U.S. Frost Dates by Weather Station

NOAA station USC00248363 · Montana

Frost Dates for Trident, MT

NOAA weather station Trident (USC00248363), Montana · elevation 4036 ft (1230 m) · hardiness zone 4b*

  • Lower-risk planting dateafter May 24only 1 spring in 10 still has a frost later
  • Typical last frostMay 950% chance of a frost after it
  • Typical first fall frostSeptember 2750% chance of a frost before it
  • Frost-free season139 daystypical year, 32°F
  • Hardiness zone4bcalculated*

The year at a glance

Frost season at Trident, MTSpring: a frost after April 29 in 9 years out of 10, after May 9 in half of the years, after May 24 in 1 year out of 10. Fall: the first frost comes before September 12 in 1 year out of 10, before September 27 in half of the years, before October 9 in 9 years out of 10. Typical frost-free season: 139 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 9First frost ~Sep 27Frost season at Trident, MTSpring: a frost after April 29 in 9 years out of 10, after May 9 in half of the years, after May 24 in 1 year out of 10. Fall: the first frost comes before September 12 in 1 year out of 10, before September 27 in half of the years, before October 9 in 9 years out of 10. Typical frost-free season: 139 days.JFMAMJJASONDLast frost ~May 9First frost ~Sep 27
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 27 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 24September 12
50% (typical)May 9September 27
90% (likely)April 29October 9
Frost-free season (32°F, 50% chance)
139 days
Hard frost (28°F), last in spring, 50% chance after
April 29
Hard frost (28°F), first in fall, 50% chance before
October 6

The last 15 years (2006–2020 normals)

Typical last frost May 7 (2 days earlier than the 30-year normal), typical first fall frost October 1 (4 days later). Frost-free season 145 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 Trident: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 24, 20% May 18, 30% May 14, 40% May 11, 50% May 9, 60% May 6, 70% May 4, 80% May 2, 90% April 29. Observed last spring frost in recent years: 2021 May 22, 2022 May 23, 2023 April 28.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 24May 9Apr 292021: May 222022: May 232023: April 28Fall at Trident: chance of a frost before this dateFall, chance of a frost before the date: 10% September 12, 20% September 18, 30% September 22, 40% September 24, 50% September 27, 60% September 29, 70% October 2, 80% October 5, 90% October 9. Observed first fall frost in recent years: 2021 September 17, 2022 October 8, 2023 October 14.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 12Sep 27Oct 92021: September 172022: October 82023: October 14
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 22September 17
2022May 23October 8
2023April 28October 14
2024no datano data
2025no datano data
2026no dataseason not over yet

What these dates mean

The cautious fall date for finishing the harvest in Trident is September 12, when there is only a 10% chance a light frost has arrived. Gardeners planning to pull in tender crops would aim to complete harvest by that day to keep risk low.

For light-frost probabilities, the 50% date is May 9 in spring and September 27 in fall; each marks the point at which a gardener faces even odds of a frost occurring later (spring) or earlier (fall). The hard-frost median dates are April 29 for spring and October 6 for fall, indicating when a 28°F frost is equally likely.

The cautious spring date for setting out tender crops is May 24, when only a 10% chance of a light frost remains. A hard frost matters more than a light frost because temperatures of 28°F can injure or kill plants that would otherwise survive a 32°F event.

How to read the probabilities

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

  • Spring: "10% after May 24" 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 12" 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 Trident, MTNumbered nearest stations: 1 — Bozeman (Gallatin Field), MT — 19 mi; 2 — Townsend, MT — 24 mi; 3 — near Four Corners, MT — 24 mi; 4 — near Bozeman, MT — 29 mi; 5 — Norris Madison Power House, MT — 33 mi.25 mi · 40 km1. Bozeman (Gallatin Field), MT — 19 mi12. Townsend, MT — 24 mi23. near Four Corners, MT — 24 mi34. near Bozeman, MT — 29 mi45. Norris Madison Power House, MT — 33 mi5
  1. Bozeman (Gallatin Field), MT 19 mi (30 km)
  2. Townsend, MT 24 mi (39 km)
  3. near Four Corners, MT 24 mi (39 km)
  4. near Bozeman, MT 29 mi (46 km)
  5. Norris Madison Power House, MT 33 mi (53 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: 45.9467, -111.4758.