U.S. Frost Dates by Weather Station

NOAA station USW00024143 · Montana

Frost Dates for Great Falls, MT

NOAA weather station Great Falls International Airport (USW00024143), Montana · elevation 3664 ft (1117 m) · hardiness zone 4b*

  • Lower-risk planting dateafter June 2only 1 spring in 10 still has a frost later
  • Typical last frostMay 1950% chance of a frost after it
  • Typical first fall frostSeptember 2450% chance of a frost before it
  • Frost-free season126 daystypical year, 32°F
  • Hardiness zone4bcalculated*

The year at a glance

Frost season at Great Falls, MTSpring: a frost after May 7 in 9 years out of 10, after May 19 in half of the years, after June 2 in 1 year out of 10. Fall: the first frost comes before September 9 in 1 year out of 10, before September 24 in half of the years, before October 5 in 9 years out of 10. Typical frost-free season: 126 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 19First frost ~Sep 24Frost season at Great Falls, MTSpring: a frost after May 7 in 9 years out of 10, after May 19 in half of the years, after June 2 in 1 year out of 10. Fall: the first frost comes before September 9 in 1 year out of 10, before September 24 in half of the years, before October 5 in 9 years out of 10. Typical frost-free season: 126 days.JFMAMJJASONDLast frost ~May 19First frost ~Sep 24
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.Today

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 2September 9
50% (typical)May 19September 24
90% (likely)May 7October 5
Frost-free season (32°F, 50% chance)
126 days
Hard frost (28°F), last in spring, 50% chance after
May 5
Hard frost (28°F), first in fall, 50% chance before
October 3

The last 15 years (2006–2020 normals)

Typical last frost May 16 (3 days earlier than the 30-year normal), typical first fall frost September 25 (1 day later). Frost-free season 130 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 Great Falls: chance of a frost after this dateSpring, chance of a frost after the date: 10% June 2, 20% May 29, 30% May 25, 40% May 22, 50% May 19, 60% May 16, 70% May 14, 80% May 11, 90% May 7. Observed last spring frost in recent years: 2021 May 22, 2022 May 31, 2023 April 23, 2024 May 31, 2025 May 22, 2026 May 18.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1Jun 2May 19May 72021: May 222022: May 312023: April 232024: May 312025: May 222026: May 18Fall at Great Falls: chance of a frost before this dateFall, chance of a frost before the date: 10% September 9, 20% September 15, 30% September 19, 40% September 22, 50% September 24, 60% September 27, 70% September 29, 80% October 1, 90% October 5. Observed first fall frost in recent years: 2021 September 17, 2022 October 13, 2023 October 6, 2024 September 30, 2025 October 5.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 9Sep 24Oct 52021: September 172022: October 132023: October 62024: September 302025: 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 22September 17
2022May 31October 13
2023April 23October 6
2024May 31September 30
2025May 22October 5
2026May 18season not over yet

What these dates mean

Setting out tender crops on the early spring date of May 7 is a gamble, because there is a 90 % chance of a light frost occurring later than that day in Great Falls.

The 50 % dates mark the point of even odds for frost: May 19 for spring and September 24 for fall. A cautious plan would wait until the June 2 date in spring, when only a 10 % chance of a later frost remains, and would finish tender crops by September 9 in fall, which carries only a 10 % chance that a frost has arrived earlier.

Hard frost at 28 °F matters more than light frost at 32 °F because it can injure plants that would survive the milder temperature, and the 50 % hard-frost dates are May 5 in spring and October 3 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 2" 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 9" 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 Great Falls, MTNumbered nearest stations: 1 — Sun River, MT — 17 mi; 2 — Cascade, MT — 23 mi; 3 — Fairfield, MT — 30 mi; 4 — Millegan, MT — 43 mi; 5 — Holter Dam, MT — 45 mi.25 mi · 40 km1. Sun River, MT — 17 mi12. Cascade, MT — 23 mi23. Fairfield, MT — 30 mi34. Millegan, MT — 43 mi45. Holter Dam, MT — 45 mi5
  1. Sun River, MT 17 mi (27 km)
  2. Cascade, MT 23 mi (38 km)
  3. Fairfield, MT 30 mi (48 km)
  4. Millegan, MT 43 mi (69 km)
  5. Holter Dam, MT 45 mi (72 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: 47.4733, -111.3822.