U.S. Frost Dates by Weather Station

NOAA station USC00241084 · Montana

Frost Dates for Brandenberg, MT

NOAA weather station Brandenberg (USC00241084), Montana · elevation 2770 ft (844 m) · hardiness zone 4b*

  • 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 2250% chance of a frost before it
  • Frost-free season126 daystypical year, 32°F
  • Hardiness zone4bcalculated*

The year at a glance

Frost season at Brandenberg, 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 8 in 1 year out of 10, before September 22 in half of the years, before October 5 in 9 years out of 10. Typical frost-free season: 126 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 18First frost ~Sep 22Frost season at Brandenberg, 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 8 in 1 year out of 10, before September 22 in half of the years, before October 5 in 9 years out of 10. Typical frost-free season: 126 days.JFMAMJJASONDLast frost ~May 18First frost ~Sep 22
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 29 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 30September 8
50% (typical)May 18September 22
90% (likely)May 6October 5
Frost-free season (32°F, 50% chance)
126 days
Hard frost (28°F), last in spring, 50% chance after
May 7
Hard frost (28°F), first in fall, 50% chance before
October 2

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 23 (1 day later). Frost-free season 127 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 Brandenberg: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 30, 20% May 25, 30% May 22, 40% May 20, 50% May 18, 60% May 16, 70% May 14, 80% May 11, 90% May 6. Observed last spring frost in recent years: 2021 May 22, 2022 May 22, 2023 May 2, 2024 May 20, 2025 May 22, 2026 May 19.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 30May 18May 62021: May 222022: May 222023: May 22024: May 202025: May 222026: May 19Fall at Brandenberg: chance of a frost before this dateFall, chance of a frost before the date: 10% September 8, 20% September 12, 30% September 16, 40% September 19, 50% September 22, 60% September 25, 70% September 28, 80% October 1, 90% October 5. Observed first fall frost in recent years: 2021 September 17, 2022 October 7, 2023 October 6, 2024 October 1, 2025 September 5.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 8Sep 22Oct 52021: September 172022: October 72023: October 62024: October 12025: September 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 22September 17
2022May 22October 7
2023May 2October 6
2024May 20October 1
2025May 22September 5
2026May 19season not over yet

What these dates mean

The cautious fall date for finishing the harvest in Brandenberg is September 8, when there is only a 10 % chance a light frost has already arrived. For planting tender crops, the cautious spring date is May 30, which carries a 10 % chance of a later light frost, while setting them out on May 6 would be a gamble with a 90 % chance of frost.

The 50% date is May 18 in spring and September 22 in fall; at each of these points the likelihood of a light frost later (spring) or earlier (fall) is even. A hard frost at 28°F, indicated by the 50% dates of May 7 in spring and October 2 in fall, can injure tender crops more severely than a light frost at 32°F. The period between the two 32°F 50% dates spans 126 days based on 29 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 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 8" 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 Brandenberg, MTNumbered nearest stations: 1 — Sonnette, MT — 37 mi; 2 — Forsyth, MT — 38 mi; 3 — Busby, MT — 40 mi; 4 — Hysham 25 SSE, MT — 44 mi; 5 — near Miles City, MT — 45 mi.MT25 mi · 40 km1. Sonnette, MT — 37 mi12. Forsyth, MT — 38 mi23. Busby, MT — 40 mi34. Hysham 25 SSE, MT — 44 mi45. near Miles City, MT — 45 mi5
  1. Sonnette, MT 37 mi (60 km)
  2. Forsyth, MT 38 mi (61 km)
  3. Busby, MT 40 mi (64 km)
  4. Hysham 25 SSE, MT 44 mi (71 km)
  5. near Miles City, MT 45 mi (73 km)

Sources

* Hardiness zone 4b: zone derived from PRISM 2023 grid at the station location (not the official USDA map). Source data: PRISM Group, Oregon State University.

Station coordinates: 45.8161, -106.2328.