U.S. Frost Dates by Weather Station

NOAA station USC00244241 · Montana

Frost Dates for Holter Dam, MT

NOAA weather station Holter Dam (USC00244241), Montana · elevation 3487 ft (1063 m) · hardiness zone 4b*

  • Lower-risk planting dateafter June 8only 1 spring in 10 still has a frost later
  • Typical last frostMay 1950% chance of a frost after it
  • Typical first fall frostSeptember 2450% chance of a frost before it
  • Frost-free season124 daystypical year, 32°F
  • Hardiness zone4bcalculated*

The year at a glance

Frost season at Holter Dam, MTSpring: a frost after May 6 in 9 years out of 10, after May 19 in half of the years, after June 8 in 1 year out of 10. Fall: the first frost comes before September 6 in 1 year out of 10, before September 24 in half of the years, before October 7 in 9 years out of 10. Typical frost-free season: 124 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 19First frost ~Sep 24Frost season at Holter Dam, MTSpring: a frost after May 6 in 9 years out of 10, after May 19 in half of the years, after June 8 in 1 year out of 10. Fall: the first frost comes before September 6 in 1 year out of 10, before September 24 in half of the years, before October 7 in 9 years out of 10. Typical frost-free season: 124 days.JFMAMJJASONDLast frost ~May 19First frost ~Sep 24
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 30 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)June 8September 6
50% (typical)May 19September 24
90% (likely)May 6October 7
Frost-free season (32°F, 50% chance)
124 days
Hard frost (28°F), last in spring, 50% chance after
May 6
Hard frost (28°F), first in fall, 50% chance before
October 4

The last 15 years (2006–2020 normals)

Typical last frost May 17 (2 days earlier than the 30-year normal), typical first fall frost September 24. 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 Holter Dam: chance of a frost after this dateSpring, chance of a frost after the date: 10% June 8, 20% June 1, 30% May 26, 40% May 23, 50% May 19, 60% May 17, 70% May 14, 80% May 10, 90% May 6. Observed last spring frost in recent years: 2021 May 30, 2022 May 23, 2023 April 23, 2024 June 1, 2025 June 3, 2026 May 22.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1Jun 8May 19May 62021: May 302022: May 232023: April 232024: June 12025: June 32026: May 22Fall at Holter Dam: chance of a frost before this dateFall, chance of a frost before the date: 10% September 6, 20% September 13, 30% September 17, 40% September 21, 50% September 24, 60% September 26, 70% September 30, 80% October 2, 90% October 7. Observed first fall frost in recent years: 2021 September 17, 2022 September 10, 2024 October 3, 2025 October 6.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 6Sep 24Oct 72021: September 172022: September 102024: October 32025: 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 23September 10
2023April 23incomplete record
2024June 1October 3
2025June 3October 6
2026May 22season not over yet

What these dates mean

September 6 marks the cautious fall date in Holter Dam for finishing the harvest of tender crops, when there is only a 10% chance that a light frost has arrived.

For a gardener, the 50% light-frost dates represent the midpoint of risk: May 19 carries a 50% chance of a frost later in spring, and September 24 carries a 50% chance of a frost earlier in fall. The hard-frost median dates are May 6 and October 4, indicating the points at which a 28 °F freeze is equally likely to have occurred or not.

A cautious plan uses June 8 as the spring date to set out tender crops, because only a 10% chance of a light frost remains later, while September 6 serves the same purpose for fall. A hard frost at 28 °F can damage plants that would survive a light frost at 32 °F.

How to read the probabilities

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

  • Spring: "10% after June 8" 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 6" 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 Holter Dam, MTNumbered nearest stations: 1 — Cascade, MT — 21 mi; 2 — Helena, MT — 27 mi; 3 — Sun River, MT — 36 mi; 4 — Millegan, MT — 41 mi; 5 — Fairfield, MT — 43 mi.25 mi · 40 km1. Cascade, MT — 21 mi12. Helena, MT — 27 mi23. Sun River, MT — 36 mi34. Millegan, MT — 41 mi45. Fairfield, MT — 43 mi5
  1. Cascade, MT 21 mi (34 km)
  2. Helena, MT 27 mi (43 km)
  3. Sun River, MT 36 mi (58 km)
  4. Millegan, MT 41 mi (66 km)
  5. Fairfield, MT 43 mi (69 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: 46.9908, -112.0117.