St. Helena is 79°F and the normal low 50°F, and the chance of measurable rain or snow is 10%.
Normals describe a typical year at the station, not a forecast. The National Weather Service forecast for St. Helena.
- Jan 59° 40°
- Feb 63° 42°
- Mar 67° 44°
- Apr 72° 46°
- May 78° 51°
- Jun 85° 54°
- Jul 89° 56°
- Aug 89° 56°
- Sep 87° 54°
- Oct 78° 49°
- Nov 66° 44°
- Dec 58° 40°
Each month: normal high (large) and low, °F. Fill: the month's normal mean, colder blue to hotter red.
What is the weather like in St. Helena each month?
July is the warmest month, with normal highs of 89°F; December the coldest, with normal lows of 40°F. The most precipitation falls in December (7.18 in), the least in July (0.01 in).
| Month | High | Low | Mean | Precip. | Wet days | 90°F days | Freezing nights | Daylight |
|---|---|---|---|---|---|---|---|---|
| January | 59°F | 40°F | 49.5°F | 6.47 | 13 | <1 | 5 | 9 h 50 min |
| February | 63°F | 42°F | 52.2°F | 6.71 | 11 | <1 | 2 | 10 h 40 min |
| March | 67°F | 44°F | 55.4°F | 4.98 | 10 | <1 | <1 | 11 h 50 min |
| April | 72°F | 46°F | 59.0°F | 1.97 | 6 | 1 | <1 | 13 h 10 min |
| May | 78°F | 51°F | 64.3°F | 1.31 | 3 | 4 | <1 | 14 h 10 min |
| June | 85°F | 54°F | 69.8°F | 0.35 | 1 | 10 | <1 | 14 h 50 min |
| July | 89°F | 56°F | 72.4°F | 0.01 | <1 | 13 | <1 | 14 h 40 min |
| August | 89°F | 56°F | 72.3°F | 0.04 | <1 | 13 | <1 | 13 h 40 min |
| September | 87°F | 54°F | 70.2°F | 0.06 | 1 | 10 | <1 | 12 h 30 min |
| October | 78°F | 49°F | 63.7°F | 1.65 | 3 | 3 | <1 | 11 h 20 min |
| November | 66°F | 44°F | 54.7°F | 3.24 | 8 | <1 | 1 | 10 h 10 min |
| December | 58°F | 40°F | 48.9°F | 7.18 | 13 | <1 | 5 | 9 h 30 min |
| Year (annual) | 74°F | 48°F | 61.0°F | 33.97 | 69 | 55 | 14 |
The same normals in °C and millimetres
| Month | High | Low | Mean | Precip. |
|---|---|---|---|---|
| January | 15°C | 5°C | 9.7°C | 164 mm |
| February | 17°C | 5°C | 11.2°C | 170 mm |
| March | 19°C | 7°C | 13.0°C | 126 mm |
| April | 22°C | 8°C | 15.0°C | 50 mm |
| May | 26°C | 10°C | 17.9°C | 33 mm |
| June | 30°C | 12°C | 21.0°C | 9 mm |
| July | 32°C | 13°C | 22.4°C | 0 mm |
| August | 32°C | 13°C | 22.4°C | 1 mm |
| September | 30°C | 12°C | 21.2°C | 2 mm |
| October | 26°C | 10°C | 17.6°C | 42 mm |
| November | 19°C | 7°C | 12.6°C | 82 mm |
| December | 15°C | 4°C | 9.4°C | 182 mm |
Source: NOAA NCEI 1991-2020 monthly normals, station USC00047643; 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.
When is the best weather to visit St. Helena?
Normal highs sit between 65°F and 85°F in March, April, May, October and November. Of those, October has the fewest days with rain (3).
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 USC00047643.
When is the last frost in St. Helena?
In St. Helena, the last spring frost (32°F) comes after February 3 in half of years and after March 13 in one year in ten; the first autumn frost comes before December 4 in half of years and before November 14 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
- February 31 year in 10 after March 13; 9 in 10 after December 28
- First autumn frost, half of years before
- December 41 year in 10 before November 14; 9 in 10 before January 1
NOAA puts the median frost-free season at 306 days, and the season without a hard freeze (28°F) at 349 days.
A hard freeze (28°F) comes after January 5 in half of springs and before December 17 in half of autumns.
The USDA's 2023 Plant Hardiness Zone Map puts St. Helena in zone 9b in ZIP code 94576, the nearest ZIP area. By the map's own method, the average of each year's coldest night over 1991-2020, St. Helena's NOAA station is in zone 9b: the coldest night of a typical year reaches 27°F (23 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 USC00047643; 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 in St. Helena?
In a normal year in St. Helena, 55 days a year reach 90°F (9 of them 100°F or more), 14 nights fall to 32°F or colder and 69 days bring measurable precipitation.
- Days at 90°F or more
- 55a normal year
- Days at 100°F or more
- 9a normal year
- Nights at 32°F or colder
- 14a normal year
- Days that stay below freezing
- fewer than 1high of 32°F or less
- Days with 0.01 inch of precipitation
- 69a normal year
The highest temperature in this station's record (since 1907) is 115°F, on July 11, 1913; the lowest is 11°F, on December 11, 1932.
The wettest calendar year on that record is 1983, with 75.40 in; the snowiest season is 1973–74, with 4.0 inches.
Source: NOAA NCEI 1991-2020 annual normals, station USC00047643; records: ACIS (NOAA Regional Climate Centers), station USC00047643. Records use complete months only; a month missing more than three days is left out.
How sunny is St. Helena?
Over a year 87% of the sunlight that would reach the ground under clear skies gets through the clouds in the area around St. Helena, 5.1 kWh per square metre a day. July is the sunniest month (95%) and December the cloudiest (72%).
| Month | Share of clear-sky sunshine | Sunshine, kWh/m² a day |
|---|---|---|
| January | 74% | 2.3 |
| February | 77% | 3.3 |
| March | 78% | 4.6 |
| April | 84% | 6.1 |
| May | 88% | 7.3 |
| June | 93% | 8.1 |
| July | 95% | 8.0 |
| August | 94% | 7.1 |
| September | 93% | 5.8 |
| October | 88% | 4.2 |
| November | 81% | 2.8 |
| December | 72% | 2.0 |
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 much heating and cooling does St. Helena need?
A normal year in St. Helena brings 2,352 heating degree days, 56% fewer than the median U.S. place (5,373), and 922 cooling degree days, 2% more than the median (908). Five months need 300 or more heating degree days, a monthly mean of about 55°F or colder. For gardens and crops, NOAA counts 4,262 growing degree days a year above 50°F.
| Month | Heating | Cooling |
|---|---|---|
| January | 481 | 0 |
| February | 359 | 1 |
| March | 302 | 3 |
| April | 195 | 16 |
| May | 87 | 65 |
| June | 16 | 160 |
| July | 1 | 229 |
| August | 1 | 226 |
| September | 10 | 166 |
| October | 92 | 53 |
| November | 311 | 3 |
| December | 498 | 0 |
| Year (annual) | 2,352 | 922 |
Source: NOAA NCEI 1991-2020 monthly normals, station USC00047643. 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.
Has St. Helena been warmer than normal lately?
Over 76 complete years from 1908 to 2018, the station's annual mean temperature rose at a fitted 2.8°F per century (+0.15°C per decade). A fitted rate summarizes the whole record; single years swing well above and below it.
Source: trend: NOAA GHCN-Monthly v4. A year counts only with all twelve months complete.
How does St. Helena's climate compare?
St. Helena is warmer than 104 of the other 218 places with their own NOAA record in California, and wetter than 174. Across the 7,247 U.S. stations with normals, its annual mean of 61.0°F is warmer than 77%.
In the Köppen classification computed from these normals, St. Helena 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 St. Helena share this climate record?
4 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.
- Kenwood, CA750 people · 7 mi SW
- Deer Park, CA740 people · 2 mi N
- Rutherford, CA89 people · 4 mi SE
- Oakville, CA0 people · 6 mi SE
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 St. Helena's climate
What is the coldest month in St. Helena?
December, with a normal high of 58°F and a normal low of 40°F. The warmest is July (89°F by day, 56°F at night).
Does it snow in St. Helena?
Rarely: NOAA's normal is no measurable snow a year.
What is the rainiest month in St. Helena?
December, with 7.18 in of precipitation in a normal year; the driest is July with 0.01 in.
What plant hardiness zone is St. Helena in?
The USDA's 2023 map puts St. Helena in zone 9b in ZIP code 94576, the nearest ZIP area. By the same method at St. Helena's NOAA station, the coldest night of a typical year reaches 27°F (23 years, 1991-2020): zone 9b.
How late can frost come in St. Helena?
In half of years the last 32°F night comes after February 3; in one year in ten it comes after March 13. Waiting until March 13 leaves about a one-in-ten chance of a later frost.
Nearby and next
- California climateThe statewide normals and every place in the state
- Angwin, CA5 miles from this station, 57.4°F a year
- Calistoga, CA9 miles from this station, 60.1°F a year
- Sonoma, CA14 miles from this station, 59.0°F a year
- Santa Rosa, CA18 miles from this station, 58.1°F a year
- Compare two placesMonth by month, side by side
Download these normals (CSV) · JSON · How these figures are made