U.S. Frost Dates by Weather Station

NOAA station USC00245754 · Montana

Frost Dates for Mizpah, MT

NOAA weather station Mizpah 4 NNW (USC00245754), Montana · elevation 2480 ft (756 m) · hardiness zone 4a*

  • Lower-risk planting dateafter June 7only 1 spring in 10 still has a frost later
  • Typical last frostMay 2250% chance of a frost after it
  • Typical first fall frostSeptember 1550% chance of a frost before it
  • Frost-free season117 daystypical year, 32°F
  • Hardiness zone4acalculated*

The year at a glance

Frost season at Mizpah, MTSpring: a frost after May 9 in 9 years out of 10, after May 22 in half of the years, after June 7 in 1 year out of 10. Fall: the first frost comes before September 2 in 1 year out of 10, before September 15 in half of the years, before October 1 in 9 years out of 10. Typical frost-free season: 117 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 22First frost ~Sep 15Frost season at Mizpah, MTSpring: a frost after May 9 in 9 years out of 10, after May 22 in half of the years, after June 7 in 1 year out of 10. Fall: the first frost comes before September 2 in 1 year out of 10, before September 15 in half of the years, before October 1 in 9 years out of 10. Typical frost-free season: 117 days.JFMAMJJASONDLast frost ~May 22First frost ~Sep 15
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 7September 2
50% (typical)May 22September 15
90% (likely)May 9October 1
Frost-free season (32°F, 50% chance)
117 days
Hard frost (28°F), last in spring, 50% chance after
May 11
Hard frost (28°F), first in fall, 50% chance before
September 25

The last 15 years (2006–2020 normals)

Typical last frost May 21 (1 day earlier than the 30-year normal), typical first fall frost September 17 (2 days later). Frost-free season 117 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 Mizpah: chance of a frost after this dateSpring, chance of a frost after the date: 10% June 7, 20% June 1, 30% May 29, 40% May 25, 50% May 22, 60% May 19, 70% May 16, 80% May 12, 90% May 9. Observed last spring frost in recent years: 2021 May 13, 2022 June 3, 2023 April 30, 2024 June 19, 2025 May 22, 2026 May 24.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1Jun 7May 22May 92021: May 132022: June 32023: April 302024: June 192025: May 222026: May 24Fall at Mizpah: chance of a frost before this dateFall, chance of a frost before the date: 10% September 2, 20% September 5, 30% September 10, 40% September 13, 50% September 15, 60% September 19, 70% September 22, 80% September 27, 90% October 1. Observed first fall frost in recent years: 2021 September 17, 2023 October 6, 2024 September 22, 2025 August 23.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 2Sep 15Oct 12021: September 172023: October 62024: September 222025: August 23
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 13September 17
2022June 3incomplete record
2023April 30October 6
2024June 19September 22
2025May 22August 23
2026May 24season not over yet

What these dates mean

September 2 is the cautious fall date in Mizpah for finishing the harvest, when there is only a 10 % chance that a light frost has already arrived. The 50 % date in fall is September 15, marking the point where a light frost has an even chance of having occurred or not, and the median hard-frost date is September 25. In spring the 50 % light-frost date is May 22, while the median hard-frost date is May 11.

For tender crops the cautious spring date is June 7, when a light frost is still only a 10 % risk later in the season, and the cautious fall date for concluding tender-crop production is September 2. A hard frost at 28 °F is more damaging than a light frost at 32 °F because the lower temperature can injure or kill plants that survive a milder freeze.

How to read the probabilities

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

  • Spring: "10% after June 7" 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 2" 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 Mizpah, MTNumbered nearest stations: 1 — near Miles City, MT — 30 mi; 2 — Powderville, MT — 32 mi; 3 — Terry, MT — 35 mi; 4 — Mildred, MT — 36 mi; 5 — Plevna, MT — 38 mi.MTNDSD25 mi · 40 km1. near Miles City, MT — 30 mi12. Powderville, MT — 32 mi23. Terry, MT — 35 mi34. Mildred, MT — 36 mi45. Plevna, MT — 38 mi5
  1. near Miles City, MT 30 mi (48 km)
  2. Powderville, MT 32 mi (52 km)
  3. Terry, MT 35 mi (57 km)
  4. Mildred, MT 36 mi (59 km)
  5. Plevna, MT 38 mi (61 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: 46.2858, -105.2919.