U.S. Frost Dates by Weather Station

NOAA station USC00243727 · Montana

Frost Dates for Grass Range, MT

NOAA weather station Grass Range (USC00243727), Montana · elevation 3490 ft (1064 m) · hardiness zone 4b*

  • Lower-risk planting dateafter June 6only 1 spring in 10 still has a frost later
  • Typical last frostMay 2050% 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 Grass Range, MTSpring: a frost after May 6 in 9 years out of 10, after May 20 in half of the years, after June 6 in 1 year out of 10. Fall: the first frost comes before September 11 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 20First frost ~Sep 24Frost season at Grass Range, MTSpring: a frost after May 6 in 9 years out of 10, after May 20 in half of the years, after June 6 in 1 year out of 10. Fall: the first frost comes before September 11 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 20First 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.

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 6September 11
50% (typical)May 20September 24
90% (likely)May 6October 5
Frost-free season (32°F, 50% chance)
126 days
Hard frost (28°F), last in spring, 50% chance after
May 6
Hard frost (28°F), first in fall, 50% chance before
October 2

The last 15 years (2006–2020 normals)

Typical last frost May 21 (1 day later than the 30-year normal), typical first fall frost September 27 (3 days later). Frost-free season 128 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 Grass Range: chance of a frost after this dateSpring, chance of a frost after the date: 10% June 6, 20% May 31, 30% May 26, 40% May 23, 50% May 20, 60% May 16, 70% May 13, 80% May 11, 90% May 6. Observed last spring frost in recent years: 2021 May 22, 2022 May 22, 2023 May 2, 2024 May 20, 2025 May 5, 2026 May 19.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1Jun 6May 20May 62021: May 222022: May 222023: May 22024: May 202025: May 52026: May 19Fall at Grass Range: chance of a frost before this dateFall, chance of a frost before the date: 10% September 11, 20% September 16, 30% September 19, 40% September 22, 50% September 24, 60% September 27, 70% September 29, 80% October 2, 90% October 5. Observed first fall frost in recent years: 2021 September 30, 2022 October 23, 2023 October 6, 2024 October 13, 2025 October 6.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 11Sep 24Oct 52021: September 302022: October 232023: October 62024: October 132025: October 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 22September 30
2022May 22October 23
2023May 2October 6
2024May 20October 13
2025May 5October 6
2026May 19season not over yet

What these dates mean

The 126 days between the 32°F 50% dates of May 20 and September 24 define the period in which light frosts are equally likely to have occurred or not, giving gardeners a mid-season window to plan most activities. In spring the cautious date for setting out tender crops is June 6, which carries a 10 % chance of a frost later; the earlier May 6 date involves a 90 % chance and is a gamble. In fall the cautious date to finish harvesting tender crops is September 11, which has a 10 % chance of a frost earlier; the later October 5 date carries a 90 % chance and is risky. The 50% hard-frost dates are May 6 in spring and October 2 in fall; hard frosts at 28 °F can damage plants more severely than light frosts at 32 °F.

How to read the probabilities

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

  • Spring: "10% after June 6" 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 11" 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 Grass Range, MTNumbered nearest stations: 1 — Flatwillow, MT — 25 mi; 2 — Valentine, MT — 26 mi; 3 — Lewistown, MT — 31 mi; 4 — Ryegate, MT — 43 mi; 5 — Roundup, MT — 43 mi.MT25 mi · 40 km1. Flatwillow, MT — 25 mi12. Valentine, MT — 26 mi23. Lewistown, MT — 31 mi34. Ryegate, MT — 43 mi45. Roundup, MT — 43 mi5
  1. Flatwillow, MT 25 mi (40 km)
  2. Valentine, MT 26 mi (42 km)
  3. Lewistown, MT 31 mi (50 km)
  4. Ryegate, MT 43 mi (68 km)
  5. Roundup, MT 43 mi (69 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: 47.0256, -108.8036.