U.S. Frost Dates by Weather Station

NOAA station USC00241737 · Montana

Frost Dates for Choteau, MT

NOAA weather station Choteau (USC00241737), Montana · elevation 3845 ft (1172 m) · hardiness zone 4b*

  • Lower-risk planting dateafter June 20only 1 spring in 10 still has a frost later
  • Typical last frostMay 2850% chance of a frost after it
  • Typical first fall frostSeptember 1250% chance of a frost before it
  • Frost-free season104 daystypical year, 32°F
  • Hardiness zone4bcalculated*

The year at a glance

Frost season at Choteau, MTSpring: a frost after May 15 in 9 years out of 10, after May 28 in half of the years, after June 20 in 1 year out of 10. Fall: the first frost comes before August 30 in 1 year out of 10, before September 12 in half of the years, before September 27 in 9 years out of 10. Typical frost-free season: 104 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 28First frost ~Sep 12Frost season at Choteau, MTSpring: a frost after May 15 in 9 years out of 10, after May 28 in half of the years, after June 20 in 1 year out of 10. Fall: the first frost comes before August 30 in 1 year out of 10, before September 12 in half of the years, before September 27 in 9 years out of 10. Typical frost-free season: 104 days.JFMAMJJASONDLast frost ~May 28First frost ~Sep 12
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 20August 30
50% (typical)May 28September 12
90% (likely)May 15September 27
Frost-free season (32°F, 50% chance)
104 days
Hard frost (28°F), last in spring, 50% chance after
May 14
Hard frost (28°F), first in fall, 50% chance before
September 25

The last 15 years (2006–2020 normals)

Typical last frost May 26 (2 days earlier than the 30-year normal), typical first fall frost September 12. Frost-free season 108 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 Choteau: chance of a frost after this dateSpring, chance of a frost after the date: 10% June 20, 20% June 10, 30% June 4, 40% June 1, 50% May 28, 60% May 25, 70% May 22, 80% May 19, 90% May 15. Observed last spring frost in recent years: 2021 May 30, 2022 May 24, 2023 May 10, 2024 June 1, 2025 June 5, 2026 May 22.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1Jul 1Jun 20May 28May 152021: May 302022: May 242023: May 102024: June 12025: June 52026: May 22Fall at Choteau: chance of a frost before this dateFall, chance of a frost before the date: 10% August 30, 20% September 3, 30% September 7, 40% September 10, 50% September 12, 60% September 15, 70% September 18, 80% September 21, 90% September 27. Observed first fall frost in recent years: 2021 September 1, 2022 October 7, 2023 September 29, 2024 September 21, 2025 October 4.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Aug 30Sep 12Sep 272021: September 12022: October 72023: September 292024: September 212025: October 4
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 30September 1
2022May 24October 7
2023May 10September 29
2024June 1September 21
2025June 5October 4
2026May 22season not over yet

What these dates mean

June 20 is the cautious spring date for setting out tender crops in Choteau; at that point the chance of a later frost is only 10 %, giving a gardener a relatively secure window to begin planting. The 50% light-frost date of May 28 marks the point where the odds of a frost later than that day are even, while the 50% hard-frost date of May 14 indicates an even chance of a temperature reaching 28 °F. In the fall the cautious date is August 30, when there is only a 10 % chance that a frost has already arrived, allowing a gardener to finish harvesting tender crops. The median light-frost date of September 12 and the hard-frost median of September 25 show where the odds balance for each type of frost. A hard frost (28 °F) can cause more severe damage than a light frost (32 °F) because it reaches a lower temperature.

How to read the probabilities

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

  • Spring: "10% after June 20" 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 30" 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 Choteau, MTNumbered nearest stations: 1 — Fairfield, MT — 17 mi; 2 — Conrad, MT — 27 mi; 3 — Sun River, MT — 32 mi; 4 — Great Falls, MT — 45 mi; 5 — Cascade, MT — 47 mi.25 mi · 40 km1. Fairfield, MT — 17 mi12. Conrad, MT — 27 mi23. Sun River, MT — 32 mi34. Great Falls, MT — 45 mi45. Cascade, MT — 47 mi5
  1. Fairfield, MT 17 mi (28 km)
  2. Conrad, MT 27 mi (43 km)
  3. Sun River, MT 32 mi (51 km)
  4. Great Falls, MT 45 mi (72 km)
  5. Cascade, MT 47 mi (76 km)

Sources

* Hardiness zone 4b: zone derived from PRISM 2023 grid at the station location (not the official USDA map). Source data: PRISM Group, Oregon State University.

Station coordinates: 47.8206, -112.1922.