U.S. Frost Dates by Weather Station

NOAA station USC00248021 · Montana

Frost Dates for Sun River, MT

NOAA weather station Sun River 4 S (USC00248021), Montana · elevation 3600 ft (1097 m) · hardiness zone 4a*

  • Lower-risk planting dateafter June 21only 1 spring in 10 still has a frost later
  • Typical last frostJune 250% chance of a frost after it
  • Typical first fall frostSeptember 1050% chance of a frost before it
  • Frost-free season96 daystypical year, 32°F
  • Hardiness zone4acalculated*

The year at a glance

Frost season at Sun River, MTSpring: a frost after May 18 in 9 years out of 10, after June 2 in half of the years, after June 21 in 1 year out of 10. Fall: the first frost comes before August 25 in 1 year out of 10, before September 10 in half of the years, before September 25 in 9 years out of 10. Typical frost-free season: 96 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~Jun 2First frost ~Sep 10Frost season at Sun River, MTSpring: a frost after May 18 in 9 years out of 10, after June 2 in half of the years, after June 21 in 1 year out of 10. Fall: the first frost comes before August 25 in 1 year out of 10, before September 10 in half of the years, before September 25 in 9 years out of 10. Typical frost-free season: 96 days.JFMAMJJASONDLast frost ~Jun 2First frost ~Sep 10
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 21August 25
50% (typical)June 2September 10
90% (likely)May 18September 25
Frost-free season (32°F, 50% chance)
96 days
Hard frost (28°F), last in spring, 50% chance after
May 18
Hard frost (28°F), first in fall, 50% chance before
September 22

The last 15 years (2006–2020 normals)

Typical last frost May 29 (4 days earlier than the 30-year normal), typical first fall frost September 9 (1 day earlier). Frost-free season 103 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 Sun River: chance of a frost after this dateSpring, chance of a frost after the date: 10% June 21, 20% June 14, 30% June 9, 40% June 5, 50% June 2, 60% May 30, 70% May 27, 80% May 23, 90% May 18. Observed last spring frost in recent years: 2021 May 28, 2022 June 2, 2023 June 22, 2026 May 21.Spring: last frostChance of a frost after this date10%50%90%Jun 1Jul 1Jun 21Jun 2May 182021: May 282022: June 22023: June 222026: May 21Fall at Sun River: chance of a frost before this dateFall, chance of a frost before the date: 10% August 25, 20% August 30, 30% September 3, 40% September 6, 50% September 10, 60% September 13, 70% September 16, 80% September 20, 90% September 25. Observed first fall frost in recent years: 2021 September 17, 2022 September 11, 2023 October 5.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Aug 25Sep 10Sep 252021: September 172022: September 112023: October 5
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 28September 17
2022June 2September 11
2023June 22October 5
2024incomplete recordincomplete record
2025incomplete recordincomplete record
2026May 21season not over yet

What these dates mean

June 21 is the cautious spring date for Sun River; with only a 10% chance of a light frost later than this date, a gardener can plan to set out tender crops. June 2 carries a 50% chance of a light frost later than this date, marking the point where frost risk is even. In the fall, August 25 is the cautious date, offering a 10% chance of a light frost earlier than this date and guiding the finish of tender-crop harvests, while September 10 has a 50% chance of a light frost earlier than this date. The 50% hard-frost dates are May 18 and September 22; each represents a 50% chance of a 28 °F frost later (spring) or earlier (fall). Hard frosts matter more than light frosts because the colder temperature can kill plant tissue that survives a 32 °F frost.

How to read the probabilities

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

  • Spring: "10% after June 21" 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 August 25" 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 Sun River, MTNumbered nearest stations: 1 — Fairfield, MT — 15 mi; 2 — Great Falls, MT — 17 mi; 3 — Cascade, MT — 18 mi; 4 — Choteau, MT — 32 mi; 5 — Holter Dam, MT — 36 mi.25 mi · 40 km1. Fairfield, MT — 15 mi12. Great Falls, MT — 17 mi23. Cascade, MT — 18 mi34. Choteau, MT — 32 mi45. Holter Dam, MT — 36 mi5
  1. Fairfield, MT 15 mi (24 km)
  2. Great Falls, MT 17 mi (27 km)
  3. Cascade, MT 18 mi (29 km)
  4. Choteau, MT 32 mi (51 km)
  5. Holter Dam, MT 36 mi (58 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: 47.4772, -111.7375.