U.S. Frost Dates by Weather Station

NOAA station USC00246862 · Montana

Frost Dates for Rapelje, MT

NOAA weather station Rapelje (USC00246862), Montana · elevation 4068 ft (1240 m) · hardiness zone 4b*

  • Lower-risk planting dateafter June 11only 1 spring in 10 still has a frost later
  • Typical last frostMay 2450% chance of a frost after it
  • Typical first fall frostSeptember 2150% chance of a frost before it
  • Frost-free season116 daystypical year, 32°F
  • Hardiness zone4bcalculated*

The year at a glance

Frost season at Rapelje, MTSpring: a frost after May 11 in 9 years out of 10, after May 24 in half of the years, after June 11 in 1 year out of 10. Fall: the first frost comes before September 4 in 1 year out of 10, before September 21 in half of the years, before October 4 in 9 years out of 10. Typical frost-free season: 116 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 24First frost ~Sep 21Frost season at Rapelje, MTSpring: a frost after May 11 in 9 years out of 10, after May 24 in half of the years, after June 11 in 1 year out of 10. Fall: the first frost comes before September 4 in 1 year out of 10, before September 21 in half of the years, before October 4 in 9 years out of 10. Typical frost-free season: 116 days.JFMAMJJASONDLast frost ~May 24First frost ~Sep 21
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)June 11September 4
50% (typical)May 24September 21
90% (likely)May 11October 4
Frost-free season (32°F, 50% chance)
116 days
Hard frost (28°F), last in spring, 50% chance after
May 11
Hard frost (28°F), first in fall, 50% chance before
October 2

The last 15 years (2006–2020 normals)

Typical last frost May 22 (2 days earlier than the 30-year normal), typical first fall frost September 24 (3 days later). Frost-free season 124 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 Rapelje: chance of a frost after this dateSpring, chance of a frost after the date: 10% June 11, 20% June 4, 30% May 31, 40% May 28, 50% May 24, 60% May 21, 70% May 18, 80% May 15, 90% May 11. Observed last spring frost in recent years: 2021 May 28.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1Jun 11May 24May 112021: May 28Fall at Rapelje: chance of a frost before this dateFall, chance of a frost before the date: 10% September 4, 20% September 10, 30% September 14, 40% September 18, 50% September 21, 60% September 24, 70% September 26, 80% September 30, 90% October 4.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 4Sep 21Oct 4
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 28incomplete record
2022incomplete recordno data
2023no datano data
2024no datano data
2025no datano data
2026no dataseason not over yet

What these dates mean

The 116 days between the 32 °F 50% dates, May 24 and September 21, frame the span when tender crops can be grown in Rapelje. During that interval a gardener can expect a roughly even chance of a light frost occurring after the spring median date or before the fall median date, giving a clear window for planting and harvesting.

For a cautious plan, the spring date to set out tender crops is June 11, when there is only a 10 % chance of a light frost later, while the fall date to finish harvesting tender crops is September 4, when there is only a 10 % chance of a light frost earlier. A hard frost (28 °F) matters more than a light frost (32 °F) because it can damage plants that would otherwise survive the milder temperature.

How to read the probabilities

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

  • Spring: "10% after June 11" 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 4" 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 Rapelje, MTNumbered nearest stations: 1 — Columbus, MT — 23 mi; 2 — Big Timber, MT — 35 mi; 3 — Billings (Logan Airport), MT — 36 mi; 4 — Ryegate, MT — 39 mi; 5 — Melville, MT — 39 mi.25 mi · 40 km1. Columbus, MT — 23 mi12. Big Timber, MT — 35 mi23. Billings (Logan Airport), MT — 36 mi34. Ryegate, MT — 39 mi45. Melville, MT — 39 mi5
  1. Columbus, MT 23 mi (36 km)
  2. Big Timber, MT 35 mi (56 km)
  3. Billings (Logan Airport), MT 36 mi (58 km)
  4. Ryegate, MT 39 mi (63 km)
  5. Melville, MT 39 mi (63 km)

Sources

* Hardiness zone 4b: zone derived from PRISM 2023 grid at the station location (not the official USDA map). Source data: PRISM Group, Oregon State University.

Station coordinates: 45.9717, -109.2536.