Frost Dates for 96122 — Portola, California

Half of springs at Portola, CA see their last 32°F night by June 11; only one in ten is past it by May 25, and one in ten still has one after July 2. Planting tender crops after June 18 leaves a 30% risk of a frost on them, after June 23 20%, and after July 2 10%. The 21 days between the even-odds date and the one-in-ten date is narrower than at 84% 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 September 10 in half of years, as early as August 22 one year in ten, and holds off past September 28 one year in ten. A hard freeze of 28°F follows 21 days later, around October 1; in spring the one-in-ten date for 28°F is June 4, 28 days before the 32°F one. The freeze-free season lasts 91 days in a typical year, the shortest of the 219 California stations; it reaches 115 days one year in ten and falls short of 62 one year in ten. On average 205 nights a year drop to 32°F or below. Severe cold of 16°F or below comes every winter, usually between November 16 and March 14.

July is the warmest month at Portola, CA, averaging 86°F by day and 45°F at night; December is the coldest at 42°F and 19°F. A freezing night is all but certain in every month from September to May (a 90% chance or more). No month is safe from a 32°F night: even the warmest has at least a 10% chance. In spring, June still brings one in 79% of years and July still brings one in 12% of years.

Warmth for growing adds up to 1,616 growing degree days a year (base 50°F), fewer than at 97% of the 283 California stations. 24% of them (395) have built up by the end of June, and July adds the most, 481. Precipitation totals 23.4 inches a year, most in December (4.3") and least in July (0.3"); May to September bring 12% of it.

The nearest other station, Doyle 4 SSE, CA, 24 miles northeast of Portola and 460 feet lower, has a last frost 21 days earlier (May 21), a first frost 21 days later (October 1), a season 39 days longer.

The first measurable snow (0.1 inch or more) in Portola usually arrives around November 28, and the last of the season around March 20. Those are the middle dates across the winters of 1991–2020 at Sierraville Rs, CA, 16.2 miles away: half of years saw snow earlier, half later. Every one of those winters saw snow. The last 32°F night of spring typically comes 83 days after the last snow.

ZIP 96122 is under a mile from Portola, CA, which stands at 4,850 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. The 2023 USDA Plant Hardiness Zone Map lists ZIP 96122 in zone 6b, the band where the coldest night of the year averaged -5 to 0°F (-20.6 to -17.8°C) over 1991–2020; a zone describes winter lows, not frost dates.