U.S. Frost Dates by Weather Station

NOAA station USC00241875 · Montana

Frost Dates for Cohagen, MT

NOAA weather station Cohagen (USC00241875), Montana · elevation 2720 ft (829 m) · hardiness zone 4a*

  • Lower-risk planting dateafter June 9only 1 spring in 10 still has a frost later
  • Typical last frostMay 2550% chance of a frost after it
  • Typical first fall frostSeptember 1450% chance of a frost before it
  • Frost-free season110 daystypical year, 32°F
  • Hardiness zone4acalculated*

The year at a glance

Frost season at Cohagen, MTSpring: a frost after May 12 in 9 years out of 10, after May 25 in half of the years, after June 9 in 1 year out of 10. Fall: the first frost comes before August 31 in 1 year out of 10, before September 14 in half of the years, before September 26 in 9 years out of 10. Typical frost-free season: 110 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 25First frost ~Sep 14Frost season at Cohagen, MTSpring: a frost after May 12 in 9 years out of 10, after May 25 in half of the years, after June 9 in 1 year out of 10. Fall: the first frost comes before August 31 in 1 year out of 10, before September 14 in half of the years, before September 26 in 9 years out of 10. Typical frost-free season: 110 days.JFMAMJJASONDLast frost ~May 25First frost ~Sep 14
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)June 9August 31
50% (typical)May 25September 14
90% (likely)May 12September 26
Frost-free season (32°F, 50% chance)
110 days
Hard frost (28°F), last in spring, 50% chance after
May 16
Hard frost (28°F), first in fall, 50% chance before
September 23

The last 15 years (2006–2020 normals)

Typical last frost May 22 (3 days earlier than the 30-year normal), typical first fall frost September 13 (1 day earlier). Frost-free season 111 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 Cohagen: chance of a frost after this dateSpring, chance of a frost after the date: 10% June 9, 20% June 3, 30% May 31, 40% May 28, 50% May 25, 60% May 23, 70% May 20, 80% May 16, 90% May 12. Observed last spring frost in recent years: 2021 May 10, 2022 May 21, 2023 May 17, 2024 May 30, 2025 May 1, 2026 May 22.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1Jun 9May 25May 122021: May 102022: May 212023: May 172024: May 302025: May 12026: May 22Fall at Cohagen: chance of a frost before this dateFall, chance of a frost before the date: 10% August 31, 20% September 5, 30% September 8, 40% September 11, 50% September 14, 60% September 16, 70% September 19, 80% September 22, 90% September 26. Observed first fall frost in recent years: 2021 October 12, 2022 October 6, 2023 October 5, 2024 September 21, 2025 September 5.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Aug 31Sep 14Sep 262021: October 122022: October 62023: October 52024: September 212025: September 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 10October 12
2022May 21October 6
2023May 17October 5
2024May 30September 21
2025May 1September 5
2026May 22season not over yet

What these dates mean

June 9 is the cautious spring light-frost date, meaning there is only a 10 % chance of a frost later and it lets a gardener set out tender crops with minimal risk. The median light-frost date, May 25, marks the point at which the chance of a frost later is even; earlier planting carries a much higher probability of frost damage.

August 31 is the cautious fall light-frost date, with a 10 % chance that a frost has arrived earlier, so it is the date to finish harvesting tender crops. The median fall light-frost date, September 14, indicates an even chance of a frost earlier, while the 90 % risk date, September 26, signals that counting on tender crops this late is a gamble. The 50 % hard-frost dates are May 16 in spring and September 23 in fall; a hard frost at 28 °F is more damaging than a light frost at 32 °F because the lower temperature can kill plants that survive a light freeze.

How to read the probabilities

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

  • Spring: "10% after June 9" 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 31" 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 Cohagen, MTNumbered nearest stations: 1 — Brockway, MT — 40 mi; 2 — Ingomar, MT — 43 mi; 3 — Circle, MT — 54 mi; 4 — Forsyth, MT — 54 mi; 5 — near Miles City, MT — 56 mi.MT25 mi · 40 km1. Brockway, MT — 40 mi12. Ingomar, MT — 43 mi23. Circle, MT — 54 mi34. Forsyth, MT — 54 mi45. near Miles City, MT — 56 mi5
  1. Brockway, MT 40 mi (65 km)
  2. Ingomar, MT 43 mi (70 km)
  3. Circle, MT 54 mi (87 km)
  4. Forsyth, MT 54 mi (87 km)
  5. near Miles City, MT 56 mi (90 km)

Sources

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

Station coordinates: 47.0564, -106.6164.