U.S. Frost Dates by Weather Station

NOAA station USC00103297 · Idaho

Frost Dates for Fort Hall, ID

NOAA weather station Fort Hall 1 NNE (USC00103297), Idaho · elevation 4465 ft (1361 m) · hardiness zone 5b*

  • Lower-risk planting dateafter June 12only 1 spring in 10 still has a frost later
  • Typical last frostMay 2050% chance of a frost after it
  • Typical first fall frostSeptember 2050% chance of a frost before it
  • Frost-free season119 daystypical year, 32°F
  • Hardiness zone5bcalculated*

The year at a glance

Frost season at Fort Hall, IDSpring: a frost after May 6 in 9 years out of 10, after May 20 in half of the years, after June 12 in 1 year out of 10. Fall: the first frost comes before September 1 in 1 year out of 10, before September 20 in half of the years, before October 7 in 9 years out of 10. Typical frost-free season: 119 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 20First frost ~Sep 20Frost season at Fort Hall, IDSpring: a frost after May 6 in 9 years out of 10, after May 20 in half of the years, after June 12 in 1 year out of 10. Fall: the first frost comes before September 1 in 1 year out of 10, before September 20 in half of the years, before October 7 in 9 years out of 10. Typical frost-free season: 119 days.JFMAMJJASONDLast frost ~May 20First frost ~Sep 20
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)June 12September 1
50% (typical)May 20September 20
90% (likely)May 6October 7
Frost-free season (32°F, 50% chance)
119 days
Hard frost (28°F), last in spring, 50% chance after
May 6
Hard frost (28°F), first in fall, 50% chance before
October 4

The last 15 years (2006–2020 normals)

Typical last frost May 21 (1 day later than the 30-year normal), typical first fall frost September 25 (5 days later). Frost-free season 125 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 Fort Hall: chance of a frost after this dateSpring, chance of a frost after the date: 10% June 12, 20% June 4, 30% May 31, 40% May 25, 50% May 20, 60% May 17, 70% May 13, 80% May 10, 90% May 6. Observed last spring frost in recent years: 2021 May 12, 2022 May 24, 2025 May 1, 2026 June 10.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1Jun 12May 20May 62021: May 122022: May 242025: May 12026: June 10Fall at Fort Hall: chance of a frost before this dateFall, chance of a frost before the date: 10% September 1, 20% September 7, 30% September 13, 40% September 17, 50% September 20, 60% September 24, 70% September 28, 80% October 2, 90% October 7. Observed first fall frost in recent years: 2021 September 18, 2025 October 5.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 1Sep 20Oct 72021: September 182025: 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
2021May 12September 18
2022May 24incomplete record
2023incomplete recordno data
2024no dataincomplete record
2025May 1October 5
2026June 10season not over yet

What these dates mean

The 119 days between the 50% light-frost dates of May 20 and September 20 define the central window in which a light frost is equally likely to have occurred or not. Within that span gardeners can time planting and harvesting with lower overall risk.

In spring the cautious date for setting out tender crops is June 12, when there is only a 10 % chance of a light frost later; planting on May 6 carries a 90 % chance and is a gamble. In fall the cautious date to finish harvesting tender crops is September 1, with a 10 % chance of a light frost earlier, while counting on crops as late as October 7 involves a 90 % chance and is a gamble.

A hard frost, measured at 28 °F, is more critical because its 50% dates—May 6 in spring and October 4 in fall—mark the point where temperatures drop far enough to damage or kill tender plants.

How to read the probabilities

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

  • Spring: "10% after June 12" 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 September 1" 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 Fort Hall, IDNumbered nearest stations: 1 — near Pocatello, ID — 10 mi; 2 — Pocatello (Airport), ID — 12 mi; 3 — Idaho Falls 46 W, ID — 43 mi; 4 — Minidoka Dam, ID — 61 mi; 5 — Preston, ID — 71 mi.WYUT25 mi · 40 km1. near Pocatello, ID — 10 mi12. Pocatello (Airport), ID — 12 mi23. Idaho Falls 46 W, ID — 43 mi34. Minidoka Dam, ID — 61 mi45. Preston, ID — 71 mi5
  1. near Pocatello, ID 10 mi (17 km)
  2. Pocatello (Airport), ID 12 mi (19 km)
  3. Idaho Falls 46 W, ID 43 mi (69 km)
  4. Minidoka Dam, ID 61 mi (98 km)
  5. Preston, ID 71 mi (114 km)

Sources

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

Station coordinates: 43.0425, -112.4144.