U.S. Frost Dates by Weather Station

NOAA station USC00244358 · Montana

Frost Dates for Hysham, MT

NOAA weather station Hysham (USC00244358), Montana · elevation 2669 ft (814 m) · hardiness zone 4b*

  • Lower-risk planting dateafter May 22only 1 spring in 10 still has a frost later
  • Typical last frostMay 1050% chance of a frost after it
  • Typical first fall frostSeptember 2950% chance of a frost before it
  • Frost-free season141 daystypical year, 32°F
  • Hardiness zone4bcalculated*

The year at a glance

Frost season at Hysham, MTSpring: a frost after April 28 in 9 years out of 10, after May 10 in half of the years, after May 22 in 1 year out of 10. Fall: the first frost comes before September 15 in 1 year out of 10, before September 29 in half of the years, before October 11 in 9 years out of 10. Typical frost-free season: 141 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 10First frost ~Sep 29Frost season at Hysham, MTSpring: a frost after April 28 in 9 years out of 10, after May 10 in half of the years, after May 22 in 1 year out of 10. Fall: the first frost comes before September 15 in 1 year out of 10, before September 29 in half of the years, before October 11 in 9 years out of 10. Typical frost-free season: 141 days.JFMAMJJASONDLast frost ~May 10First 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 22September 15
50% (typical)May 10September 29
90% (likely)April 28October 11
Frost-free season (32°F, 50% chance)
141 days
Hard frost (28°F), last in spring, 50% chance after
April 30
Hard frost (28°F), first in fall, 50% chance before
October 7

The last 15 years (2006–2020 normals)

Typical last frost May 10 (same as the 30-year normal), typical first fall frost October 1 (2 days later). Frost-free season 143 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: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 22, 20% May 18, 30% May 15, 40% May 12, 50% May 10, 60% May 7, 70% May 5, 80% May 1, 90% April 28. Observed last spring frost in recent years: 2021 May 11, 2023 April 25, 2025 April 24, 2026 May 20.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 22May 10Apr 282021: May 112023: April 252025: April 242026: May 20Fall at Hysham: chance of a frost before this dateFall, chance of a frost before the date: 10% September 15, 20% September 21, 30% September 24, 40% September 27, 50% September 29, 60% October 1, 70% October 4, 80% October 6, 90% October 11. Observed first fall frost in recent years: 2021 October 12, 2022 October 17, 2023 October 7, 2024 October 14, 2025 October 15.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 15Sep 29Oct 112021: October 122022: October 172023: October 72024: October 142025: October 15
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
2022incomplete recordOctober 17
2023April 25October 7
2024incomplete recordOctober 14
2025April 24October 15
2026May 20season not over yet

What these dates mean

The cautious fall date in Hysham for finishing the harvest is September 15, when there is only a 10 % chance that a light frost has already arrived.

For a gardener, the 50% light-frost dates—May 10 in spring and September 29 in fall—mark the point where a frost is equally likely to occur later (spring) or earlier (fall). The median hard-frost dates are April 30 and October 7; a hard frost at 28 °F can damage plants more severely than a light frost at 32 °F.

A cautious plan therefore uses the May 22 spring date, which carries only a 10 % chance of a later light frost, for setting out tender crops, and the September 15 fall date for completing the harvest, while the April 28 (90 % chance) and October 11 (90 % chance) dates remain gambles.

How to read the probabilities

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

  • Spring: "10% after May 22" 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 15" 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, MTNumbered nearest stations: 1 — Ingomar, MT — 19 mi; 2 — Hysham 25 SSE, MT — 25 mi; 3 — Forsyth, MT — 27 mi; 4 — Melstone, MT — 37 mi; 5 — Hardin, MT — 43 mi.25 mi · 40 km1. Ingomar, MT — 19 mi12. Hysham 25 SSE, MT — 25 mi23. Forsyth, MT — 27 mi34. Melstone, MT — 37 mi45. Hardin, MT — 43 mi5
  1. Ingomar, MT 19 mi (31 km)
  2. Hysham 25 SSE, MT 25 mi (40 km)
  3. Forsyth, MT 27 mi (43 km)
  4. Melstone, MT 37 mi (60 km)
  5. Hardin, MT 43 mi (69 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: 46.2897, -107.2306.