U.S. Frost Dates by Weather Station

NOAA station USC00047085 · California

Frost Dates for Portola, CA

NOAA weather station Portola (USC00047085), California · elevation 4850 ft (1478 m) · hardiness zone 6b*

  • Lower-risk planting dateafter July 2only 1 spring in 10 still has a frost later
  • Typical last frostJune 1150% chance of a frost after it
  • Typical first fall frostSeptember 1050% chance of a frost before it
  • Frost-free season91 daystypical year, 32°F
  • Hardiness zone6bcalculated*

The year at a glance

Frost season at Portola, CASpring: a frost after May 25 in 9 years out of 10, after June 11 in half of the years, after July 2 in 1 year out of 10. Fall: the first frost comes before August 22 in 1 year out of 10, before September 10 in half of the years, before September 28 in 9 years out of 10. Typical frost-free season: 91 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~Jun 11First frost ~Sep 10Frost season at Portola, CASpring: a frost after May 25 in 9 years out of 10, after June 11 in half of the years, after July 2 in 1 year out of 10. Fall: the first frost comes before August 22 in 1 year out of 10, before September 10 in half of the years, before September 28 in 9 years out of 10. Typical frost-free season: 91 days.JFMAMJJASONDLast frost ~Jun 11First frost ~Sep 10
frost likely chance fading 90% → 10% frost-free in a typical yearThe year at a glance. Blue: frost is likely; the gradient fades as the chance drops from 90% to 10%; green: the frost-free part of a typical year.

Frost dates (32°F)

Probability of frost, based on 29 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)July 2August 22
50% (typical)June 11September 10
90% (likely)May 25September 28
Frost-free season (32°F, 50% chance)
91 days
Hard frost (28°F), last in spring, 50% chance after
May 17
Hard frost (28°F), first in fall, 50% chance before
October 1

The last 15 years (2006–2020 normals)

Typical last frost June 9 (2 days earlier than the 30-year normal), typical first fall frost September 13 (3 days later). Frost-free season 95 days.

A shorter, more recent NOAA period. The 30-year normals above remain the main reference: 15 years carry more year-to-year noise.

Add these dates to your calendar Yearly reminders for the typical last frost, the lower-risk planting date and the typical first fall frost (.ics).

How the chance of frost changes with the date

Spring at Portola: chance of a frost after this dateSpring, chance of a frost after the date: 10% July 2, 20% June 23, 30% June 18, 40% June 14, 50% June 11, 60% June 7, 70% June 4, 80% June 1, 90% May 25. Observed last spring frost in recent years: 2021 June 10, 2022 July 10, 2023 May 11, 2024 June 18, 2025 June 23.Spring: last frostChance of a frost after this date10%50%90%Jun 1Jul 1Jul 2Jun 11May 252021: June 102022: July 102023: May 112024: June 182025: June 23Fall at Portola: chance of a frost before this dateFall, chance of a frost before the date: 10% August 22, 20% August 29, 30% September 3, 40% September 7, 50% September 10, 60% September 13, 70% September 17, 80% September 21, 90% September 28. Observed first fall frost in recent years: 2021 September 27, 2022 September 15, 2023 September 13, 2024 September 18, 2025 October 6.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Aug 22Sep 10Sep 282021: September 272022: September 152023: September 132024: September 182025: October 6
Two charts, because spring and fall answer different questions: in spring, the chance that a frost still comes after a date; in fall, the chance that the first frost has already come before it. NOAA publishes these odds from 10% to 90%, so the lines stop there. ◆ marks the actual date in each recent year.

What actually happened in recent years

Observed dates of the last spring and first fall frost (32°F) at this station; ◆ marks them on the charts above
YearLast spring frostFirst fall frost
2021June 10September 27
2022July 10September 15
2023May 11September 13
2024June 18September 18
2025June 23October 6
2026incomplete recordseason not over yet

What these dates mean

August 22 is the cautious fall date to finish harvesting tender crops in Portola, because only a 10% chance of a frost has occurred earlier. The 50% fall light-frost date is September 10; there is a 50% chance of a frost earlier than this date. A hard-frost median for fall falls on October 1, marking when a 28 °F event is equally likely to have happened or not.

June 11 is the 50% spring light-frost date, meaning there is a 50% chance of a frost later than this date. The cautious spring date for setting out tender crops is July 2, when the risk of a later frost drops to 10%; planting on May 25 carries a 90% risk. The hard-frost median for spring is May 17. A hard frost at 28 °F matters more than a light frost at 32 °F because it can injure plants that survive milder freezes.

How to read the probabilities

The dates are probabilities from 30 years of weather, not a forecast for this year.

  • Spring: "10% after July 2" means that in about one spring out of ten a frost still happens after that date. The 90% date is the earliest: a frost after it happens in most years.
  • Fall: "10% before August 22" means that in about one fall out of ten the first frost arrives before that date.
  • 50% is the typical date: half of the years are earlier, half are later.
  • A light frost (32°F) damages tender plants; a hard frost (28°F) also stops most hardy vegetables.

Nearby weather stations

Map of weather stations near Portola, CANumbered nearest stations: 1 — Strawberry Valley, CA — 38 mi; 2 — near Greenville, CA — 41 mi; 3 — Sparks, NV — 43 mi; 4 — near Reno, NV — 43 mi; 5 — Tahoe City, CA — 47 mi.NV25 mi · 40 km1. Strawberry Valley, CA — 38 mi12. near Greenville, CA — 41 mi23. Sparks, NV — 43 mi34. near Reno, NV — 43 mi45. Tahoe City, CA — 47 mi5
  1. Strawberry Valley, CA 38 mi (61 km)
  2. near Greenville, CA 41 mi (66 km)
  3. Sparks, NV 43 mi (69 km)
  4. near Reno, NV 43 mi (70 km)
  5. Tahoe City, CA 47 mi (76 km)

Sources

* Hardiness zone 6b: zone derived from PRISM 2023 grid at the station location (not the official USDA map). Source data: PRISM Group, Oregon State University.

Station coordinates: 39.8053, -120.4719.