U.S. Frost Dates by Weather Station

NOAA station USC00244364 · Montana

Frost Dates for Hysham 25 SSE, MT

NOAA weather station Hysham 25 SSE (USC00244364), Montana · elevation 3100 ft (945 m) · hardiness zone 4a*

  • Lower-risk planting dateafter June 13only 1 spring in 10 still has a frost later
  • Typical last frostMay 2650% chance of a frost after it
  • Typical first fall frostSeptember 1650% chance of a frost before it
  • Frost-free season111 daystypical year, 32°F
  • Hardiness zone4acalculated*

The year at a glance

Frost season at Hysham 25 SSE, MTSpring: a frost after May 12 in 9 years out of 10, after May 26 in half of the years, after June 13 in 1 year out of 10. Fall: the first frost comes before September 1 in 1 year out of 10, before September 16 in half of the years, before October 1 in 9 years out of 10. Typical frost-free season: 111 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 26First frost ~Sep 16Frost season at Hysham 25 SSE, MTSpring: a frost after May 12 in 9 years out of 10, after May 26 in half of the years, after June 13 in 1 year out of 10. Fall: the first frost comes before September 1 in 1 year out of 10, before September 16 in half of the years, before October 1 in 9 years out of 10. Typical frost-free season: 111 days.JFMAMJJASONDLast frost ~May 26First frost ~Sep 16
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 25 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)June 13September 1
50% (typical)May 26September 16
90% (likely)May 12October 1
Frost-free season (32°F, 50% chance)
111 days
Hard frost (28°F), last in spring, 50% chance after
May 14
Hard frost (28°F), first in fall, 50% chance before
September 27

The last 15 years (2006–2020 normals)

Typical last frost May 25 (1 day earlier than the 30-year normal), typical first fall frost September 17 (1 day later). Frost-free season 113 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 Hysham 25 SSE: chance of a frost after this dateSpring, chance of a frost after the date: 10% June 13, 20% June 5, 30% June 1, 40% May 29, 50% May 26, 60% May 23, 70% May 20, 80% May 17, 90% May 12. Observed last spring frost in recent years: 2022 May 24, 2023 May 19, 2024 June 19, 2025 May 22, 2026 May 20.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1Jun 13May 26May 122022: May 242023: May 192024: June 192025: May 222026: May 20Fall at Hysham 25 SSE: chance of a frost before this dateFall, chance of a frost before the date: 10% September 1, 20% September 6, 30% September 9, 40% September 13, 50% September 16, 60% September 19, 70% September 22, 80% September 26, 90% October 1. Observed first fall frost in recent years: 2022 October 7, 2023 October 6, 2024 October 1, 2025 September 5.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 1Sep 16Oct 12022: October 72023: October 62024: October 12025: 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
2021incomplete recordincomplete record
2022May 24October 7
2023May 19October 6
2024June 19October 1
2025May 22September 5
2026May 20season not over yet

What these dates mean

June 13 is the cautious spring light-frost date, with a 10% chance of frost later than that day, and it lets a gardener schedule setting out tender crops with minimal risk.

The 50% date is May 26, indicating a 50% chance of a frost later than this date, while the 50% date is September 16, indicating a 50% chance of a frost earlier than this date. Hard-frost median dates are May 14 in spring and September 27 in fall; a hard frost at 28 °F can injure plants that survive a light frost at 32 °F. September 1 is the cautious fall light-frost date, with a 10% chance of frost earlier than that day, and it guides the latest time to finish harvesting tender crops. The 25-year record supplies a solid statistical foundation for these probabilities.

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 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 Hysham 25 SSE, MTNumbered nearest stations: 1 — Hysham, MT — 25 mi; 2 — Hardin, MT — 27 mi; 3 — Busby, MT — 29 mi; 4 — Forsyth, MT — 32 mi; 5 — Ingomar, MT — 44 mi.25 mi · 40 km1. Hysham, MT — 25 mi12. Hardin, MT — 27 mi23. Busby, MT — 29 mi34. Forsyth, MT — 32 mi45. Ingomar, MT — 44 mi5
  1. Hysham, MT 25 mi (40 km)
  2. Hardin, MT 27 mi (43 km)
  3. Busby, MT 29 mi (46 km)
  4. Forsyth, MT 32 mi (52 km)
  5. Ingomar, MT 44 mi (70 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: 45.9353, -107.1375.