U.S. Frost Dates by Weather Station

NOAA station USC00243098 · Montana

Frost Dates for Forsyth, MT

NOAA weather station Forsyth (USC00243098), Montana · elevation 2515 ft (767 m) · hardiness zone 4b*

  • Lower-risk planting dateafter May 21only 1 spring in 10 still has a frost later
  • Typical last frostMay 950% chance of a frost after it
  • Typical first fall frostSeptember 2950% chance of a frost before it
  • Frost-free season142 daystypical year, 32°F
  • Hardiness zone4bcalculated*

The year at a glance

Frost season at Forsyth, MTSpring: a frost after April 28 in 9 years out of 10, after May 9 in half of the years, after May 21 in 1 year out of 10. Fall: the first frost comes before September 16 in 1 year out of 10, before September 29 in half of the years, before October 10 in 9 years out of 10. Typical frost-free season: 142 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 9First frost ~Sep 29Frost season at Forsyth, MTSpring: a frost after April 28 in 9 years out of 10, after May 9 in half of the years, after May 21 in 1 year out of 10. Fall: the first frost comes before September 16 in 1 year out of 10, before September 29 in half of the years, before October 10 in 9 years out of 10. Typical frost-free season: 142 days.JFMAMJJASONDLast frost ~May 9First frost ~Sep 29
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)May 21September 16
50% (typical)May 9September 29
90% (likely)April 28October 10
Frost-free season (32°F, 50% chance)
142 days
Hard frost (28°F), last in spring, 50% chance after
April 29
Hard frost (28°F), first in fall, 50% chance before
October 8

The last 15 years (2006–2020 normals)

Typical last frost May 8 (1 day earlier than the 30-year normal), typical first fall frost October 3 (4 days later). Frost-free season 146 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 Forsyth: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 21, 20% May 17, 30% May 14, 40% May 12, 50% May 9, 60% May 6, 70% May 4, 80% May 1, 90% April 28. Observed last spring frost in recent years: 2021 May 11, 2022 May 22, 2023 April 30, 2024 May 4, 2025 May 2, 2026 May 19.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 21May 9Apr 282021: May 112022: May 222023: April 302024: May 42025: May 22026: May 19Fall at Forsyth: chance of a frost before this dateFall, chance of a frost before the date: 10% September 16, 20% September 21, 30% September 24, 40% September 26, 50% September 29, 60% October 1, 70% October 4, 80% October 7, 90% October 10. Observed first fall frost in recent years: 2021 October 12, 2022 October 16, 2023 October 6, 2024 October 1, 2025 October 12.Fall: first frostChance of a frost before this date10%50%90%Oct 1Sep 16Sep 29Oct 102021: October 122022: October 162023: October 62024: October 12025: October 12
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 11October 12
2022May 22October 16
2023April 30October 6
2024May 4October 1
2025May 2October 12
2026May 19season not over yet

What these dates mean

The 142 days between the 32°F 50% dates in Forsyth, Montana, bracket the window when a light frost is as likely as not. The 50% spring date is May 9 and the 50% fall date is September 29; these dates are not frost dates but simply the median points in the 30-year record where the chance of a light frost is 50 %.

For a cautious planting plan, the spring date to set out tender crops is May 21, when there is only a 10 % chance of a frost later, while the fall date to finish harvesting tender crops is September 16, when there is only a 10 % chance of a frost earlier. The hard-frost median dates are April 29 in spring and October 8 in fall, and a hard frost at 28 °F can damage plants that 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 May 21" 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 16" 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 Forsyth, MTNumbered nearest stations: 1 — Hysham, MT — 27 mi; 2 — Ingomar, MT — 32 mi; 3 — Hysham 25 SSE, MT — 32 mi; 4 — Brandenberg, MT — 38 mi; 5 — near Miles City, MT — 39 mi.25 mi · 40 km1. Hysham, MT — 27 mi12. Ingomar, MT — 32 mi23. Hysham 25 SSE, MT — 32 mi34. Brandenberg, MT — 38 mi45. near Miles City, MT — 39 mi5
  1. Hysham, MT 27 mi (43 km)
  2. Ingomar, MT 32 mi (52 km)
  3. Hysham 25 SSE, MT 32 mi (52 km)
  4. Brandenberg, MT 38 mi (61 km)
  5. near Miles City, MT 39 mi (63 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: 46.2722, -106.6756.