U.S. Frost Dates by Weather Station

NOAA station USC00248498 · Montana

Frost Dates for Valentine, MT

NOAA weather station Valentine (USC00248498), Montana · elevation 2905 ft (885 m) · hardiness zone 4a*

  • Lower-risk planting dateafter May 31only 1 spring in 10 still has a frost later
  • Typical last frostMay 1850% chance of a frost after it
  • Typical first fall frostSeptember 2150% chance of a frost before it
  • Frost-free season124 daystypical year, 32°F
  • Hardiness zone4acalculated*

The year at a glance

Frost season at Valentine, MTSpring: a frost after May 7 in 9 years out of 10, after May 18 in half of the years, after May 31 in 1 year out of 10. Fall: the first frost comes before September 7 in 1 year out of 10, before September 21 in half of the years, before October 2 in 9 years out of 10. Typical frost-free season: 124 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 18First frost ~Sep 21Frost season at Valentine, MTSpring: a frost after May 7 in 9 years out of 10, after May 18 in half of the years, after May 31 in 1 year out of 10. Fall: the first frost comes before September 7 in 1 year out of 10, before September 21 in half of the years, before October 2 in 9 years out of 10. Typical frost-free season: 124 days.JFMAMJJASONDLast frost ~May 18First frost ~Sep 21
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 29 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 31September 7
50% (typical)May 18September 21
90% (likely)May 7October 2
Frost-free season (32°F, 50% chance)
124 days
Hard frost (28°F), last in spring, 50% chance after
May 7
Hard frost (28°F), first in fall, 50% chance before
September 29

The last 15 years (2006–2020 normals)

Typical last frost May 17 (1 day earlier than the 30-year normal), typical first fall frost September 24 (3 days later). Frost-free season 129 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 Valentine: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 31, 20% May 26, 30% May 23, 40% May 20, 50% May 18, 60% May 16, 70% May 13, 80% May 10, 90% May 7. Observed last spring frost in recent years: 2021 May 22, 2022 June 1, 2023 April 29, 2025 May 22, 2026 May 22.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 31May 18May 72021: May 222022: June 12023: April 292025: May 222026: May 22Fall at Valentine: chance of a frost before this dateFall, chance of a frost before the date: 10% September 7, 20% September 12, 30% September 16, 40% September 19, 50% September 21, 60% September 23, 70% September 25, 80% September 28, 90% October 2. Observed first fall frost in recent years: 2021 September 17, 2022 September 10, 2023 October 6, 2024 October 1, 2025 October 6.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 7Sep 21Oct 22021: September 172022: September 102023: October 62024: 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 17
2022June 1September 10
2023April 29October 6
2024incomplete recordOctober 1
2025May 22October 6
2026May 22season not over yet

What these dates mean

Setting out tender crops on the early spring date, May 7, carries a 90 % chance of a light frost later, making it a gamble for growers in Valentine.

A cautious plan would wait until the late spring date, May 31, where only a 10 % chance of a later light frost remains. In the fall, the cautious date to finish tender crops is September 7, which has a 10 % chance of an earlier light frost. The 50 % dates—May 18 in spring and September 21 in fall—mark the point where a light frost is equally likely to occur before or after, giving gardeners a midpoint reference. The 50 % hard-frost dates are May 7 and September 29, and a hard frost can damage plants that survive a light frost, so it is a more critical threshold.

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 7" 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 Valentine, MTNumbered nearest stations: 1 — Grass Range, MT — 26 mi; 2 — Flatwillow, MT — 36 mi; 3 — Malta, MT — 43 mi; 4 — Lewistown, MT — 49 mi; 5 — Melstone, MT — 59 mi.25 mi · 40 km1. Grass Range, MT — 26 mi12. Flatwillow, MT — 36 mi23. Malta, MT — 43 mi34. Lewistown, MT — 49 mi45. Melstone, MT — 59 mi5
  1. Grass Range, MT 26 mi (42 km)
  2. Flatwillow, MT 36 mi (58 km)
  3. Malta, MT 43 mi (69 km)
  4. Lewistown, MT 49 mi (79 km)
  5. Melstone, MT 59 mi (95 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: 47.3392, -108.4992.