U.S. Frost Dates by Weather Station

NOAA station USC00241518 · Montana

Frost Dates for Carlyle, MT

NOAA weather station Carlyle 13 NW (USC00241518), Montana · elevation 3145 ft (959 m) · hardiness zone 4b*

  • Lower-risk planting dateafter May 31only 1 spring in 10 still has a frost later
  • Typical last frostMay 1850% chance of a frost after it
  • Typical first fall frostSeptember 2950% chance of a frost before it
  • Frost-free season134 daystypical year, 32°F
  • Hardiness zone4bcalculated*

The year at a glance

Frost season at Carlyle, MTSpring: a frost after May 4 in 9 years out of 10, after May 18 in half of the years, after May 31 in 1 year out of 10. Fall: the first frost comes before September 13 in 1 year out of 10, before September 29 in half of the years, before October 12 in 9 years out of 10. Typical frost-free season: 134 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 18First frost ~Sep 29Frost season at Carlyle, MTSpring: a frost after May 4 in 9 years out of 10, after May 18 in half of the years, after May 31 in 1 year out of 10. Fall: the first frost comes before September 13 in 1 year out of 10, before September 29 in half of the years, before October 12 in 9 years out of 10. Typical frost-free season: 134 days.JFMAMJJASONDLast frost ~May 18First frost ~Sep 29
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 26 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 31September 13
50% (typical)May 18September 29
90% (likely)May 4October 12
Frost-free season (32°F, 50% chance)
134 days
Hard frost (28°F), last in spring, 50% chance after
May 6
Hard frost (28°F), first in fall, 50% chance before
October 6

The last 15 years (2006–2020 normals)

Typical last frost May 13 (5 days earlier than the 30-year normal), typical first fall frost October 3 (4 days later). Frost-free season 141 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 Carlyle: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 31, 20% May 26, 30% May 23, 40% May 20, 50% May 18, 60% May 14, 70% May 11, 80% May 7, 90% May 4. Observed last spring frost in recent years: 2021 May 10, 2022 May 22, 2023 May 2, 2024 May 4, 2025 May 20, 2026 May 19.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 31May 18May 42021: May 102022: May 222023: May 22024: May 42025: May 202026: May 19Fall at Carlyle: chance of a frost before this dateFall, chance of a frost before the date: 10% September 13, 20% September 18, 30% September 22, 40% September 25, 50% September 29, 60% October 1, 70% October 3, 80% October 7, 90% October 12. Observed first fall frost in recent years: 2021 October 12, 2022 October 7, 2023 October 6, 2024 October 30, 2025 September 3.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Nov 1Sep 13Sep 29Oct 122021: October 122022: October 72023: October 62024: October 302025: September 3
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 10October 12
2022May 22October 7
2023May 2October 6
2024May 4October 30
2025May 20September 3
2026May 19season not over yet

What these dates mean

The 134 days between the 50% light-frost dates in Carlyle frame the window when tender crops can be grown without a light frost risk.

The median light-frost dates are May 18 in spring and September 29 in fall; each represents a 50 % chance of a frost occurring afterward (spring) or earlier (fall). For a cautious plan, the spring date to set out tender crops is May 31, which carries only a 10 % chance of a later frost, while the fall cautious date to finish harvesting tender crops is September 13, which has just a 10 % chance of an earlier frost.

Hard-frost median dates are May 6 and October 6; a hard frost at 28 °F can injure or kill plants that might survive a light frost at 32 °F, making it a more critical threshold for tender crops.

How to read the probabilities

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

  • Spring: "10% after May 31" 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 13" 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 Carlyle, MTNumbered nearest stations: 1 — Plevna, MT — 25 mi; 2 — Mildred, MT — 31 mi; 3 — Glendive, MT — 32 mi; 4 — Medora, ND — 39 mi; 5 — Terry, MT — 47 mi.MTND25 mi · 40 km1. Plevna, MT — 25 mi12. Mildred, MT — 31 mi23. Glendive, MT — 32 mi34. Medora, ND — 39 mi45. Terry, MT — 47 mi5
  1. Plevna, MT 25 mi (40 km)
  2. Mildred, MT 31 mi (50 km)
  3. Glendive, MT 32 mi (51 km)
  4. Medora, ND 39 mi (63 km)
  5. Terry, MT 47 mi (76 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: 46.7444, -104.3092.