U.S. Frost Dates by Weather Station

NOAA station USC00105462 · Idaho

Frost Dates for Mackay Lost River Ranger Station, ID

NOAA weather station Mackay Lost River Ranger Station (USC00105462), Idaho · elevation 5897 ft (1797 m) · hardiness zone 5b*

  • Lower-risk planting dateafter June 21only 1 spring in 10 still has a frost later
  • Typical last frostJune 150% chance of a frost after it
  • Typical first fall frostSeptember 2050% chance of a frost before it
  • Frost-free season109 daystypical year, 32°F
  • Hardiness zone5bcalculated*

The year at a glance

Frost season at Mackay Lost River Ranger Station, IDSpring: a frost after May 13 in 9 years out of 10, after June 1 in half of the years, after June 21 in 1 year out of 10. Fall: the first frost comes before September 4 in 1 year out of 10, before September 20 in half of the years, before October 3 in 9 years out of 10. Typical frost-free season: 109 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~Jun 1First frost ~Sep 20Frost season at Mackay Lost River Ranger Station, IDSpring: a frost after May 13 in 9 years out of 10, after June 1 in half of the years, after June 21 in 1 year out of 10. Fall: the first frost comes before September 4 in 1 year out of 10, before September 20 in half of the years, before October 3 in 9 years out of 10. Typical frost-free season: 109 days.JFMAMJJASONDLast frost ~Jun 1First 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 27 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)June 21September 4
50% (typical)June 1September 20
90% (likely)May 13October 3
Frost-free season (32°F, 50% chance)
109 days
Hard frost (28°F), last in spring, 50% chance after
May 13
Hard frost (28°F), first in fall, 50% chance before
October 1

The last 15 years (2006–2020 normals)

Typical last frost May 31 (1 day earlier than the 30-year normal), typical first fall frost September 24 (4 days later). Frost-free season 114 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 Mackay Lost River Ranger Station: chance of a frost after this dateSpring, chance of a frost after the date: 10% June 21, 20% June 15, 30% June 9, 40% June 5, 50% June 1, 60% May 29, 70% May 24, 80% May 20, 90% May 13. Observed last spring frost in recent years: 2022 May 23, 2023 May 11, 2024 May 18, 2025 April 22, 2026 May 19.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1Jul 1Jun 21Jun 1May 132022: May 232023: May 112024: May 182025: April 222026: May 19Fall at Mackay Lost River Ranger Station: chance of a frost before this dateFall, chance of a frost before the date: 10% September 4, 20% September 9, 30% September 13, 40% September 16, 50% September 20, 60% September 22, 70% September 25, 80% September 28, 90% October 3. Observed first fall frost in recent years: 2022 October 23, 2023 October 26, 2024 October 19.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Nov 1Sep 4Sep 20Oct 32022: October 232023: October 262024: October 19
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
2021incomplete recordincomplete record
2022May 23October 23
2023May 11October 26
2024May 18October 19
2025April 22incomplete record
2026May 19season not over yet

What these dates mean

June 21 is the cautious light-frost date for spring at Mackay Lost River Ranger Station; on that day there is only a 10 % chance of a frost later, so a gardener can plan to set out tender crops with minimal risk. The 50% date is June 1, which marks an even odds point for a frost later than that day, while the 90 % date of May 13 signals a gamble for early planting.

In the fall the cautious light-frost date is September 4, when there is only a 10 % chance that a frost has arrived earlier, allowing harvest of tender crops to be finished safely. The 50% date is September 20, indicating an even chance of a frost earlier than that day, and October 3 carries a 90 % chance of a frost earlier, making late planting risky.

Hard-frost thresholds (28 °F) are more damaging than light-frost thresholds (32 °F); the 50% hard-frost dates are May 13 in spring and October 1 in fall.

How to read the probabilities

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

  • Spring: "10% after June 21" 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 4" 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 Mackay Lost River Ranger Station, IDNumbered nearest stations: 1 — Chilly Barton Flat, ID — 11 mi; 2 — Idaho Falls 46 W, ID — 43 mi; 3 — Richfield, ID — 66 mi; 4 — Dubois, ID — 74 mi; 5 — Shoshone, ID — 79 mi.ID25 mi · 40 km1. Chilly Barton Flat, ID — 11 mi12. Idaho Falls 46 W, ID — 43 mi23. Richfield, ID — 66 mi34. Dubois, ID — 74 mi45. Shoshone, ID — 79 mi5
  1. Chilly Barton Flat, ID 11 mi (18 km)
  2. Idaho Falls 46 W, ID 43 mi (69 km)
  3. Richfield, ID 66 mi (106 km)
  4. Dubois, ID 74 mi (119 km)
  5. Shoshone, ID 79 mi (127 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: 43.9178, -113.6153.