U.S. Frost Dates by Weather Station

NOAA station USC00247263 · Montana

Frost Dates for Ryegate, MT

NOAA weather station Ryegate 18 NNW (USC00247263), Montana · elevation 4440 ft (1353 m) · hardiness zone 4b*

  • Lower-risk planting dateafter June 13only 1 spring in 10 still has a frost later
  • Typical last frostMay 2650% chance of a frost after it
  • Typical first fall frostSeptember 1850% chance of a frost before it
  • Frost-free season112 daystypical year, 32°F
  • Hardiness zone4bcalculated*

The year at a glance

Frost season at Ryegate, MTSpring: a frost after May 12 in 9 years out of 10, after May 26 in half of the years, after June 13 in 1 year out of 10. Fall: the first frost comes before September 1 in 1 year out of 10, before September 18 in half of the years, before October 1 in 9 years out of 10. Typical frost-free season: 112 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 26First frost ~Sep 18Frost season at Ryegate, MTSpring: a frost after May 12 in 9 years out of 10, after May 26 in half of the years, after June 13 in 1 year out of 10. Fall: the first frost comes before September 1 in 1 year out of 10, before September 18 in half of the years, before October 1 in 9 years out of 10. Typical frost-free season: 112 days.JFMAMJJASONDLast frost ~May 26First frost ~Sep 18
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 29 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)June 13September 1
50% (typical)May 26September 18
90% (likely)May 12October 1
Frost-free season (32°F, 50% chance)
112 days
Hard frost (28°F), last in spring, 50% chance after
May 11
Hard frost (28°F), first in fall, 50% chance before
September 29

The last 15 years (2006–2020 normals)

Typical last frost May 28 (2 days later than the 30-year normal), typical first fall frost September 22 (4 days later). Frost-free season 116 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 Ryegate: chance of a frost after this dateSpring, chance of a frost after the date: 10% June 13, 20% June 7, 30% June 2, 40% May 30, 50% May 26, 60% May 23, 70% May 20, 80% May 17, 90% May 12. Observed last spring frost in recent years: 2021 May 30, 2022 May 23, 2023 June 22, 2024 May 31, 2025 June 3, 2026 June 14.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1Jun 13May 26May 122021: May 302022: May 232023: June 222024: May 312025: June 32026: June 14Fall at Ryegate: chance of a frost before this dateFall, chance of a frost before the date: 10% September 1, 20% September 7, 30% September 11, 40% September 15, 50% September 18, 60% September 21, 70% September 24, 80% September 27, 90% October 1. Observed first fall frost in recent years: 2021 September 17, 2022 October 7, 2023 October 6, 2024 September 21, 2025 October 6.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 1Sep 18Oct 12021: September 172022: October 72023: October 62024: September 212025: October 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 30September 17
2022May 23October 7
2023June 22October 6
2024May 31September 21
2025June 3October 6
2026June 14season not over yet

What these dates mean

September 1 is the cautious fall date for finishing the harvest in Ryegate, indicating a 10 % chance that a light frost has already arrived. The risky late-season date is October 1, where there is a 90 % chance of a light frost, so planning tender crops beyond September 1 raises the risk significantly. In spring the cautious date for setting out tender crops is June 13, which carries only a 10 % chance of a later light frost, while the earlier May 12 date involves a 90 % gamble.

For a gardener, the 50 % spring light-frost date is May 26, meaning there is an even chance of a frost later in the season, and the 50 % fall light-frost date is September 18, meaning there is an even chance of a frost earlier in the season. The 50 % hard-frost dates are May 11 and September 29; a hard frost at 28 °F is more damaging than a light frost at 32 °F because it can kill plant tissue that survives milder freezes.

How to read the probabilities

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

  • Spring: "10% after June 13" 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 Ryegate, MTNumbered nearest stations: 1 — Lewistown, MT — 36 mi; 2 — Rapelje, MT — 39 mi; 3 — Roundup, MT — 39 mi; 4 — Grass Range, MT — 43 mi; 5 — Melville, MT — 45 mi.25 mi · 40 km1. Lewistown, MT — 36 mi12. Rapelje, MT — 39 mi23. Roundup, MT — 39 mi34. Grass Range, MT — 43 mi45. Melville, MT — 45 mi5
  1. Lewistown, MT 36 mi (58 km)
  2. Rapelje, MT 39 mi (63 km)
  3. Roundup, MT 39 mi (63 km)
  4. Grass Range, MT 43 mi (68 km)
  5. Melville, MT 45 mi (72 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: 46.5333, -109.3442.