U.S. Frost Dates by Weather Station

NOAA station USW00024144 · Montana

Frost Dates for Helena, MT

NOAA weather station Helena Regional Airport (USW00024144), Montana · elevation 3828 ft (1167 m) · hardiness zone 5a*

  • Lower-risk planting dateafter May 25only 1 spring in 10 still has a frost later
  • Typical last frostMay 950% chance of a frost after it
  • Typical first fall frostSeptember 3050% chance of a frost before it
  • Frost-free season142 daystypical year, 32°F
  • Hardiness zone5acalculated*

The year at a glance

Frost season at Helena, MTSpring: a frost after April 29 in 9 years out of 10, after May 9 in half of the years, after May 25 in 1 year out of 10. Fall: the first frost comes before September 17 in 1 year out of 10, before September 30 in half of the years, before October 11 in 9 years out of 10. Typical frost-free season: 142 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 9First frost ~Sep 30Frost season at Helena, MTSpring: a frost after April 29 in 9 years out of 10, after May 9 in half of the years, after May 25 in 1 year out of 10. Fall: the first frost comes before September 17 in 1 year out of 10, before September 30 in half of the years, before October 11 in 9 years out of 10. Typical frost-free season: 142 days.JFMAMJJASONDLast frost ~May 9First frost ~Sep 30
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 25 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 25September 17
50% (typical)May 9September 30
90% (likely)April 29October 11
Frost-free season (32°F, 50% chance)
142 days
Hard frost (28°F), last in spring, 50% chance after
April 29
Hard frost (28°F), first in fall, 50% chance before
October 7

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 2 (2 days later). Frost-free season 148 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 Helena: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 25, 20% May 19, 30% May 15, 40% May 12, 50% May 9, 60% May 7, 70% May 4, 80% May 2, 90% April 29. Observed last spring frost in recent years: 2021 May 22, 2022 May 11, 2023 April 28, 2024 May 20, 2025 April 25, 2026 May 19.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 25May 9Apr 292021: May 222022: May 112023: April 282024: May 202025: April 252026: May 19Fall at Helena: chance of a frost before this dateFall, chance of a frost before the date: 10% September 17, 20% September 22, 30% September 25, 40% September 27, 50% September 30, 60% October 1, 70% October 4, 80% October 7, 90% October 11. Observed first fall frost in recent years: 2021 September 30, 2022 October 19, 2023 October 24, 2024 October 1, 2025 October 5.Fall: first frostChance of a frost before this date10%50%90%Oct 1Sep 17Sep 30Oct 112021: September 302022: October 192023: October 242024: October 12025: 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 22September 30
2022May 11October 19
2023April 28October 24
2024May 20October 1
2025April 25October 5
2026May 19season not over yet

What these dates mean

The 142 days between the 50% dates in Helena, May 9 and September 30, mark the interval when a light frost (32 °F) is less likely than not. For a gardener these median dates are not frost dates; they simply indicate the point where there is a 50% chance of a light frost later in spring and a 50% chance of a light frost earlier in fall.

When planning tender crops, the cautious spring date is May 25, which carries only a 10% chance of a light frost later, while planting before April 29 would be a gamble with a 90% chance of frost. In the fall the cautious date to finish harvesting tender crops is September 17, reflecting a 10% chance that a light frost has already arrived; extending to October 11 would be a gamble with a 90% chance of an earlier frost. A hard frost at 28 °F matters more than a light frost because it can cause more severe damage to plants.

How to read the probabilities

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

  • Spring: "10% after May 25" 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 17" 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 Helena, MTNumbered nearest stations: 1 — Holter Dam, MT — 27 mi; 2 — Boulder, MT — 27 mi; 3 — Townsend, MT — 32 mi; 4 — Millegan, MT — 42 mi; 5 — Cascade, MT — 44 mi.MT25 mi · 40 km1. Holter Dam, MT — 27 mi12. Boulder, MT — 27 mi23. Townsend, MT — 32 mi34. Millegan, MT — 42 mi45. Cascade, MT — 44 mi5
  1. Holter Dam, MT 27 mi (43 km)
  2. Boulder, MT 27 mi (43 km)
  3. Townsend, MT 32 mi (52 km)
  4. Millegan, MT 42 mi (68 km)
  5. Cascade, MT 44 mi (71 km)

Sources

* Hardiness zone 5a: zone derived from PRISM 2023 grid at the station location (not the official USDA map). Source data: PRISM Group, Oregon State University.

Station coordinates: 46.6056, -111.9636.