U.S. Frost Dates by Weather Station

NOAA station USC00480140 · Wyoming

Frost Dates for Alta, WY

NOAA weather station Alta 1 NNW (USC00480140), Wyoming · elevation 6440 ft (1963 m) · hardiness zone 5a*

  • Lower-risk planting dateafter July 4only 1 spring in 10 still has a frost later
  • Typical last frostJune 1450% chance of a frost after it
  • Typical first fall frostSeptember 1150% chance of a frost before it
  • Frost-free season85 daystypical year, 32°F
  • Hardiness zone5acalculated*

The year at a glance

Frost season at Alta, WYSpring: a frost after May 23 in 9 years out of 10, after June 14 in half of the years, after July 4 in 1 year out of 10. Fall: the first frost comes before August 28 in 1 year out of 10, before September 11 in half of the years, before September 26 in 9 years out of 10. Typical frost-free season: 85 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~Jun 14First frost ~Sep 11Frost season at Alta, WYSpring: a frost after May 23 in 9 years out of 10, after June 14 in half of the years, after July 4 in 1 year out of 10. Fall: the first frost comes before August 28 in 1 year out of 10, before September 11 in half of the years, before September 26 in 9 years out of 10. Typical frost-free season: 85 days.JFMAMJJASONDLast frost ~Jun 14First frost ~Sep 11
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 28 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)July 4August 28
50% (typical)June 14September 11
90% (likely)May 23September 26
Frost-free season (32°F, 50% chance)
85 days
Hard frost (28°F), last in spring, 50% chance after
May 24
Hard frost (28°F), first in fall, 50% chance before
September 23

The last 15 years (2006–2020 normals)

Typical last frost June 13 (1 day earlier than the 30-year normal), typical first fall frost September 12 (1 day later). Frost-free season 92 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 Alta: chance of a frost after this dateSpring, chance of a frost after the date: 10% July 4, 20% June 26, 30% June 21, 40% June 18, 50% June 14, 60% June 10, 70% June 6, 80% May 31, 90% May 23. Observed last spring frost in recent years: 2021 June 11, 2022 June 15, 2023 June 22, 2024 June 21, 2025 June 5, 2026 June 11.Spring: last frostChance of a frost after this date10%50%90%Jun 1Jul 1Jul 4Jun 14May 232021: June 112022: June 152023: June 222024: June 212025: June 52026: June 11Fall at Alta: chance of a frost before this dateFall, chance of a frost before the date: 10% August 28, 20% September 1, 30% September 4, 40% September 8, 50% September 11, 60% September 14, 70% September 18, 80% September 21, 90% September 26. Observed first fall frost in recent years: 2021 September 17, 2022 September 10, 2023 September 21, 2024 August 29, 2025 October 5.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Aug 28Sep 11Sep 262021: September 172022: September 102023: September 212024: August 292025: October 5
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 11September 17
2022June 15September 10
2023June 22September 21
2024June 21August 29
2025June 5October 5
2026June 11season not over yet

What these dates mean

The cautious spring date for light frost in Alta is July 4; with only a 10 % chance of a frost later than that, a gardener can schedule the first planting of tender crops after this day.

The median light-frost dates are June 14 and September 11; each marks a 50 % chance of a frost occurring later (spring) or earlier (fall). Using the cautious fall date of August 28, which carries a 10 % chance of a frost earlier than that, a gardener can plan the final harvest of tender crops before frost becomes likely. The 50 % hard-frost dates are May 24 in spring and September 23 in fall. The record spans 28 years, giving confidence in these probabilities.

A hard frost at 28 °F can damage plants that would otherwise survive a light frost at 32 °F.

How to read the probabilities

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

  • Spring: "10% after July 4" 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 28" 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 Alta, WYNumbered nearest stations: 1 — Dubois, ID — 66 mi; 2 — Yellowstone Park Mammoth, WY — 85 mi; 3 — Fort Hall, ID — 86 mi; 4 — near Pocatello, ID — 92 mi; 5 — Burris, WY — 92 mi.WYIDMT50 mi · 80 km1. Dubois, ID — 66 mi12. Yellowstone Park Mammoth, WY — 85 mi23. Fort Hall, ID — 86 mi34. near Pocatello, ID — 92 mi45. Burris, WY — 92 mi5
  1. Dubois, ID 66 mi (107 km)
  2. Yellowstone Park Mammoth, WY 85 mi (137 km)
  3. Fort Hall, ID 86 mi (138 km)
  4. near Pocatello, ID 92 mi (148 km)
  5. Burris, WY 92 mi (149 km)

Sources

* Hardiness zone 5a: zone derived from PRISM 2023 grid at the station location (not the official USDA map). Source data: PRISM Group, Oregon State University.

Station coordinates: 43.7728, -111.0339.