U.S. Frost Dates by Weather Station

NOAA station USC00241047 · Montana

Frost Dates near Four Corners, MT

NOAA weather station Bozeman 6 W Experiment Farm (USC00241047), Montana · elevation 4775 ft (1455 m) · hardiness zone 5a*

Four Corners is the nearest town to the station (1.7 mi away) in GeoNames (CC BY 4.0). The dates below are measured at the station, not in the town itself.

  • Lower-risk planting dateafter June 12only 1 spring in 10 still has a frost later
  • Typical last frostMay 2350% chance of a frost after it
  • Typical first fall frostSeptember 2350% chance of a frost before it
  • Frost-free season123 daystypical year, 32°F
  • Hardiness zone5acalculated*

The year at a glance

Frost season near Four Corners, MTSpring: a frost after May 5 in 9 years out of 10, after May 23 in half of the years, after June 12 in 1 year out of 10. Fall: the first frost comes before September 4 in 1 year out of 10, before September 23 in half of the years, before October 6 in 9 years out of 10. Typical frost-free season: 123 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 23First frost ~Sep 23Frost season near Four Corners, MTSpring: a frost after May 5 in 9 years out of 10, after May 23 in half of the years, after June 12 in 1 year out of 10. Fall: the first frost comes before September 4 in 1 year out of 10, before September 23 in half of the years, before October 6 in 9 years out of 10. Typical frost-free season: 123 days.JFMAMJJASONDLast frost ~May 23First frost ~Sep 23
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 12September 4
50% (typical)May 23September 23
90% (likely)May 5October 6
Frost-free season (32°F, 50% chance)
123 days
Hard frost (28°F), last in spring, 50% chance after
May 2
Hard frost (28°F), first in fall, 50% chance before
October 3

The last 15 years (2006–2020 normals)

Typical last frost May 18 (5 days earlier than the 30-year normal), typical first fall frost September 28 (5 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 near Four Corners: chance of a frost after this dateSpring, chance of a frost after the date: 10% June 12, 20% June 5, 30% May 31, 40% May 27, 50% May 23, 60% May 18, 70% May 15, 80% May 10, 90% May 5. Observed last spring frost in recent years: 2021 June 6, 2022 May 23, 2023 June 22, 2024 May 31, 2025 May 7, 2026 June 8.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1Jun 12May 23May 52021: June 62022: May 232023: June 222024: May 312025: May 72026: June 8Fall near Four Corners: chance of a frost before this dateFall, chance of a frost before the date: 10% September 4, 20% September 12, 30% September 16, 40% September 20, 50% September 23, 60% September 26, 70% September 30, 80% October 2, 90% October 6. Observed first fall frost in recent years: 2021 September 17, 2022 September 10, 2023 October 24, 2024 October 1, 2025 October 6.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 4Sep 23Oct 62021: September 172022: September 102023: October 242024: October 12025: 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
2021June 6September 17
2022May 23September 10
2023June 22October 24
2024May 31October 1
2025May 7October 6
2026June 8season not over yet

What these dates mean

Planting tender crops on the early spring date of May 5 is a gamble in the Four Corners area, because there is a 90 % chance that a light frost will still occur later that season.

The 50% date is May 23, meaning a gardener faces an even chance of a light frost after that point in spring, while the 50% date is September 23 for fall, indicating an even chance of a light frost before that day. A cautious plan would wait until the June 12 date, which carries only a 10 % chance of a later frost, and would finish tender crops by the September 4 date, which has only a 10 % chance that a frost has arrived earlier. The 50% hard-frost date is May 2 and the 50% hard-frost date is October 3; hard frost at 28 °F can damage plants more severely than a light frost 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 12" 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 4" 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 Four Corners, MTNumbered nearest stations: 1 — near Bozeman, MT — 5 mi; 2 — Bozeman (Gallatin Field), MT — 8 mi; 3 — Trident, MT — 24 mi; 4 — Norris Madison Power House, MT — 26 mi; 5 — Ennis, MT — 35 mi.MTWY25 mi · 40 km1. near Bozeman, MT — 5 mi12. Bozeman (Gallatin Field), MT — 8 mi23. Trident, MT — 24 mi34. Norris Madison Power House, MT — 26 mi45. Ennis, MT — 35 mi5
  1. near Bozeman, MT 5 mi (8 km)
  2. Bozeman (Gallatin Field), MT 8 mi (13 km)
  3. Trident, MT 24 mi (39 km)
  4. Norris Madison Power House, MT 26 mi (43 km)
  5. Ennis, MT 35 mi (57 km)

Sources

* Hardiness zone 5a: zone derived from PRISM 2023 grid at the station location (not the official USDA map). Source data: PRISM Group, Oregon State University.

Station coordinates: 45.6728, -111.1547.