U.S. Frost Dates by Weather Station

NOAA station USC00240743 · Montana

Frost Dates for Biddle, MT

NOAA weather station Biddle 8 SW (USC00240743), Montana · elevation 3631 ft (1107 m) · hardiness zone 5a*

  • Lower-risk planting dateafter May 27only 1 spring in 10 still has a frost later
  • Typical last frostMay 1450% chance of a frost after it
  • Typical first fall frostSeptember 2850% chance of a frost before it
  • Frost-free season134 daystypical year, 32°F
  • Hardiness zone5acalculated*

The year at a glance

Frost season at Biddle, MTSpring: a frost after May 2 in 9 years out of 10, after May 14 in half of the years, after May 27 in 1 year out of 10. Fall: the first frost comes before September 13 in 1 year out of 10, before September 28 in half of the years, before October 12 in 9 years out of 10. Typical frost-free season: 134 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 14First frost ~Sep 28Frost season at Biddle, MTSpring: a frost after May 2 in 9 years out of 10, after May 14 in half of the years, after May 27 in 1 year out of 10. Fall: the first frost comes before September 13 in 1 year out of 10, before September 28 in half of the years, before October 12 in 9 years out of 10. Typical frost-free season: 134 days.JFMAMJJASONDLast frost ~May 14First frost ~Sep 28
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 27September 13
50% (typical)May 14September 28
90% (likely)May 2October 12
Frost-free season (32°F, 50% chance)
134 days
Hard frost (28°F), last in spring, 50% chance after
May 3
Hard frost (28°F), first in fall, 50% chance before
October 5

The last 15 years (2006–2020 normals)

Typical last frost May 13 (1 day earlier than the 30-year normal), typical first fall frost October 1 (3 days later). Frost-free season 138 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 Biddle: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 27, 20% May 23, 30% May 20, 40% May 17, 50% May 14, 60% May 12, 70% May 10, 80% May 6, 90% May 2. Observed last spring frost in recent years: 2021 May 10, 2022 May 22, 2023 May 1, 2025 May 22, 2026 May 19.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 27May 14May 22021: May 102022: May 222023: May 12025: May 222026: May 19Fall at Biddle: chance of a frost before this dateFall, chance of a frost before the date: 10% September 13, 20% September 18, 30% September 22, 40% September 25, 50% September 28, 60% October 1, 70% October 3, 80% October 6, 90% October 12. Observed first fall frost in recent years: 2021 September 17, 2022 October 9, 2023 October 6, 2024 September 22, 2025 October 7.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 13Sep 28Oct 122021: September 172022: October 92023: October 62024: September 222025: October 7
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 10September 17
2022May 22October 9
2023May 1October 6
2024incomplete recordSeptember 22
2025May 22October 7
2026May 19season not over yet

What these dates mean

May 27 is the cautious spring light-frost date for Biddle; with only a 10 % chance of a frost later, a gardener can schedule the first planting of tender vegetables after that day. The 50 % light-frost date of May 14 marks the point where a frost is equally likely to occur later or not, giving a midpoint reference for timing decisions. In the fall, the cautious date is September 13, when a frost earlier than that point is only a 10 % chance, so finishing tender-crop harvests after this day reduces risk. The fall 50 % date of September 28 likewise serves as a median marker. The hard-frost 50 % dates, May 3 in spring and October 5 in fall, show when a temperature of 28 °F is equally probable, and a hard frost can damage or kill plants more severely than a light frost. Using the cautious dates helps align planting and harvesting with the lowest frost risk while the 50 % dates give a balanced reference point.

How to read the probabilities

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

  • Spring: "10% after May 27" 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 13" 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 Biddle, MTNumbered nearest stations: 1 — Moorhead, MT — 16 mi; 2 — Sonnette, MT — 29 mi; 3 — Leiter, WY — 42 mi; 4 — Devils Tower #2, WY — 49 mi; 5 — Clearmont, WY — 57 mi.WYMT25 mi · 40 km1. Moorhead, MT — 16 mi12. Sonnette, MT — 29 mi23. Leiter, WY — 42 mi34. Devils Tower #2, WY — 49 mi45. Clearmont, WY — 57 mi5
  1. Moorhead, MT 16 mi (26 km)
  2. Sonnette, MT 29 mi (46 km)
  3. Leiter, WY 42 mi (67 km)
  4. Devils Tower #2, WY 49 mi (79 km)
  5. Clearmont, WY 57 mi (92 km)

Sources

* Hardiness zone 5a: zone derived from PRISM 2023 grid at the station location (not the official USDA map). Source data: PRISM Group, Oregon State University.

Station coordinates: 45.0428, -105.48.