U.S. Frost Dates by Weather Station

NOAA station USC00241169 · Montana

Frost Dates for Brockway, MT

NOAA weather station Brockway 3 WSW (USC00241169), Montana · elevation 2630 ft (802 m) · hardiness zone 4a*

  • Lower-risk planting dateafter June 5only 1 spring in 10 still has a frost later
  • Typical last frostMay 2050% chance of a frost after it
  • Typical first fall frostSeptember 1950% chance of a frost before it
  • Frost-free season121 daystypical year, 32°F
  • Hardiness zone4acalculated*

The year at a glance

Frost season at Brockway, MTSpring: a frost after May 6 in 9 years out of 10, after May 20 in half of the years, after June 5 in 1 year out of 10. Fall: the first frost comes before September 4 in 1 year out of 10, before September 19 in half of the years, before October 2 in 9 years out of 10. Typical frost-free season: 121 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 20First frost ~Sep 19Frost season at Brockway, MTSpring: a frost after May 6 in 9 years out of 10, after May 20 in half of the years, after June 5 in 1 year out of 10. Fall: the first frost comes before September 4 in 1 year out of 10, before September 19 in half of the years, before October 2 in 9 years out of 10. Typical frost-free season: 121 days.JFMAMJJASONDLast frost ~May 20First frost ~Sep 19
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.Today

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)June 5September 4
50% (typical)May 20September 19
90% (likely)May 6October 2
Frost-free season (32°F, 50% chance)
121 days
Hard frost (28°F), last in spring, 50% chance after
May 9
Hard frost (28°F), first in fall, 50% chance before
September 28

The last 15 years (2006–2020 normals)

Typical last frost May 18 (2 days earlier than the 30-year normal), typical first fall frost September 20 (1 day later). Frost-free season 124 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 Brockway: chance of a frost after this dateSpring, chance of a frost after the date: 10% June 5, 20% May 30, 30% May 25, 40% May 23, 50% May 20, 60% May 17, 70% May 14, 80% May 11, 90% May 6. Observed last spring frost in recent years: 2021 May 15, 2022 June 3, 2023 May 19, 2024 May 24, 2025 May 2.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1Jun 5May 20May 62021: May 152022: June 32023: May 192024: May 242025: May 2Fall at Brockway: chance of a frost before this dateFall, chance of a frost before the date: 10% September 4, 20% September 9, 30% September 13, 40% September 16, 50% September 19, 60% September 22, 70% September 24, 80% September 28, 90% October 2. Observed first fall frost in recent years: 2021 October 13, 2022 September 21, 2023 October 6, 2024 October 1.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 4Sep 19Oct 22021: October 132022: September 212023: October 62024: October 1
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 15October 13
2022June 3September 21
2023May 19October 6
2024May 24October 1
2025May 2incomplete record
2026incomplete recordseason not over yet

What these dates mean

September 4 is the cautious fall date in Brockway, marking a 10 % chance that a light frost has occurred earlier, and it signals when tender crops should be finished. The 50% date is May 20; there is a 50 % chance of a light frost later than this date. The 50% date is September 19; there is a 50 % chance of a light frost earlier than this date.

For spring, the cautious date for setting out tender crops is June 5, when the chance of a light frost later than that date is only 10 %. A hard frost at 28 °F matters more than a light frost because plants that survive 32 °F can still be damaged by the colder temperature. Hard-frost 50% dates are May 9 and September 28, indicating the midpoint when a 28 °F frost is equally likely. The climate normals represent 30 years of record.

How to read the probabilities

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

  • Spring: "10% after June 5" 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 4" 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 Brockway, MTNumbered nearest stations: 1 — Circle, MT — 14 mi; 2 — Cohagen, MT — 40 mi; 3 — Terry, MT — 42 mi; 4 — Vida, MT — 46 mi; 5 — Glendive, MT — 54 mi.25 mi · 40 km1. Circle, MT — 14 mi12. Cohagen, MT — 40 mi23. Terry, MT — 42 mi34. Vida, MT — 46 mi45. Glendive, MT — 54 mi5
  1. Circle, MT 14 mi (22 km)
  2. Cohagen, MT 40 mi (65 km)
  3. Terry, MT 42 mi (68 km)
  4. Vida, MT 46 mi (74 km)
  5. Glendive, MT 54 mi (86 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: 47.2875, -105.83.