U.S. Frost Dates by Weather Station

NOAA station USC00241722 · Montana

Frost Dates for Chinook, MT

NOAA weather station Chinook (USC00241722), Montana · elevation 2420 ft (738 m) · hardiness zone 3b*

  • 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 2350% chance of a frost before it
  • Frost-free season130 daystypical year, 32°F
  • Hardiness zone3bcalculated*

The year at a glance

Frost season at Chinook, MTSpring: a frost after May 1 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 11 in 1 year out of 10, before September 23 in half of the years, before October 3 in 9 years out of 10. Typical frost-free season: 130 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 14First frost ~Sep 23Frost season at Chinook, MTSpring: a frost after May 1 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 11 in 1 year out of 10, before September 23 in half of the years, before October 3 in 9 years out of 10. Typical frost-free season: 130 days.JFMAMJJASONDLast frost ~May 14First 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)May 27September 11
50% (typical)May 14September 23
90% (likely)May 1October 3
Frost-free season (32°F, 50% chance)
130 days
Hard frost (28°F), last in spring, 50% chance after
May 3
Hard frost (28°F), first in fall, 50% chance before
October 1

The last 15 years (2006–2020 normals)

Typical last frost May 15 (1 day later than the 30-year normal), typical first fall frost September 27 (4 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 Chinook: 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 11, 70% May 9, 80% May 6, 90% May 1. Observed last spring frost in recent years: 2024 May 5.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 27May 14May 12024: May 5Fall at Chinook: chance of a frost before this dateFall, chance of a frost before the date: 10% September 11, 20% September 15, 30% September 19, 40% September 21, 50% September 23, 60% September 25, 70% September 27, 80% September 30, 90% October 3. Observed first fall frost in recent years: 2021 September 17, 2022 September 21, 2023 October 6, 2024 October 1.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 11Sep 23Oct 32021: September 172022: September 212023: October 62024: October 1
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
2021incomplete recordSeptember 17
2022incomplete recordSeptember 21
2023incomplete recordOctober 6
2024May 5October 1
2025incomplete recordincomplete record
2026incomplete recordseason not over yet

What these dates mean

There are 130 days between the 32 °F 50% dates of May 14 and September 23, a stretch that defines the central portion of Chinook’s growing season.

The 50% date of May 14 marks the point at which there is an even chance of a light frost occurring, while the 50% date of September 23 marks the point at which a light frost is equally likely to have happened. Gardeners who prefer a low risk in spring would wait until the cautious date of May 27, which carries a 10 % chance of a frost later, and in fall would aim to finish tender crops by the cautious date of September 11, which carries a 10 % chance of a frost earlier.

A hard frost of 28 °F, represented by the dates May 3 and October 1, can injure plants that survive a 32 °F light frost, so those dates are especially important for protecting tender crops.

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 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 Chinook, MTNumbered nearest stations: 1 — Havre, MT — 25 mi; 2 — Fort Assiniboine, MT — 27 mi; 3 — Turner, MT — 48 mi; 4 — Gildford, MT — 49 mi; 5 — Simpson 6 N - Wildhorse, MT — 53 mi.25 mi · 40 km1. Havre, MT — 25 mi12. Fort Assiniboine, MT — 27 mi23. Turner, MT — 48 mi34. Gildford, MT — 49 mi45. Simpson 6 N - Wildhorse, MT — 53 mi5
  1. Havre, MT 25 mi (40 km)
  2. Fort Assiniboine, MT 27 mi (43 km)
  3. Turner, MT 48 mi (76 km)
  4. Gildford, MT 49 mi (79 km)
  5. Simpson 6 N - Wildhorse, MT 53 mi (86 km)

Sources

* Hardiness zone 3b: zone derived from PRISM 2023 grid at the station location (not the official USDA map). Source data: PRISM Group, Oregon State University.

Station coordinates: 48.5883, -109.2267.