U.S. Frost Dates by Weather Station

NOAA station USC00243110 · Montana

Frost Dates for Fort Assiniboine, MT

NOAA weather station Fort Assiniboine (USC00243110), Montana · elevation 2675 ft (815 m) · hardiness zone 4a*

  • Lower-risk planting dateafter May 28only 1 spring in 10 still has a frost later
  • Typical last frostMay 1550% chance of a frost after it
  • Typical first fall frostSeptember 2450% chance of a frost before it
  • Frost-free season130 daystypical year, 32°F
  • Hardiness zone4acalculated*

The year at a glance

Frost season at Fort Assiniboine, MTSpring: a frost after May 4 in 9 years out of 10, after May 15 in half of the years, after May 28 in 1 year out of 10. Fall: the first frost comes before September 8 in 1 year out of 10, before September 24 in half of the years, before October 4 in 9 years out of 10. Typical frost-free season: 130 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 15First frost ~Sep 24Frost season at Fort Assiniboine, MTSpring: a frost after May 4 in 9 years out of 10, after May 15 in half of the years, after May 28 in 1 year out of 10. Fall: the first frost comes before September 8 in 1 year out of 10, before September 24 in half of the years, before October 4 in 9 years out of 10. Typical frost-free season: 130 days.JFMAMJJASONDLast frost ~May 15First 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 29 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 28September 8
50% (typical)May 15September 24
90% (likely)May 4October 4
Frost-free season (32°F, 50% chance)
130 days
Hard frost (28°F), last in spring, 50% chance after
May 4
Hard frost (28°F), first in fall, 50% chance before
October 2

The last 15 years (2006–2020 normals)

Typical last frost May 15 (same as the 30-year normal), typical first fall frost September 29 (5 days later). Frost-free season 137 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 Fort Assiniboine: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 28, 20% May 24, 30% May 20, 40% May 18, 50% May 15, 60% May 13, 70% May 11, 80% May 8, 90% May 4. Observed last spring frost in recent years: 2021 May 23, 2022 May 22, 2023 April 28, 2024 May 4, 2025 May 22, 2026 May 23.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 28May 15May 42021: May 232022: May 222023: April 282024: May 42025: May 222026: May 23Fall at Fort Assiniboine: chance of a frost before this dateFall, chance of a frost before the date: 10% September 8, 20% September 13, 30% September 18, 40% September 21, 50% September 24, 60% September 26, 70% September 29, 80% October 1, 90% October 4. Observed first fall frost in recent years: 2021 September 17, 2022 September 21, 2023 October 6, 2024 October 3, 2025 October 6.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 8Sep 24Oct 42021: September 172022: September 212023: October 62024: October 32025: 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 23September 17
2022May 22September 21
2023April 28October 6
2024May 4October 3
2025May 22October 6
2026May 23season not over yet

What these dates mean

Setting out tender crops on the early spring date, May 4, is a gamble, carrying a 90% chance of a frost later than that day. For a more cautious approach, the spring date to use is May 28, when only a 10% chance remains. The 50% date is May 15; it marks the point at which the odds of a light frost are even.

In the fall, the cautious date for finishing tender crops is September 8, with a 10% chance that a frost has already arrived. The 50% date is September 24, the median point for frost risk. Hard-frost 50% dates are May 4 and October 2, indicating when a 28 °F freeze is as likely as not. A hard frost at 28 °F can damage plant tissue that would survive 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 28" 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 8" 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 Fort Assiniboine, MTNumbered nearest stations: 1 — Havre, MT — 4 mi; 2 — Gildford, MT — 23 mi; 3 — Chinook, MT — 27 mi; 4 — Simpson 6 N - Wildhorse, MT — 39 mi; 5 — Loma, MT — 50 mi.MT25 mi · 40 km1. Havre, MT — 4 mi12. Gildford, MT — 23 mi23. Chinook, MT — 27 mi34. Simpson 6 N - Wildhorse, MT — 39 mi45. Loma, MT — 50 mi5
  1. Havre, MT 4 mi (6 km)
  2. Gildford, MT 23 mi (38 km)
  3. Chinook, MT 27 mi (43 km)
  4. Simpson 6 N - Wildhorse, MT 39 mi (63 km)
  5. Loma, MT 50 mi (81 km)

Sources

* Hardiness zone 4a: zone derived from PRISM 2023 grid at the station location (not the official USDA map). Source data: PRISM Group, Oregon State University.

Station coordinates: 48.4983, -109.8014.