U.S. Frost Dates by Weather Station

NOAA station USW00024135 · Montana

Frost Dates near Butte, MT

NOAA weather station Butte Bert Mooney Airport (USW00024135), Montana · elevation 5506 ft (1678 m) · hardiness zone 4a*

Butte is the nearest town to the station (3.2 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 July 1only 1 spring in 10 still has a frost later
  • Typical last frostJune 1350% chance of a frost after it
  • Typical first fall frostSeptember 550% chance of a frost before it
  • Frost-free season83 daystypical year, 32°F
  • Hardiness zone4acalculated*

The year at a glance

Frost season near Butte, MTSpring: a frost after May 27 in 9 years out of 10, after June 13 in half of the years, after July 1 in 1 year out of 10. Fall: the first frost comes before August 24 in 1 year out of 10, before September 5 in half of the years, before September 19 in 9 years out of 10. Typical frost-free season: 83 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~Jun 13First frost ~Sep 5Frost season near Butte, MTSpring: a frost after May 27 in 9 years out of 10, after June 13 in half of the years, after July 1 in 1 year out of 10. Fall: the first frost comes before August 24 in 1 year out of 10, before September 5 in half of the years, before September 19 in 9 years out of 10. Typical frost-free season: 83 days.JFMAMJJASONDLast frost ~Jun 13First frost ~Sep 5
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)July 1August 24
50% (typical)June 13September 5
90% (likely)May 27September 19
Frost-free season (32°F, 50% chance)
83 days
Hard frost (28°F), last in spring, 50% chance after
May 21
Hard frost (28°F), first in fall, 50% chance before
September 17

The last 15 years (2006–2020 normals)

Typical last frost June 10 (3 days earlier than the 30-year normal), typical first fall frost September 7 (2 days later). Frost-free season 88 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 Butte: chance of a frost after this dateSpring, chance of a frost after the date: 10% July 1, 20% June 25, 30% June 21, 40% June 17, 50% June 13, 60% June 10, 70% June 5, 80% June 1, 90% May 27. Observed last spring frost in recent years: 2021 June 11, 2022 June 25, 2023 June 22, 2024 June 19, 2025 June 23, 2026 June 14.Spring: last frostChance of a frost after this date10%50%90%Jun 1Jul 1Jul 1Jun 13May 272021: June 112022: June 252023: June 222024: June 192025: June 232026: June 14Fall near Butte: chance of a frost before this dateFall, chance of a frost before the date: 10% August 24, 20% August 28, 30% August 31, 40% September 3, 50% September 5, 60% September 8, 70% September 11, 80% September 14, 90% September 19. Observed first fall frost in recent years: 2021 September 1, 2022 September 9, 2023 September 20, 2024 September 21, 2025 September 23.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Aug 24Sep 5Sep 192021: September 12022: September 92023: September 202024: September 212025: 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 1
2022June 25September 9
2023June 22September 20
2024June 19September 21
2025June 23September 23
2026June 14season not over yet

What these dates mean

August 24 marks the cautious fall date for finishing the harvest of tender crops in the Butte area, with only a 10% chance that a light frost has arrived earlier.

The 50% dates are June 13 in spring and September 5 in fall; they represent the point at which a light frost is equally likely to have occurred or not, but they are not themselves frost dates. The two 32°F 50% dates are 83 days apart, showing the central span of even odds for a light frost. A cautious planting plan uses July 1 for setting out tender crops in spring and August 24 for ending the tender-crop harvest in fall.

Hard frost at 28 °F is more damaging than a light frost at 32 °F because it can kill tissues that survive a milder freeze.

How to read the probabilities

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

  • Spring: "10% after July 1" 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 24" 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 Butte, MTNumbered nearest stations: 1 — Boulder, MT — 26 mi; 2 — Twin Bridges, MT — 30 mi; 3 — Dillon (Airport), MT — 49 mi; 4 — Trident, MT — 49 mi; 5 — Helena, MT — 51 mi.MT25 mi · 40 km1. Boulder, MT — 26 mi12. Twin Bridges, MT — 30 mi23. Dillon (Airport), MT — 49 mi34. Trident, MT — 49 mi45. Helena, MT — 51 mi5
  1. Boulder, MT 26 mi (42 km)
  2. Twin Bridges, MT 30 mi (48 km)
  3. Dillon (Airport), MT 49 mi (79 km)
  4. Trident, MT 49 mi (79 km)
  5. Helena, MT 51 mi (82 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: 45.9647, -112.5006.