U.S. Frost Dates by Weather Station

NOAA station USC00242857 · Montana

Frost Dates for Fairfield, MT

NOAA weather station Fairfield (USC00242857), Montana · elevation 3983 ft (1214 m) · hardiness zone 4b*

  • Lower-risk planting dateafter June 1only 1 spring in 10 still has a frost later
  • Typical last frostMay 1350% chance of a frost after it
  • Typical first fall frostSeptember 2750% chance of a frost before it
  • Frost-free season135 daystypical year, 32°F
  • Hardiness zone4bcalculated*

The year at a glance

Frost season at Fairfield, MTSpring: a frost after April 28 in 9 years out of 10, after May 13 in half of the years, after June 1 in 1 year out of 10. Fall: the first frost comes before September 14 in 1 year out of 10, before September 27 in half of the years, before October 9 in 9 years out of 10. Typical frost-free season: 135 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 13First frost ~Sep 27Frost season at Fairfield, MTSpring: a frost after April 28 in 9 years out of 10, after May 13 in half of the years, after June 1 in 1 year out of 10. Fall: the first frost comes before September 14 in 1 year out of 10, before September 27 in half of the years, before October 9 in 9 years out of 10. Typical frost-free season: 135 days.JFMAMJJASONDLast frost ~May 13First frost ~Sep 27
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 1September 14
50% (typical)May 13September 27
90% (likely)April 28October 9
Frost-free season (32°F, 50% chance)
135 days
Hard frost (28°F), last in spring, 50% chance after
April 30
Hard frost (28°F), first in fall, 50% chance before
October 7

The last 15 years (2006–2020 normals)

Typical last frost May 11 (2 days earlier than the 30-year normal), typical first fall frost September 29 (2 days later). Frost-free season 139 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 Fairfield: chance of a frost after this dateSpring, chance of a frost after the date: 10% June 1, 20% May 25, 30% May 20, 40% May 16, 50% May 13, 60% May 10, 70% May 6, 80% May 3, 90% April 28. Observed last spring frost in recent years: 2021 May 23, 2022 May 22, 2023 April 23, 2024 May 8, 2025 May 6, 2026 May 18.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1Jun 1May 13Apr 282021: May 232022: May 222023: April 232024: May 82025: May 62026: May 18Fall at Fairfield: chance of a frost before this dateFall, chance of a frost before the date: 10% September 14, 20% September 18, 30% September 21, 40% September 24, 50% September 27, 60% September 29, 70% October 1, 80% October 5, 90% October 9. Observed first fall frost in recent years: 2022 October 22, 2023 October 23, 2024 September 30, 2025 October 5.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 14Sep 27Oct 92022: October 222023: October 232024: 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 23incomplete record
2022May 22October 22
2023April 23October 23
2024May 8September 30
2025May 6October 5
2026May 18season not over yet

What these dates mean

Setting out tender crops on April 28 in Fairfield carries a 90% chance of a frost later that season, making it a clear gamble. The cautious spring date for tender crops is June 1, when the chance of a frost later is only 10%. In the fall the cautious date to finish harvesting tender crops is September 14, which carries a 10% chance of a frost earlier; counting on crops through October 9 would be a gamble with a 90% chance of an early frost. The 50% light-frost dates are May 13 and September 27; each marks an even chance of a frost occurring later (spring) or earlier (fall).

The median hard-frost dates are April 30 in spring and October 7 in fall, and a hard frost at 28 °F can damage plants that survive a light frost at 32 °F, so it represents a greater risk.

How to read the probabilities

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

  • Spring: "10% after June 1" 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 14" 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 Fairfield, MTNumbered nearest stations: 1 — Sun River, MT — 15 mi; 2 — Choteau, MT — 17 mi; 3 — Great Falls, MT — 30 mi; 4 — Cascade, MT — 30 mi; 5 — Conrad, MT — 39 mi.25 mi · 40 km1. Sun River, MT — 15 mi12. Choteau, MT — 17 mi23. Great Falls, MT — 30 mi34. Cascade, MT — 30 mi45. Conrad, MT — 39 mi5
  1. Sun River, MT 15 mi (24 km)
  2. Choteau, MT 17 mi (28 km)
  3. Great Falls, MT 30 mi (48 km)
  4. Cascade, MT 30 mi (49 km)
  5. Conrad, MT 39 mi (62 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: 47.615, -111.9861.