U.S. Frost Dates by Weather Station

NOAA station USC00245712 · Montana

Frost Dates for Millegan, MT

NOAA weather station Millegan 14 SE (USC00245712), Montana · elevation 4970 ft (1515 m) · hardiness zone 4a*

  • Lower-risk planting dateafter July 3only 1 spring in 10 still has a frost later
  • Typical last frostJune 1150% chance of a frost after it
  • Typical first fall frostSeptember 650% chance of a frost before it
  • Frost-free season86 daystypical year, 32°F
  • Hardiness zone4acalculated*

The year at a glance

Frost season at Millegan, MTSpring: a frost after May 27 in 9 years out of 10, after June 11 in half of the years, after July 3 in 1 year out of 10. Fall: the first frost comes before August 20 in 1 year out of 10, before September 6 in half of the years, before September 21 in 9 years out of 10. Typical frost-free season: 86 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~Jun 11First frost ~Sep 6Frost season at Millegan, MTSpring: a frost after May 27 in 9 years out of 10, after June 11 in half of the years, after July 3 in 1 year out of 10. Fall: the first frost comes before August 20 in 1 year out of 10, before September 6 in half of the years, before September 21 in 9 years out of 10. Typical frost-free season: 86 days.JFMAMJJASONDLast frost ~Jun 11First frost ~Sep 6
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 25 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)July 3August 20
50% (typical)June 11September 6
90% (likely)May 27September 21
Frost-free season (32°F, 50% chance)
86 days
Hard frost (28°F), last in spring, 50% chance after
May 20
Hard frost (28°F), first in fall, 50% chance before
September 22

The last 15 years (2006–2020 normals)

Typical last frost June 9 (2 days earlier than the 30-year normal), typical first fall frost September 12 (6 days later). Frost-free season 92 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 Millegan: chance of a frost after this dateSpring, chance of a frost after the date: 10% July 3, 20% June 23, 30% June 17, 40% June 14, 50% June 11, 60% June 7, 70% June 4, 80% June 1, 90% May 27. Observed last spring frost in recent years: 2021 June 11, 2022 June 26, 2025 June 23.Spring: last frostChance of a frost after this date10%50%90%Jun 1Jul 1Jul 3Jun 11May 272021: June 112022: June 262025: June 23Fall at Millegan: chance of a frost before this dateFall, chance of a frost before the date: 10% August 20, 20% August 27, 30% August 31, 40% September 3, 50% September 6, 60% September 9, 70% September 12, 80% September 16, 90% September 21. Observed first fall frost in recent years: 2021 September 14, 2023 September 20, 2024 September 21.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Aug 20Sep 6Sep 212021: September 142023: September 202024: September 21
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 14
2022June 26incomplete record
2023incomplete recordSeptember 20
2024incomplete recordSeptember 21
2025June 23incomplete record
2026incomplete recordseason not over yet

What these dates mean

August 20 is the cautious fall date for Millegan, marking only a 10 % chance that a light frost has arrived earlier, so it is the date to aim for when finishing the harvest of tender crops. In spring the cautious date for setting out tender crops is July 3, when there is just a 10 % chance of a light frost later.

The 50% light-frost dates—June 11 in spring and September 6 in fall—represent the median point where a frost is equally likely to have occurred or not; they give a garden-wide reference of the frost-free window. The median hard-frost dates are May 20 in spring and September 22 in fall, and a hard frost at 28 °F can damage plants 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 July 3" 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 20" 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 Millegan, MTNumbered nearest stations: 1 — Cascade, MT — 35 mi; 2 — Holter Dam, MT — 41 mi; 3 — Townsend, MT — 42 mi; 4 — Helena, MT — 42 mi; 5 — Great Falls, MT — 43 mi.MT25 mi · 40 km1. Cascade, MT — 35 mi12. Holter Dam, MT — 41 mi23. Townsend, MT — 42 mi34. Helena, MT — 42 mi45. Great Falls, MT — 43 mi5
  1. Cascade, MT 35 mi (56 km)
  2. Holter Dam, MT 41 mi (66 km)
  3. Townsend, MT 42 mi (68 km)
  4. Helena, MT 42 mi (68 km)
  5. Great Falls, MT 43 mi (69 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: 46.8747, -111.1642.