U.S. Frost Dates by Weather Station

NOAA station USC00241297 · Montana

Frost Dates for Busby, MT

NOAA weather station Busby (USC00241297), Montana · elevation 3430 ft (1046 m) · hardiness zone 4a*

  • Lower-risk planting dateafter June 7only 1 spring in 10 still has a frost later
  • Typical last frostMay 2350% chance of a frost after it
  • Typical first fall frostSeptember 1550% chance of a frost before it
  • Frost-free season113 daystypical year, 32°F
  • Hardiness zone4acalculated*

The year at a glance

Frost season at Busby, MTSpring: a frost after May 12 in 9 years out of 10, after May 23 in half of the years, after June 7 in 1 year out of 10. Fall: the first frost comes before September 1 in 1 year out of 10, before September 15 in half of the years, before October 1 in 9 years out of 10. Typical frost-free season: 113 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 23First frost ~Sep 15Frost season at Busby, MTSpring: a frost after May 12 in 9 years out of 10, after May 23 in half of the years, after June 7 in 1 year out of 10. Fall: the first frost comes before September 1 in 1 year out of 10, before September 15 in half of the years, before October 1 in 9 years out of 10. Typical frost-free season: 113 days.JFMAMJJASONDLast frost ~May 23First frost ~Sep 15
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 30 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)June 7September 1
50% (typical)May 23September 15
90% (likely)May 12October 1
Frost-free season (32°F, 50% chance)
113 days
Hard frost (28°F), last in spring, 50% chance after
May 11
Hard frost (28°F), first in fall, 50% chance before
September 27

The last 15 years (2006–2020 normals)

Typical last frost May 24 (1 day later than the 30-year normal), typical first fall frost September 14 (1 day earlier). Frost-free season 113 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 Busby: chance of a frost after this dateSpring, chance of a frost after the date: 10% June 7, 20% June 1, 30% May 29, 40% May 26, 50% May 23, 60% May 21, 70% May 18, 80% May 15, 90% May 12. Observed last spring frost in recent years: 2021 May 30, 2022 June 1, 2023 May 24, 2025 May 27.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1Jun 7May 23May 122021: May 302022: June 12023: May 242025: May 27Fall at Busby: chance of a frost before this dateFall, chance of a frost before the date: 10% September 1, 20% September 5, 30% September 9, 40% September 12, 50% September 15, 60% September 18, 70% September 21, 80% September 25, 90% October 1. Observed first fall frost in recent years: 2021 September 21, 2022 October 8, 2023 September 29, 2025 October 7.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 1Sep 15Oct 12021: September 212022: October 82023: September 292025: October 7
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
2021May 30September 21
2022June 1October 8
2023May 24September 29
2024incomplete recordincomplete record
2025May 27October 7
2026incomplete recordseason not over yet

What these dates mean

The 113 days between the 50% dates in Busby, May 23 and September 15, hold the window in which a light frost is equally likely to have occurred or not.

For a gardener the May 23 date marks the median point for a spring light-frost risk, meaning there is a 50% chance of a frost later than this date. The June 7 date carries a 10% chance of a later frost and is the cautious point for setting out tender crops, while the May 12 date carries a 90% chance and represents a gamble. In the fall the September 15 date is the median for a light-frost risk, with a 50% chance of a frost earlier than this date. The September 1 date, with only a 10% chance of an earlier frost, is the cautious point for finishing tender harvests, whereas October 1, at 90% chance, is a gamble. The median hard-frost dates are May 11 and September 27.

A hard frost at 28 °F can damage plant tissue that survives a light frost at 32 °F, so the hard-frost dates are critical thresholds for garden planning.

How to read the probabilities

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

  • Spring: "10% after June 7" 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 1" 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 Busby, MTNumbered nearest stations: 1 — Hysham 25 SSE, MT — 29 mi; 2 — Hardin, MT — 34 mi; 3 — Brandenberg, MT — 40 mi; 4 — Yellowtail Dam, MT — 50 mi; 5 — Forsyth, MT — 52 mi.MT25 mi · 40 km1. Hysham 25 SSE, MT — 29 mi12. Hardin, MT — 34 mi23. Brandenberg, MT — 40 mi34. Yellowtail Dam, MT — 50 mi45. Forsyth, MT — 52 mi5
  1. Hysham 25 SSE, MT 29 mi (46 km)
  2. Hardin, MT 34 mi (55 km)
  3. Brandenberg, MT 40 mi (64 km)
  4. Yellowtail Dam, MT 50 mi (80 km)
  5. Forsyth, MT 52 mi (84 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: 45.5397, -106.9597.