U.S. Frost Dates by Weather Station

NOAA station USW00024138 · Montana

Frost Dates for Dillon (Airport), MT

NOAA weather station Dillon Airport (USW00024138), Montana · elevation 5200 ft (1585 m) · hardiness zone 5a*

  • Lower-risk planting dateafter June 21only 1 spring in 10 still has a frost later
  • Typical last frostJune 250% chance of a frost after it
  • Typical first fall frostSeptember 1350% chance of a frost before it
  • Frost-free season101 daystypical year, 32°F
  • Hardiness zone5acalculated*

The year at a glance

Frost season at Dillon (Airport), MTSpring: a frost after May 17 in 9 years out of 10, after June 2 in half of the years, after June 21 in 1 year out of 10. Fall: the first frost comes before August 30 in 1 year out of 10, before September 13 in half of the years, before September 28 in 9 years out of 10. Typical frost-free season: 101 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~Jun 2First frost ~Sep 13Frost season at Dillon (Airport), MTSpring: a frost after May 17 in 9 years out of 10, after June 2 in half of the years, after June 21 in 1 year out of 10. Fall: the first frost comes before August 30 in 1 year out of 10, before September 13 in half of the years, before September 28 in 9 years out of 10. Typical frost-free season: 101 days.JFMAMJJASONDLast frost ~Jun 2First 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.Today

Frost dates (32°F)

Probability of frost, based on 25 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)June 21August 30
50% (typical)June 2September 13
90% (likely)May 17September 28
Frost-free season (32°F, 50% chance)
101 days
Hard frost (28°F), last in spring, 50% chance after
May 14
Hard frost (28°F), first in fall, 50% chance before
September 27

The last 15 years (2006–2020 normals)

Typical last frost June 1 (1 day earlier than the 30-year normal), typical first fall frost September 16 (3 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 at Dillon (Airport): chance of a frost after this dateSpring, chance of a frost after the date: 10% June 21, 20% June 15, 30% June 10, 40% June 6, 50% June 2, 60% May 31, 70% May 27, 80% May 22, 90% May 17. Observed last spring frost in recent years: 2021 June 11, 2022 May 23, 2023 June 22, 2024 May 31, 2025 May 23, 2026 May 22.Spring: last frostChance of a frost after this date10%50%90%Jun 1Jul 1Jun 21Jun 2May 172021: June 112022: May 232023: June 222024: May 312025: May 232026: May 22Fall at Dillon (Airport): chance of a frost before this dateFall, chance of a frost before the date: 10% August 30, 20% September 4, 30% September 8, 40% September 11, 50% September 13, 60% September 17, 70% September 20, 80% September 23, 90% September 28. Observed first fall frost in recent years: 2021 September 17, 2022 September 9, 2023 September 28, 2024 September 22, 2025 September 23.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Aug 30Sep 13Sep 282021: September 172022: September 92023: September 282024: September 222025: September 23
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
2021June 11September 17
2022May 23September 9
2023June 22September 28
2024May 31September 22
2025May 23September 23
2026May 22season not over yet

What these dates mean

Planting tender crops on May 17 in Dillon (Airport) carries a 90 % chance that a light frost will still occur, so it is a gamble for growers. The chance drops to 10 % only by June 21, which is the cautious spring date for setting out those crops.

For a gardener, the 50 % date of June 2 means there is a 50 % chance of a light frost later than that day, while the 50 % date of September 13 signifies a 50 % chance of a light frost earlier than that day. The cautious plan therefore uses June 21 to start tender crops and August 30 to finish harvesting them, each with only a 10 % frost risk. Hard-frost dates—May 14 in spring and September 27 in fall—are noted because a temperature of 28 °F can damage plants more severely than the 32 °F light-frost threshold.

How to read the probabilities

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

  • Spring: "10% after June 21" 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 30" 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 (Airport), MTNumbered nearest stations: 1 — near Dillon, MT — 5 mi; 2 — Twin Bridges, MT — 23 mi; 3 — Ennis, MT — 41 mi; 4 — Norris Madison Power House, MT — 47 mi; 5 — near Butte, MT — 49 mi.MTIDWY25 mi · 40 km1. near Dillon, MT — 5 mi12. Twin Bridges, MT — 23 mi23. Ennis, MT — 41 mi34. Norris Madison Power House, MT — 47 mi45. near Butte, MT — 49 mi5
  1. near Dillon, MT 5 mi (9 km)
  2. Twin Bridges, MT 23 mi (37 km)
  3. Ennis, MT 41 mi (67 km)
  4. Norris Madison Power House, MT 47 mi (76 km)
  5. near Butte, MT 49 mi (79 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: 45.2575, -112.5545.