U.S. Frost Dates by Weather Station

NOAA station USC00248430 · Montana

Frost Dates for Twin Bridges, MT

NOAA weather station Twin Bridges (USC00248430), Montana · elevation 4625 ft (1410 m) · hardiness zone 5a*

  • Lower-risk planting dateafter July 1only 1 spring in 10 still has a frost later
  • Typical last frostJune 850% chance of a frost after it
  • Typical first fall frostSeptember 250% chance of a frost before it
  • Frost-free season82 daystypical year, 32°F
  • Hardiness zone5acalculated*

The year at a glance

Frost season at Twin Bridges, MTSpring: a frost after May 22 in 9 years out of 10, after June 8 in half of the years, after July 1 in 1 year out of 10. Fall: the first frost comes before August 16 in 1 year out of 10, before September 2 in half of the years, before September 15 in 9 years out of 10. Typical frost-free season: 82 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~Jun 8First frost ~Sep 2Frost season at Twin Bridges, MTSpring: a frost after May 22 in 9 years out of 10, after June 8 in half of the years, after July 1 in 1 year out of 10. Fall: the first frost comes before August 16 in 1 year out of 10, before September 2 in half of the years, before September 15 in 9 years out of 10. Typical frost-free season: 82 days.JFMAMJJASONDLast frost ~Jun 8First frost ~Sep 2
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)July 1August 16
50% (typical)June 8September 2
90% (likely)May 22September 15
Frost-free season (32°F, 50% chance)
82 days
Hard frost (28°F), last in spring, 50% chance after
May 20
Hard frost (28°F), first in fall, 50% chance before
September 15

The last 15 years (2006–2020 normals)

Typical last frost June 8 (same as the 30-year normal), typical first fall frost September 1 (1 day earlier). Frost-free season 83 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 Twin Bridges: chance of a frost after this dateSpring, chance of a frost after the date: 10% July 1, 20% June 23, 30% June 17, 40% June 12, 50% June 8, 60% June 4, 70% June 1, 80% May 28, 90% May 22. Observed last spring frost in recent years: 2021 June 12, 2022 May 24, 2023 June 22, 2024 June 19, 2025 June 24, 2026 June 15.Spring: last frostChance of a frost after this date10%50%90%Jun 1Jul 1Jul 1Jun 8May 222021: June 122022: May 242023: June 222024: June 192025: June 242026: June 15Fall at Twin Bridges: chance of a frost before this dateFall, chance of a frost before the date: 10% August 16, 20% August 23, 30% August 27, 40% August 31, 50% September 2, 60% September 4, 70% September 8, 80% September 11, 90% September 15. Observed first fall frost in recent years: 2021 August 29, 2022 September 9, 2023 September 15, 2024 August 29, 2025 September 23.Fall: first frostChance of a frost before this date10%50%90%Sep 1Aug 16Sep 2Sep 152021: August 292022: September 92023: September 152024: August 292025: September 23
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
2021June 12August 29
2022May 24September 9
2023June 22September 15
2024June 19August 29
2025June 24September 23
2026June 15season not over yet

What these dates mean

Twin Bridges has an 82-day span between the 32°F 50% dates, June 8 and September 2, which frames the period when light frosts are equally likely to have occurred or not. For a gardener this means that around the middle of this window the risk of a light frost is balanced, while earlier or later dates carry higher odds of frost.

In spring the cautious date for setting out tender crops is July 1, when only a 10% chance of a later light frost remains; planting on May 22 would be a gamble with a 90% chance of frost later. In fall the cautious date to finish harvesting tender crops is August 16, offering a 10% chance that a light frost has arrived earlier; aiming for September 15 would be a gamble with a 90% chance of frost earlier.

A hard frost, recorded at 28°F, can damage plants more severely than a light frost, and the 50% hard-frost dates are May 20 in spring and September 15 in fall.

How to read the probabilities

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

  • Spring: "10% after July 1" 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 August 16" 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 Twin Bridges, MTNumbered nearest stations: 1 — Dillon (Airport), MT — 23 mi; 2 — near Dillon, MT — 28 mi; 3 — near Butte, MT — 30 mi; 4 — Ennis, MT — 33 mi; 5 — Norris Madison Power House, MT — 34 mi.MTIDWY25 mi · 40 km1. Dillon (Airport), MT — 23 mi12. near Dillon, MT — 28 mi23. near Butte, MT — 30 mi34. Ennis, MT — 33 mi45. Norris Madison Power House, MT — 34 mi5
  1. Dillon (Airport), MT 23 mi (37 km)
  2. near Dillon, MT 28 mi (45 km)
  3. near Butte, MT 30 mi (48 km)
  4. Ennis, MT 33 mi (53 km)
  5. Norris Madison Power House, MT 34 mi (54 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: 45.5472, -112.3261.