U.S. Frost Dates by Weather Station

NOAA station USC00247894 · Montana

Frost Dates for Stevensville, MT

NOAA weather station Stevensville (USC00247894), Montana · elevation 3375 ft (1029 m) · hardiness zone 5b*

  • Lower-risk planting dateafter June 13only 1 spring in 10 still has a frost later
  • Typical last frostMay 2250% chance of a frost after it
  • Typical first fall frostSeptember 1950% chance of a frost before it
  • Frost-free season115 daystypical year, 32°F
  • Hardiness zone5bcalculated*

The year at a glance

Frost season at Stevensville, MTSpring: a frost after May 7 in 9 years out of 10, after May 22 in half of the years, after June 13 in 1 year out of 10. Fall: the first frost comes before September 3 in 1 year out of 10, before September 19 in half of the years, before October 2 in 9 years out of 10. Typical frost-free season: 115 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 22First frost ~Sep 19Frost season at Stevensville, MTSpring: a frost after May 7 in 9 years out of 10, after May 22 in half of the years, after June 13 in 1 year out of 10. Fall: the first frost comes before September 3 in 1 year out of 10, before September 19 in half of the years, before October 2 in 9 years out of 10. Typical frost-free season: 115 days.JFMAMJJASONDLast frost ~May 22First frost ~Sep 19
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 28 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)June 13September 3
50% (typical)May 22September 19
90% (likely)May 7October 2
Frost-free season (32°F, 50% chance)
115 days
Hard frost (28°F), last in spring, 50% chance after
May 5
Hard frost (28°F), first in fall, 50% chance before
October 1

The last 15 years (2006–2020 normals)

Typical last frost May 13 (9 days earlier than the 30-year normal), typical first fall frost September 26 (7 days later). Frost-free season 132 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 Stevensville: chance of a frost after this dateSpring, chance of a frost after the date: 10% June 13, 20% June 4, 30% May 30, 40% May 26, 50% May 22, 60% May 18, 70% May 14, 80% May 11, 90% May 7. Observed last spring frost in recent years: 2021 May 29, 2022 May 11, 2023 April 28, 2024 May 30, 2025 May 18.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1Jun 13May 22May 72021: May 292022: May 112023: April 282024: May 302025: May 18Fall at Stevensville: chance of a frost before this dateFall, chance of a frost before the date: 10% September 3, 20% September 8, 30% September 13, 40% September 16, 50% September 19, 60% September 22, 70% September 24, 80% September 28, 90% October 2. Observed first fall frost in recent years: 2021 September 17, 2022 October 9, 2023 October 19, 2024 October 3, 2025 September 26.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 3Sep 19Oct 22021: September 172022: October 92023: October 192024: October 32025: September 26
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 29September 17
2022May 11October 9
2023April 28October 19
2024May 30October 3
2025May 18September 26
2026incomplete recordseason not over yet

What these dates mean

The 115 days between the 50% dates in Stevensville—May 22 and September 19—frame the interval where a light frost has an even chance of occurring. For a gardener, the May 22 date marks the median point in spring, while the September 19 date is the median point in fall, each representing a 50 % chance of a frost later (spring) or earlier (fall).

In spring, the cautious date for setting out tender crops is June 13, which carries only a 10 % chance of a later frost; planting on May 7 would be a gamble with a 90 % chance of frost later. In fall, the cautious date to finish harvesting tender crops is September 3, with only a 10 % chance that a frost has arrived earlier; waiting until October 2 would be a gamble with a 90 % chance of an earlier frost.

A hard frost at 28 °F is more damaging than a light frost at 32 °F because it can kill plant tissue that survives just-above-freezing temperatures.

How to read the probabilities

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

  • Spring: "10% after June 13" 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 3" 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 Stevensville, MTNumbered nearest stations: 1 — Lindbergh Lake, MT — 64 mi; 2 — near Butte, MT — 85 mi; 3 — near Thompson Falls, MT — 96 mi; 4 — Boulder, MT — 96 mi; 5 — Helena, MT — 101 mi.MTID50 mi · 80 km1. Lindbergh Lake, MT — 64 mi12. near Butte, MT — 85 mi23. near Thompson Falls, MT — 96 mi34. Boulder, MT — 96 mi45. Helena, MT — 101 mi5
  1. Lindbergh Lake, MT 64 mi (104 km)
  2. near Butte, MT 85 mi (137 km)
  3. near Thompson Falls, MT 96 mi (154 km)
  4. Boulder, MT 96 mi (155 km)
  5. Helena, MT 101 mi (163 km)

Sources

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

Station coordinates: 46.5136, -114.0911.