U.S. Frost Dates by Weather Station

NOAA station USW00094008 · Montana

Frost Dates for Glasgow, MT

NOAA weather station Glasgow International Airport (USW00094008), Montana · elevation 2285 ft (697 m) · hardiness zone 4a*

  • Lower-risk planting dateafter May 18only 1 spring in 10 still has a frost later
  • Typical last frostMay 650% chance of a frost after it
  • Typical first fall frostSeptember 3050% chance of a frost before it
  • Frost-free season145 daystypical year, 32°F
  • Hardiness zone4acalculated*

The year at a glance

Frost season at Glasgow, MTSpring: a frost after April 24 in 9 years out of 10, after May 6 in half of the years, after May 18 in 1 year out of 10. Fall: the first frost comes before September 17 in 1 year out of 10, before September 30 in half of the years, before October 13 in 9 years out of 10. Typical frost-free season: 145 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 6First frost ~Sep 30Frost season at Glasgow, MTSpring: a frost after April 24 in 9 years out of 10, after May 6 in half of the years, after May 18 in 1 year out of 10. Fall: the first frost comes before September 17 in 1 year out of 10, before September 30 in half of the years, before October 13 in 9 years out of 10. Typical frost-free season: 145 days.JFMAMJJASONDLast frost ~May 6First frost ~Sep 30
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 18September 17
50% (typical)May 6September 30
90% (likely)April 24October 13
Frost-free season (32°F, 50% chance)
145 days
Hard frost (28°F), last in spring, 50% chance after
April 26
Hard frost (28°F), first in fall, 50% chance before
October 9

The last 15 years (2006–2020 normals)

Typical last frost May 6 (same as the 30-year normal), typical first fall frost October 3 (3 days later). Frost-free season 148 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 Glasgow: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 18, 20% May 14, 30% May 11, 40% May 9, 50% May 6, 60% May 4, 70% May 1, 80% April 29, 90% April 24. Observed last spring frost in recent years: 2021 May 10, 2022 May 22, 2023 April 23, 2024 May 4, 2025 April 25, 2026 May 19.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 18May 6Apr 242021: May 102022: May 222023: April 232024: May 42025: April 252026: May 19Fall at Glasgow: chance of a frost before this dateFall, chance of a frost before the date: 10% September 17, 20% September 22, 30% September 25, 40% September 28, 50% September 30, 60% October 3, 70% October 5, 80% October 9, 90% October 13. Observed first fall frost in recent years: 2021 October 14, 2022 October 7, 2023 October 6, 2024 October 13, 2025 October 5.Fall: first frostChance of a frost before this date10%50%90%Oct 1Sep 17Sep 30Oct 132021: October 142022: October 72023: October 62024: October 132025: October 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
2021May 10October 14
2022May 22October 7
2023April 23October 6
2024May 4October 13
2025April 25October 5
2026May 19season not over yet

What these dates mean

The 145-day span between the 32°F 50% dates of May 6 and September 30 defines the period in which a light frost is as likely to arrive as to stay away, giving Glasgow’s gardeners a window for tender crops.

In spring the cautious date for setting out tender crops is the later May 18, which carries only a 10 % chance of a frost later. Planting on April 24 would be a gamble with a 90 % chance of a frost later. In fall the cautious date to finish harvesting tender crops is the early September 17, with only a 10 % chance that a frost has already come; waiting until October 13 would be a gamble with a 90 % chance a frost has arrived.

A hard frost, indicated by the 28°F 50% dates of April 26 and October 9, can damage plants more severely than a light frost at 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 18" 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 17" 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 Glasgow, MTNumbered nearest stations: 1 — Glasgow #2, MT — 2 mi; 2 — Fort Peck Power Plant, MT — 17 mi; 3 — Opheim, MT — 36 mi; 4 — Saco, MT — 38 mi; 5 — Content, MT — 43 mi.MT25 mi · 40 km1. Glasgow #2, MT — 2 mi12. Fort Peck Power Plant, MT — 17 mi23. Opheim, MT — 36 mi34. Saco, MT — 38 mi45. Content, MT — 43 mi5
  1. Glasgow #2, MT 2 mi (3 km)
  2. Fort Peck Power Plant, MT 17 mi (27 km)
  3. Opheim, MT 36 mi (58 km)
  4. Saco, MT 38 mi (61 km)
  5. Content, MT 43 mi (69 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: 48.2138, -106.6213.