U.S. Frost Dates by Weather Station

NOAA station USC00240780 · Montana

Frost Dates for Big Timber, MT

NOAA weather station Big Timber (USC00240780), Montana · elevation 4100 ft (1250 m) · hardiness zone 5a*

  • Lower-risk planting dateafter May 27only 1 spring in 10 still has a frost later
  • Typical last frostMay 1450% chance of a frost after it
  • Typical first fall frostSeptember 2650% chance of a frost before it
  • Frost-free season133 daystypical year, 32°F
  • Hardiness zone5acalculated*

The year at a glance

Frost season at Big Timber, MTSpring: a frost after May 2 in 9 years out of 10, after May 14 in half of the years, after May 27 in 1 year out of 10. Fall: the first frost comes before September 10 in 1 year out of 10, before September 26 in half of the years, before October 8 in 9 years out of 10. Typical frost-free season: 133 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 14First frost ~Sep 26Frost season at Big Timber, MTSpring: a frost after May 2 in 9 years out of 10, after May 14 in half of the years, after May 27 in 1 year out of 10. Fall: the first frost comes before September 10 in 1 year out of 10, before September 26 in half of the years, before October 8 in 9 years out of 10. Typical frost-free season: 133 days.JFMAMJJASONDLast frost ~May 14First frost ~Sep 26
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)May 27September 10
50% (typical)May 14September 26
90% (likely)May 2October 8
Frost-free season (32°F, 50% chance)
133 days
Hard frost (28°F), last in spring, 50% chance after
May 2
Hard frost (28°F), first in fall, 50% chance before
October 5

The last 15 years (2006–2020 normals)

Typical last frost May 13 (1 day earlier than the 30-year normal), typical first fall frost September 28 (2 days later). Frost-free season 135 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 Big Timber: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 27, 20% May 23, 30% May 20, 40% May 17, 50% May 14, 60% May 12, 70% May 9, 80% May 6, 90% May 2. Observed last spring frost in recent years: 2021 May 30, 2022 May 23, 2023 May 10, 2024 May 23, 2025 May 6.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 27May 14May 22021: May 302022: May 232023: May 102024: May 232025: May 6Fall at Big Timber: chance of a frost before this dateFall, chance of a frost before the date: 10% September 10, 20% September 16, 30% September 21, 40% September 24, 50% September 26, 60% September 29, 70% October 1, 80% October 3, 90% October 8. Observed first fall frost in recent years: 2021 September 17, 2022 October 13, 2023 October 15, 2024 October 1, 2025 October 6.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 10Sep 26Oct 82021: September 172022: October 132023: October 152024: 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
2021May 30September 17
2022May 23October 13
2023May 10October 15
2024May 23October 1
2025May 6October 6
2026incomplete recordseason not over yet

What these dates mean

September 10 is the cautious fall date in Big Timber for finishing the harvest, when there is only a 10 % chance that a light frost has arrived.

The 50% light-frost dates are May 14 in spring and September 26 in fall; at each of these dates the probability of a frost is exactly even. A cautious spring plan uses the later date, May 27, because it carries only a 10 % chance of a frost later than that day, while the early date, May 2, carries a 90 % chance and is a gamble for tender crops. The hard-frost median dates are May 2 and October 5, and a hard frost at 28 °F can damage or kill tender 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 May 27" 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 10" 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 Big Timber, MTNumbered nearest stations: 1 — Melville, MT — 19 mi; 2 — Nye #2, MT — 28 mi; 3 — Rapelje, MT — 35 mi; 4 — Columbus, MT — 35 mi; 5 — Mystic Lake, MT — 42 mi.MTWY25 mi · 40 km1. Melville, MT — 19 mi12. Nye #2, MT — 28 mi23. Rapelje, MT — 35 mi34. Columbus, MT — 35 mi45. Mystic Lake, MT — 42 mi5
  1. Melville, MT 19 mi (31 km)
  2. Nye #2, MT 28 mi (46 km)
  3. Rapelje, MT 35 mi (56 km)
  4. Columbus, MT 35 mi (57 km)
  5. Mystic Lake, MT 42 mi (68 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: 45.8325, -109.9511.