Frost Dates for 94503 — American Canyon, California

Half of springs at Napa Co Airport, CA see their last 32°F night by March 5; only one in ten is past it by January 26, and one in ten still has one after April 16. Planting tender crops after March 25 leaves a 30% risk of a frost on them, after April 4 20%, and after April 16 10%. The 42 days between the even-odds date and the one-in-ten date is wider than at 90% of the 220 California stations behind this site's ZIP pages, so a fixed calendar date is a gamble here.

In fall the first 32°F night comes by November 21 in half of years, as early as November 3 one year in ten, and holds off past December 13 one year in ten. A hard freeze of 28°F follows 14 days later, around December 5; in spring the one-in-ten date for 28°F is February 28, 47 days before the 32°F one. The freeze-free season lasts 256 days in a typical year, shorter than at 65% of the 219 California stations; it reaches 305 days one year in ten and falls short of 212 one year in ten. On average 34 nights a year drop to 32°F or below. Nights of 36°F, when frost can still settle on leaves in low, open spots, continue 43 days longer, to about April 17, and return around November 4. Nights of 24°F, cold enough to damage hardier plants, come in 52% of winters.

August is the warmest month at Napa Co Airport, CA, averaging 80°F by day and 54°F at night; December is the coldest at 57°F and 38°F. From May to October the chance of a 32°F night stays under 10% each month. In spring, February still brings one in 67% of years and March still brings one in 44% of years and April still brings one in 24% of years.

Warmth for growing adds up to 3,153 growing degree days a year (base 50°F), fewer than at 81% of the 283 California stations. 37% of them (1,156) have built up by the end of June, and August adds the most, 533. Precipitation totals 20.3 inches a year, most in January (4.2") and least in July (0.0"); May to September bring 5% of it.

The nearest other station, Vallejo, CA, 6 miles southeast of American Canyon and 327 feet higher, has a last frost 47 days earlier (January 17), a first frost 36 days later (December 27), a season 90 days longer.

Measurable snow is rare here: only 0 of 20 winters from 1991 to 2020 recorded 0.1 inch or more at Napa State Hospital, CA, 7.4 miles away, so there is no typical first-snow date to plan around.

ZIP 94503 is 3 miles from Napa Co Airport, CA, which stands at 14 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 1 other ZIP code shares the station; its figures are presented in full for 94589 (Vallejo). The 2023 USDA Plant Hardiness Zone Map lists ZIP 94503 in zone 9b, the band where the coldest night of the year averaged 25 to 30°F (-3.9 to -1.1°C) over 1991–2020; a zone describes winter lows, not frost dates.