U.S. Frost Dates by Weather Station

NOAA station USC00248777 · Montana

Frost Dates for Westby, MT

NOAA weather station Westby (USC00248777), Montana · elevation 2103 ft (641 m) · hardiness zone 3b*

  • Lower-risk planting dateafter May 30only 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 season125 daystypical year, 32°F
  • Hardiness zone3bcalculated*

The year at a glance

Frost season at Westby, MTSpring: a frost after May 6 in 9 years out of 10, after May 18 in half of the years, after May 30 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 1 in 9 years out of 10. Typical frost-free season: 125 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 18First frost ~Sep 21Frost season at Westby, MTSpring: a frost after May 6 in 9 years out of 10, after May 18 in half of the years, after May 30 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 1 in 9 years out of 10. Typical frost-free season: 125 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 27 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 30September 7
50% (typical)May 18September 21
90% (likely)May 6October 1
Frost-free season (32°F, 50% chance)
125 days
Hard frost (28°F), last in spring, 50% chance after
May 8
Hard frost (28°F), first in fall, 50% chance before
September 28

The last 15 years (2006–2020 normals)

Typical last frost May 16 (2 days earlier than the 30-year normal), typical first fall frost September 22 (1 day later). Frost-free season 125 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 Westby: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 30, 20% May 26, 30% May 23, 40% May 20, 50% May 18, 60% May 15, 70% May 13, 80% May 11, 90% May 6. Observed last spring frost in recent years: 2021 May 11, 2023 May 20, 2024 May 2, 2025 May 6, 2026 May 19.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 30May 18May 62021: May 112023: May 202024: May 22025: May 62026: May 19Fall at Westby: chance of a frost before this dateFall, chance of a frost before the date: 10% September 7, 20% September 13, 30% September 16, 40% September 19, 50% September 21, 60% September 23, 70% September 25, 80% September 27, 90% October 1. Observed first fall frost in recent years: 2021 September 24, 2024 October 1, 2025 October 6.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 7Sep 21Oct 12021: September 242024: 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 11September 24
2022incomplete recordincomplete record
2023May 20incomplete record
2024May 2October 1
2025May 6October 6
2026May 19season not over yet

What these dates mean

The cautious fall date for finishing the harvest in Westby is September 7, when there is only a 10% chance that a light frost has arrived earlier.

In spring, the cautious date for setting out tender crops is May 30, because only a 10% chance of a light frost remains after that day; planting on May 6 carries a 90% chance of frost and is a gamble. The 50% light-frost dates are May 18 in spring and September 21 in fall, each representing an even-odds point for a frost. The 50% hard-frost dates are May 8 and September 28; a hard frost matters more than a light frost because temperatures of 28 °F can damage plant tissue that survives 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 30" 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 Westby, MTNumbered nearest stations: 1 — Fortuna, ND — 11 mi; 2 — Crosby, ND — 34 mi; 3 — Williston (Sloulin Airport), ND — 52 mi; 4 — near Williston, ND — 53 mi; 5 — Culbertson, MT — 54 mi.NDMT25 mi · 40 km1. Fortuna, ND — 11 mi12. Crosby, ND — 34 mi23. Williston (Sloulin Airport), ND — 52 mi34. near Williston, ND — 53 mi45. Culbertson, MT — 54 mi5
  1. Fortuna, ND 11 mi (18 km)
  2. Crosby, ND 34 mi (55 km)
  3. Williston (Sloulin Airport), ND 52 mi (84 km)
  4. near Williston, ND 53 mi (85 km)
  5. Culbertson, MT 54 mi (87 km)

Sources

* Hardiness zone 3b: zone derived from PRISM 2023 grid at the station location (not the official USDA map). Source data: PRISM Group, Oregon State University.

Station coordinates: 48.8744, -104.0503.