U.S. Frost Dates by Weather Station

NOAA station USW00024036 · Montana

Frost Dates for Lewistown, MT

NOAA weather station Lewistown Municipal Airport (USW00024036), Montana · elevation 4145 ft (1263 m) · hardiness zone 4b*

  • Lower-risk planting dateafter June 8only 1 spring in 10 still has a frost later
  • Typical last frostMay 2350% chance of a frost after it
  • Typical first fall frostSeptember 2150% chance of a frost before it
  • Frost-free season119 daystypical year, 32°F
  • Hardiness zone4bcalculated*

The year at a glance

Frost season at Lewistown, MTSpring: a frost after May 11 in 9 years out of 10, after May 23 in half of the years, after June 8 in 1 year out of 10. Fall: the first frost comes before September 8 in 1 year out of 10, before September 21 in half of the years, before October 3 in 9 years out of 10. Typical frost-free season: 119 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 23First frost ~Sep 21Frost season at Lewistown, MTSpring: a frost after May 11 in 9 years out of 10, after May 23 in half of the years, after June 8 in 1 year out of 10. Fall: the first frost comes before September 8 in 1 year out of 10, before September 21 in half of the years, before October 3 in 9 years out of 10. Typical frost-free season: 119 days.JFMAMJJASONDLast frost ~May 23First frost ~Sep 21
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 8
50% (typical)May 23September 21
90% (likely)May 11October 3
Frost-free season (32°F, 50% chance)
119 days
Hard frost (28°F), last in spring, 50% chance after
May 9
Hard frost (28°F), first in fall, 50% chance before
October 1

The last 15 years (2006–2020 normals)

Typical last frost May 24 (1 day later than the 30-year normal), typical first fall frost September 23 (2 days later). Frost-free season 121 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 Lewistown: chance of a frost after this dateSpring, chance of a frost after the date: 10% June 8, 20% June 2, 30% May 29, 40% May 26, 50% May 23, 60% May 21, 70% May 18, 80% May 15, 90% May 11. Observed last spring frost in recent years: 2021 May 30, 2022 May 24, 2023 May 10, 2024 June 1, 2025 June 3, 2026 May 22.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1Jun 8May 23May 112021: May 302022: May 242023: May 102024: June 12025: June 32026: May 22Fall at Lewistown: chance of a frost before this dateFall, chance of a frost before the date: 10% September 8, 20% September 12, 30% September 17, 40% September 19, 50% September 21, 60% September 23, 70% September 26, 80% September 29, 90% October 3. Observed first fall frost in recent years: 2021 September 16, 2022 October 13, 2023 October 6, 2024 September 30, 2025 September 5.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 8Sep 21Oct 32021: September 162022: October 132023: October 62024: September 302025: September 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 30September 16
2022May 24October 13
2023May 10October 6
2024June 1September 30
2025June 3September 5
2026May 22season not over yet

What these dates mean

September 8 is the cautious fall date in Lewistown for finishing the harvest of tender crops, because there is only a 10% chance that a light frost has arrived before that day. The 50% date for light frost in fall is September 21, which means a gardener faces an even chance of a frost earlier than that date. In spring the cautious date for setting out tender crops is June 8, also with a 10% chance of a later light frost, while the 50% spring date is May 23.

For the gardener, the 50% dates—May 23 in spring and September 21 in fall—represent the points at which a light frost is equally likely to occur later (spring) or earlier (fall). The hard-frost median dates are May 9 and October 1; a hard frost at 28 °F can damage or kill plants that would survive a 32 °F light frost, making it a more critical threshold.

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 8" 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 Lewistown, MTNumbered nearest stations: 1 — Grass Range, MT — 31 mi; 2 — Ryegate, MT — 36 mi; 3 — Stanford, MT — 37 mi; 4 — Valentine, MT — 49 mi; 5 — Flatwillow, MT — 54 mi.MT25 mi · 40 km1. Grass Range, MT — 31 mi12. Ryegate, MT — 36 mi23. Stanford, MT — 37 mi34. Valentine, MT — 49 mi45. Flatwillow, MT — 54 mi5
  1. Grass Range, MT 31 mi (50 km)
  2. Ryegate, MT 36 mi (58 km)
  3. Stanford, MT 37 mi (59 km)
  4. Valentine, MT 49 mi (79 km)
  5. Flatwillow, MT 54 mi (86 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: 47.0492, -109.4578.