U.S. Frost Dates by Weather Station

NOAA station USC00247620 · Montana

Frost Dates for Simpson 6 N - Wildhorse, MT

NOAA weather station Simpson 6 N - Wildhorse (USC00247620), Montana · elevation 2815 ft (858 m) · hardiness zone 3b*

  • Lower-risk planting dateafter June 8only 1 spring in 10 still has a frost later
  • Typical last frostMay 2650% chance of a frost after it
  • Typical first fall frostSeptember 1550% chance of a frost before it
  • Frost-free season110 daystypical year, 32°F
  • Hardiness zone3bcalculated*

The year at a glance

Frost season at Simpson 6 N - Wildhorse, MTSpring: a frost after May 12 in 9 years out of 10, after May 26 in half of the years, after June 8 in 1 year out of 10. Fall: the first frost comes before September 1 in 1 year out of 10, before September 15 in half of the years, before September 25 in 9 years out of 10. Typical frost-free season: 110 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 26First frost ~Sep 15Frost season at Simpson 6 N - Wildhorse, MTSpring: a frost after May 12 in 9 years out of 10, after May 26 in half of the years, after June 8 in 1 year out of 10. Fall: the first frost comes before September 1 in 1 year out of 10, before September 15 in half of the years, before September 25 in 9 years out of 10. Typical frost-free season: 110 days.JFMAMJJASONDLast frost ~May 26First 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 26 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)June 8September 1
50% (typical)May 26September 15
90% (likely)May 12September 25
Frost-free season (32°F, 50% chance)
110 days
Hard frost (28°F), last in spring, 50% chance after
May 14
Hard frost (28°F), first in fall, 50% chance before
September 23

The last 15 years (2006–2020 normals)

Typical last frost May 22 (4 days earlier than the 30-year normal), typical first fall frost September 14 (1 day earlier). Frost-free season 115 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 Simpson 6 N - Wildhorse: chance of a frost after this dateSpring, chance of a frost after the date: 10% June 8, 20% June 4, 30% May 31, 40% May 28, 50% May 26, 60% May 23, 70% May 20, 80% May 17, 90% May 12. Observed last spring frost in recent years: 2021 May 22, 2022 May 22, 2023 April 28, 2024 May 4, 2025 May 6, 2026 May 21.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1Jun 8May 26May 122021: May 222022: May 222023: April 282024: May 42025: May 62026: May 21Fall at Simpson 6 N - Wildhorse: chance of a frost before this dateFall, chance of a frost before the date: 10% September 1, 20% September 4, 30% September 9, 40% September 12, 50% September 15, 60% September 17, 70% September 19, 80% September 22, 90% September 25. Observed first fall frost in recent years: 2021 September 16, 2022 September 10, 2023 September 29, 2024 October 1, 2025 October 6.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 1Sep 15Sep 252021: September 162022: September 102023: September 292024: October 12025: 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 22September 16
2022May 22September 10
2023April 28September 29
2024May 4October 1
2025May 6October 6
2026May 21season not over yet

What these dates mean

The 110 days between the 32°F 50% dates of May 26 and September 15 in Simpson 6 N – Wildhorse frame the window when a light frost is no more likely than not, giving gardeners a period with balanced frost risk. The 50% dates themselves mark the point at which the probability of a frost later (in spring) or earlier (in fall) reaches an even split, rather than a guaranteed frost event.

For a cautious planting plan, the spring cautious date is June 8, when there is only a 10% chance of a light frost later, while the fall cautious date is September 1, with only a 10% chance that a light frost has arrived earlier. A hard frost, recorded at 28 °F, matters more than a light frost because temperatures that low can cause more severe damage to tender plant tissue, and its 50% dates are May 14 in spring and September 23 in fall.

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 1" 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 Simpson 6 N - Wildhorse, MTNumbered nearest stations: 1 — Gildford, MT — 30 mi; 2 — Havre, MT — 38 mi; 3 — Fort Assiniboine, MT — 39 mi; 4 — Chinook, MT — 53 mi; 5 — Loma, MT — 75 mi.25 mi · 40 km1. Gildford, MT — 30 mi12. Havre, MT — 38 mi23. Fort Assiniboine, MT — 39 mi34. Chinook, MT — 53 mi45. Loma, MT — 75 mi5
  1. Gildford, MT 30 mi (49 km)
  2. Havre, MT 38 mi (61 km)
  3. Fort Assiniboine, MT 39 mi (63 km)
  4. Chinook, MT 53 mi (86 km)
  5. Loma, MT 75 mi (120 km)

Sources

* Hardiness zone 3b: zone derived from PRISM 2023 grid at the station location (not the official USDA map). Source data: PRISM Group, Oregon State University.

Station coordinates: 48.9981, -110.2158.