U.S. Frost Dates by Weather Station

NOAA station USC00243176 · Montana

Frost Dates for Fort Peck Power Plant, MT

NOAA weather station Fort Peck Power Plant (USC00243176), Montana · elevation 2071 ft (631 m) · hardiness zone 4a*

  • Lower-risk planting dateafter May 20only 1 spring in 10 still has a frost later
  • Typical last frostMay 850% chance of a frost after it
  • Typical first fall frostOctober 150% chance of a frost before it
  • Frost-free season145 daystypical year, 32°F
  • Hardiness zone4acalculated*

The year at a glance

Frost season at Fort Peck Power Plant, MTSpring: a frost after April 25 in 9 years out of 10, after May 8 in half of the years, after May 20 in 1 year out of 10. Fall: the first frost comes before September 18 in 1 year out of 10, before October 1 in half of the years, before October 16 in 9 years out of 10. Typical frost-free season: 145 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 8First frost ~Oct 1Frost season at Fort Peck Power Plant, MTSpring: a frost after April 25 in 9 years out of 10, after May 8 in half of the years, after May 20 in 1 year out of 10. Fall: the first frost comes before September 18 in 1 year out of 10, before October 1 in half of the years, before October 16 in 9 years out of 10. Typical frost-free season: 145 days.JFMAMJJASONDLast frost ~May 8First 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 30 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 20September 18
50% (typical)May 8October 1
90% (likely)April 25October 16
Frost-free season (32°F, 50% chance)
145 days
Hard frost (28°F), last in spring, 50% chance after
April 28
Hard frost (28°F), first in fall, 50% chance before
October 10

The last 15 years (2006–2020 normals)

Typical last frost May 6 (2 days earlier than the 30-year normal), typical first fall frost October 5 (4 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 Fort Peck Power Plant: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 20, 20% May 15, 30% May 13, 40% May 10, 50% May 8, 60% May 5, 70% May 3, 80% April 30, 90% April 25. Observed last spring frost in recent years: 2021 May 12, 2022 April 26, 2023 April 28, 2024 May 4, 2025 April 23, 2026 May 6.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 20May 8Apr 252021: May 122022: April 262023: April 282024: May 42025: April 232026: May 6Fall at Fort Peck Power Plant: chance of a frost before this dateFall, chance of a frost before the date: 10% September 18, 20% September 23, 30% September 26, 40% September 29, 50% October 1, 60% October 4, 70% October 7, 80% October 10, 90% October 16. Observed first fall frost in recent years: 2021 October 13, 2023 October 6, 2024 October 13, 2025 October 6.Fall: first frostChance of a frost before this date10%50%90%Oct 1Sep 18Oct 1Oct 162021: October 132023: October 62024: October 132025: 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 12October 13
2022April 26incomplete record
2023April 28October 6
2024May 4October 13
2025April 23October 6
2026May 6season not over yet

What these dates mean

May 20 is the cautious spring date, giving a gardener only a 10 % chance that a light frost will occur later and allowing the setting out of tender crops with confidence. The 50 % date is May 8, indicating an even chance of a light frost occurring, while April 25 carries a 90 % chance and is a gamble for early planting.

In the fall, September 18 serves as the cautious date, providing a 10 % chance that a light frost has already arrived and letting the gardener plan the final harvest of tender crops. The 50 % date is October 1, showing an even chance of a light frost occurring, and October 16 has a 90 % chance, making late-season planting risky.

The hard-frost 50 % dates are April 28 in spring and October 10 in fall; a hard frost at 28 °F can damage plants more severely than a light frost at 32 °F, so these dates are especially important to watch.

How to read the probabilities

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

  • Spring: "10% after May 20" 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 18" 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 Fort Peck Power Plant, MTNumbered nearest stations: 1 — Glasgow #2, MT — 16 mi; 2 — Glasgow, MT — 17 mi; 3 — Opheim, MT — 47 mi; 4 — Vida, MT — 49 mi; 5 — Content, MT — 52 mi.MT25 mi · 40 km1. Glasgow #2, MT — 16 mi12. Glasgow, MT — 17 mi23. Opheim, MT — 47 mi34. Vida, MT — 49 mi45. Content, MT — 52 mi5
  1. Glasgow #2, MT 16 mi (26 km)
  2. Glasgow, MT 17 mi (27 km)
  3. Opheim, MT 47 mi (76 km)
  4. Vida, MT 49 mi (79 km)
  5. Content, MT 52 mi (83 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: 48.0119, -106.4117.