U.S. Frost Dates by Weather Station

NOAA station USC00242122 · Montana

Frost Dates for Culbertson, MT

NOAA weather station Culbertson (USC00242122), Montana · elevation 1940 ft (591 m) · hardiness zone 3b*

  • Lower-risk planting dateafter June 3only 1 spring in 10 still has a frost later
  • Typical last frostMay 2150% chance of a frost after it
  • Typical first fall frostSeptember 2050% chance of a frost before it
  • Frost-free season120 daystypical year, 32°F
  • Hardiness zone3bcalculated*

The year at a glance

Frost season at Culbertson, MTSpring: a frost after May 8 in 9 years out of 10, after May 21 in half of the years, after June 3 in 1 year out of 10. Fall: the first frost comes before September 6 in 1 year out of 10, before September 20 in half of the years, before September 30 in 9 years out of 10. Typical frost-free season: 120 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 21First frost ~Sep 20Frost season at Culbertson, MTSpring: a frost after May 8 in 9 years out of 10, after May 21 in half of the years, after June 3 in 1 year out of 10. Fall: the first frost comes before September 6 in 1 year out of 10, before September 20 in half of the years, before September 30 in 9 years out of 10. Typical frost-free season: 120 days.JFMAMJJASONDLast frost ~May 21First 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 30 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)June 3September 6
50% (typical)May 21September 20
90% (likely)May 8September 30
Frost-free season (32°F, 50% chance)
120 days
Hard frost (28°F), last in spring, 50% chance after
May 10
Hard frost (28°F), first in fall, 50% chance before
September 27

The last 15 years (2006–2020 normals)

Typical last frost May 21 (same as the 30-year normal), typical first fall frost September 20. Frost-free season 122 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 Culbertson: chance of a frost after this dateSpring, chance of a frost after the date: 10% June 3, 20% May 29, 30% May 26, 40% May 24, 50% May 21, 60% May 18, 70% May 16, 80% May 12, 90% May 8. Observed last spring frost in recent years: 2021 May 13, 2022 June 3, 2024 May 24, 2025 May 22, 2026 May 19.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1Jun 3May 21May 82021: May 132022: June 32024: May 242025: May 222026: May 19Fall at Culbertson: chance of a frost before this dateFall, chance of a frost before the date: 10% September 6, 20% September 10, 30% September 14, 40% September 17, 50% September 20, 60% September 22, 70% September 24, 80% September 26, 90% September 30. Observed first fall frost in recent years: 2021 September 17, 2022 October 7, 2024 October 1, 2025 September 6.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 6Sep 20Sep 302021: September 172022: October 72024: October 12025: September 6
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 13September 17
2022June 3October 7
2023incomplete recordincomplete record
2024May 24October 1
2025May 22September 6
2026May 19season not over yet

What these dates mean

June 3 is the cautious spring light-frost date for Culbertson, offering only a 10 % chance of a frost later and giving a gardener a reliable point to set out tender crops.

The 50 % light-frost dates fall on May 21 and September 20; at each of these dates the chance of a frost is exactly even, marking the midpoint of the frost-risk window. The 50 % hard-frost dates are May 10 and September 27, and a hard frost at 28 °F can damage plants more severely than a light frost at 32 °F. During the 30-year record, the span between May 21 and September 20 amounts to 120 days, indicating the central window of balanced frost probability.

For a cautious fall plan, September 6 is the date with only a 10 % chance that a frost has arrived earlier, providing a sensible deadline to finish harvesting tender crops.

How to read the probabilities

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

  • Spring: "10% after June 3" 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 6" 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 Culbertson, MTNumbered nearest stations: 1 — Sidney, MT — 34 mi; 2 — near Williston, ND — 36 mi; 3 — Williston (Sloulin Airport), ND — 40 mi; 4 — Vida, MT — 44 mi; 5 — Bredette, MT — 44 mi.MT25 mi · 40 km1. Sidney, MT — 34 mi12. near Williston, ND — 36 mi23. Williston (Sloulin Airport), ND — 40 mi34. Vida, MT — 44 mi45. Bredette, MT — 44 mi5
  1. Sidney, MT 34 mi (54 km)
  2. near Williston, ND 36 mi (57 km)
  3. Williston (Sloulin Airport), ND 40 mi (65 km)
  4. Vida, MT 44 mi (71 km)
  5. Bredette, MT 44 mi (71 km)

Sources

* Hardiness zone 3b: zone derived from PRISM 2023 grid at the station location (not the official USDA map). Source data: PRISM Group, Oregon State University.

Station coordinates: 48.1503, -104.5089.