U.S. Frost Dates by Weather Station

NOAA station USC00247034 · Montana

Frost Dates for Ridgeway, MT

NOAA weather station Ridgeway 1 S (USC00247034), Montana · elevation 3316 ft (1011 m) · hardiness zone 4a*

  • Lower-risk planting dateafter June 1only 1 spring in 10 still has a frost later
  • Typical last frostMay 1750% chance of a frost after it
  • Typical first fall frostSeptember 2250% chance of a frost before it
  • Frost-free season127 daystypical year, 32°F
  • Hardiness zone4acalculated*

The year at a glance

Frost season at Ridgeway, MTSpring: a frost after May 5 in 9 years out of 10, after May 17 in half of the years, after June 1 in 1 year out of 10. Fall: the first frost comes before September 5 in 1 year out of 10, before September 22 in half of the years, before October 5 in 9 years out of 10. Typical frost-free season: 127 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 17First frost ~Sep 22Frost season at Ridgeway, MTSpring: a frost after May 5 in 9 years out of 10, after May 17 in half of the years, after June 1 in 1 year out of 10. Fall: the first frost comes before September 5 in 1 year out of 10, before September 22 in half of the years, before October 5 in 9 years out of 10. Typical frost-free season: 127 days.JFMAMJJASONDLast frost ~May 17First frost ~Sep 22
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)June 1September 5
50% (typical)May 17September 22
90% (likely)May 5October 5
Frost-free season (32°F, 50% chance)
127 days
Hard frost (28°F), last in spring, 50% chance after
May 6
Hard frost (28°F), first in fall, 50% chance before
October 1

The last 15 years (2006–2020 normals)

Typical last frost May 16 (1 day earlier than the 30-year normal), typical first fall frost September 24 (2 days later). Frost-free season 129 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 Ridgeway: chance of a frost after this dateSpring, chance of a frost after the date: 10% June 1, 20% May 27, 30% May 24, 40% May 21, 50% May 17, 60% May 15, 70% May 12, 80% May 9, 90% May 5. Observed last spring frost in recent years: 2021 May 18, 2022 May 22, 2023 May 19, 2024 June 19, 2025 May 21, 2026 May 23.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1Jun 1May 17May 52021: May 182022: May 222023: May 192024: June 192025: May 212026: May 23Fall at Ridgeway: chance of a frost before this dateFall, chance of a frost before the date: 10% September 5, 20% September 11, 30% September 15, 40% September 18, 50% September 22, 60% September 25, 70% September 29, 80% October 1, 90% October 5. Observed first fall frost in recent years: 2022 October 8, 2023 October 6, 2024 September 22, 2025 September 6.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 5Sep 22Oct 52022: October 82023: October 62024: September 222025: September 6
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 18incomplete record
2022May 22October 8
2023May 19October 6
2024June 19September 22
2025May 21September 6
2026May 23season not over yet

What these dates mean

June 1 is the cautious spring light-frost date, with only a 10 % chance of a later frost, and it signals when tender crops can be set out with low risk.

The 50% date is May 17 for spring and September 22 for fall; each marks an even-odds point for a light frost. The cautious fall light-frost date is September 5, when there is only a 10 % chance that a frost has already arrived, guiding the finish of tender-crop harvests. The record spans 26 years, and the interval between the two 32 °F 50% dates is 127 days.

The hard-frost 50% dates are May 6 and October 1; a hard frost at 28 °F can kill plants that would survive a light frost at 32 °F, so it represents a higher risk threshold.

How to read the probabilities

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

  • Spring: "10% after June 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 September 5" 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 Ridgeway, MTNumbered nearest stations: 1 — Camp Crook, SD — 23 mi; 2 — Ekalaka, MT — 27 mi; 3 — Powderville, MT — 37 mi; 4 — Biddle, MT — 59 mi; 5 — Plevna, MT — 63 mi.MTSDND25 mi · 40 km1. Camp Crook, SD — 23 mi12. Ekalaka, MT — 27 mi23. Powderville, MT — 37 mi34. Biddle, MT — 59 mi45. Plevna, MT — 63 mi5
  1. Camp Crook, SD 23 mi (37 km)
  2. Ekalaka, MT 27 mi (43 km)
  3. Powderville, MT 37 mi (60 km)
  4. Biddle, MT 59 mi (96 km)
  5. Plevna, MT 63 mi (102 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: 45.5022, -104.4478.