U.S. Frost Dates by Weather Station

NOAA station USC00243013 · Montana

Frost Dates for Flatwillow, MT

NOAA weather station Flatwillow 4 ENE (USC00243013), Montana · elevation 3292 ft (1003 m) · hardiness zone 4b*

  • Lower-risk planting dateafter May 31only 1 spring in 10 still has a frost later
  • Typical last frostMay 1650% chance of a frost after it
  • Typical first fall frostSeptember 2850% chance of a frost before it
  • Frost-free season132 daystypical year, 32°F
  • Hardiness zone4bcalculated*

The year at a glance

Frost season at Flatwillow, MTSpring: a frost after May 3 in 9 years out of 10, after May 16 in half of the years, after May 31 in 1 year out of 10. Fall: the first frost comes before September 15 in 1 year out of 10, before September 28 in half of the years, before October 10 in 9 years out of 10. Typical frost-free season: 132 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 16First frost ~Sep 28Frost season at Flatwillow, MTSpring: a frost after May 3 in 9 years out of 10, after May 16 in half of the years, after May 31 in 1 year out of 10. Fall: the first frost comes before September 15 in 1 year out of 10, before September 28 in half of the years, before October 10 in 9 years out of 10. Typical frost-free season: 132 days.JFMAMJJASONDLast frost ~May 16First frost ~Sep 28
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.Today

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)May 31September 15
50% (typical)May 16September 28
90% (likely)May 3October 10
Frost-free season (32°F, 50% chance)
132 days
Hard frost (28°F), last in spring, 50% chance after
May 3
Hard frost (28°F), first in fall, 50% chance before
October 5

The last 15 years (2006–2020 normals)

Typical last frost May 15 (1 day earlier than the 30-year normal), typical first fall frost September 30 (2 days later). Frost-free season 135 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 Flatwillow: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 31, 20% May 25, 30% May 22, 40% May 18, 50% May 16, 60% May 13, 70% May 10, 80% May 7, 90% May 3. Observed last spring frost in recent years: 2021 May 22, 2022 May 26, 2023 May 2, 2024 May 20, 2025 May 22, 2026 May 19.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 31May 16May 32021: May 222022: May 262023: May 22024: May 202025: May 222026: May 19Fall at Flatwillow: chance of a frost before this dateFall, chance of a frost before the date: 10% September 15, 20% September 19, 30% September 23, 40% September 25, 50% September 28, 60% September 30, 70% October 2, 80% October 5, 90% October 10. Observed first fall frost in recent years: 2021 October 10, 2022 October 7, 2023 October 6, 2024 October 13, 2025 October 7.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 15Sep 28Oct 102021: October 102022: October 72023: October 62024: October 132025: October 7
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 22October 10
2022May 26October 7
2023May 2October 6
2024May 20October 13
2025May 22October 7
2026May 19season not over yet

What these dates mean

The 132 days between the two 32°F 50% dates in Flatwillow frame the period where light frosts are equally likely to occur. The 50% date is May 16, indicating a 50% chance of a frost later than this date in spring, and the 50% date is September 28, indicating a 50% chance of a frost earlier than this date in fall. Gardeners can use these benchmarks to gauge the central window for tender-crop activity.

The cautious spring date for setting out tender crops is May 31, with a 10% chance of a frost later, while the early-spring gamble date is May 3, which carries a 90% chance of a frost later. In fall, the cautious date to finish harvesting tender crops is September 15, with a 10% chance of a frost earlier, and the late-fall gamble date is October 10, which has a 90% chance of a frost earlier. A hard frost at 28°F is more damaging than a light frost at 32°F, so the 50% hard-frost dates—May 3 in spring and October 5 in fall—are key reference points.

How to read the probabilities

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

  • Spring: "10% after May 31" 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 Flatwillow, MTNumbered nearest stations: 1 — Grass Range, MT — 25 mi; 2 — Roundup, MT — 28 mi; 3 — Melstone, MT — 28 mi; 4 — Valentine, MT — 36 mi; 5 — Ryegate, MT — 50 mi.MT25 mi · 40 km1. Grass Range, MT — 25 mi12. Roundup, MT — 28 mi23. Melstone, MT — 28 mi34. Valentine, MT — 36 mi45. Ryegate, MT — 50 mi5
  1. Grass Range, MT 25 mi (40 km)
  2. Roundup, MT 28 mi (44 km)
  3. Melstone, MT 28 mi (45 km)
  4. Valentine, MT 36 mi (58 km)
  5. Ryegate, MT 50 mi (81 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: 46.8217, -108.3725.