U.S. Frost Dates by Weather Station

NOAA station USC00243557 · Montana

Frost Dates for Glasgow #2, MT

NOAA weather station Glasgow #2 (USC00243557), Montana · elevation 2090 ft (637 m) · hardiness zone 4a*

  • Lower-risk planting dateafter May 15only 1 spring in 10 still has a frost later
  • Typical last frostMay 350% chance of a frost after it
  • Typical first fall frostOctober 150% chance of a frost before it
  • Frost-free season151 daystypical year, 32°F
  • Hardiness zone4acalculated*

The year at a glance

Frost season at Glasgow #2, MTSpring: a frost after April 19 in 9 years out of 10, after May 3 in half of the years, after May 15 in 1 year out of 10. Fall: the first frost comes before September 19 in 1 year out of 10, before October 1 in half of the years, before October 15 in 9 years out of 10. Typical frost-free season: 151 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 3First frost ~Oct 1Frost season at Glasgow #2, MTSpring: a frost after April 19 in 9 years out of 10, after May 3 in half of the years, after May 15 in 1 year out of 10. Fall: the first frost comes before September 19 in 1 year out of 10, before October 1 in half of the years, before October 15 in 9 years out of 10. Typical frost-free season: 151 days.JFMAMJJASONDLast frost ~May 3First frost ~Oct 1
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 26 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 15September 19
50% (typical)May 3October 1
90% (likely)April 19October 15
Frost-free season (32°F, 50% chance)
151 days
Hard frost (28°F), last in spring, 50% chance after
April 23
Hard frost (28°F), first in fall, 50% chance before
October 11

The last 15 years (2006–2020 normals)

Typical last frost May 4 (1 day later than the 30-year normal), typical first fall frost October 5 (4 days later). Frost-free season 151 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 #2: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 15, 20% May 11, 30% May 8, 40% May 6, 50% May 3, 60% May 1, 70% April 29, 80% April 25, 90% April 19. Observed last spring frost in recent years: 2021 May 10, 2022 May 22, 2023 April 23, 2024 May 4.Spring: last frostChance of a frost after this date10%50%90%May 1May 15May 3Apr 192021: May 102022: May 222023: April 232024: May 4Fall at Glasgow #2: chance of a frost before this dateFall, chance of a frost before the date: 10% September 19, 20% September 23, 30% September 26, 40% September 28, 50% October 1, 60% October 4, 70% October 6, 80% October 10, 90% October 15. Observed first fall frost in recent years: 2021 October 14, 2022 October 7, 2023 October 6, 2024 October 13.Fall: first frostChance of a frost before this date10%50%90%Oct 1Sep 19Oct 1Oct 152021: October 142022: October 72023: October 62024: October 13
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
2025no datano data
2026no dataseason not over yet

What these dates mean

May 15 is the cautious spring light-frost date, giving a gardener a ten-percent chance that a frost will occur later and allowing tender crops to be set out. The 50% light-frost date is May 3, indicating an even-odds chance of a later frost, while the 50% hard-frost date in spring is April 23.

September 19 serves as the cautious fall light-frost date, carrying a ten-percent chance that a frost has already arrived and marking the time to finish harvesting tender crops. The 50% fall light-frost date is October 1, meaning an even-odds chance of an earlier frost, and the 50% hard-frost date in fall is October 11. A hard frost at 28 °F can injure plants that survive a 32 °F light frost, making it a more severe 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 15" 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 19" 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 #2, MTNumbered nearest stations: 1 — Glasgow, MT — 2 mi; 2 — Fort Peck Power Plant, MT — 16 mi; 3 — Opheim, MT — 38 mi; 4 — Saco, MT — 38 mi; 5 — Content, MT — 42 mi.MT25 mi · 40 km1. Glasgow, MT — 2 mi12. Fort Peck Power Plant, MT — 16 mi23. Opheim, MT — 38 mi34. Saco, MT — 38 mi45. Content, MT — 42 mi5
  1. Glasgow, MT 2 mi (3 km)
  2. Fort Peck Power Plant, MT 16 mi (26 km)
  3. Opheim, MT 38 mi (61 km)
  4. Saco, MT 38 mi (61 km)
  5. Content, MT 42 mi (67 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: 48.1925, -106.6383.