U.S. Frost Dates by Weather Station

NOAA station USC00242409 · Montana

Frost Dates near Dillon, MT

NOAA weather station Dillon University of Montana Western (USC00242409), Montana · elevation 5129 ft (1563 m) · hardiness zone 5a*

Dillon is the nearest town to the station (0.5 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 23only 1 spring in 10 still has a frost later
  • Typical last frostJune 550% chance of a frost after it
  • Typical first fall frostSeptember 1350% chance of a frost before it
  • Frost-free season98 daystypical year, 32°F
  • Hardiness zone5acalculated*

The year at a glance

Frost season near Dillon, MTSpring: a frost after May 18 in 9 years out of 10, after June 5 in half of the years, after June 23 in 1 year out of 10. Fall: the first frost comes before August 29 in 1 year out of 10, before September 13 in half of the years, before September 27 in 9 years out of 10. Typical frost-free season: 98 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~Jun 5First frost ~Sep 13Frost season near Dillon, MTSpring: a frost after May 18 in 9 years out of 10, after June 5 in half of the years, after June 23 in 1 year out of 10. Fall: the first frost comes before August 29 in 1 year out of 10, before September 13 in half of the years, before September 27 in 9 years out of 10. Typical frost-free season: 98 days.JFMAMJJASONDLast frost ~Jun 5First frost ~Sep 13
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)June 23August 29
50% (typical)June 5September 13
90% (likely)May 18September 27
Frost-free season (32°F, 50% chance)
98 days
Hard frost (28°F), last in spring, 50% chance after
May 16
Hard frost (28°F), first in fall, 50% chance before
September 25

The last 15 years (2006–2020 normals)

Typical last frost May 31 (5 days earlier than the 30-year normal), typical first fall frost September 18 (5 days later). Frost-free season 106 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 Dillon: chance of a frost after this dateSpring, chance of a frost after the date: 10% June 23, 20% June 18, 30% June 13, 40% June 9, 50% June 5, 60% June 2, 70% May 29, 80% May 24, 90% May 18. Observed last spring frost in recent years: 2023 June 22, 2024 May 25, 2025 May 19.Spring: last frostChance of a frost after this date10%50%90%Jun 1Jul 1Jun 23Jun 5May 182023: June 222024: May 252025: May 19Fall near Dillon: chance of a frost before this dateFall, chance of a frost before the date: 10% August 29, 20% September 3, 30% September 7, 40% September 10, 50% September 13, 60% September 16, 70% September 19, 80% September 22, 90% September 27. Observed first fall frost in recent years: 2021 September 17, 2023 September 28, 2024 October 3, 2025 October 5.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Aug 29Sep 13Sep 272021: September 172023: September 282024: October 32025: 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
2021incomplete recordSeptember 17
2022incomplete recordincomplete record
2023June 22September 28
2024May 25October 3
2025May 19October 5
2026incomplete recordseason not over yet

What these dates mean

The cautious fall date for finishing the harvest in the Dillon area is August 29, when there is only a 10 % chance that a light frost has arrived. For spring tender crops a cautious plan waits until the June 23 light-frost date, which also carries a 10 % chance of a later frost.

The 50 % date of June 5 marks the median point for light-frost risk in spring, while the 50 % date of September 13 does the same for fall; on those days a light frost is equally likely to have occurred or not. The median hard-frost dates are May 16 in spring and September 25 in fall, indicating when a 28 °F frost is as probable as it is unlikely.

Hard frost at 28 °F can damage or kill plants that might 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 June 23" 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 August 29" 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 Dillon, MTNumbered nearest stations: 1 — Dillon (Airport), MT — 5 mi; 2 — Twin Bridges, MT — 28 mi; 3 — Ennis, MT — 46 mi; 4 — Norris Madison Power House, MT — 52 mi; 5 — near Butte, MT — 53 mi.MTIDWY25 mi · 40 km1. Dillon (Airport), MT — 5 mi12. Twin Bridges, MT — 28 mi23. Ennis, MT — 46 mi34. Norris Madison Power House, MT — 52 mi45. near Butte, MT — 53 mi5
  1. Dillon (Airport), MT 5 mi (9 km)
  2. Twin Bridges, MT 28 mi (45 km)
  3. Ennis, MT 46 mi (74 km)
  4. Norris Madison Power House, MT 52 mi (84 km)
  5. near Butte, MT 53 mi (85 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: 45.2089, -112.6389.