U.S. Frost Dates by Weather Station

NOAA station USC00247740 · Montana

Frost Dates for Sonnette, MT

NOAA weather station Sonnette 7 SW (USC00247740), Montana · elevation 3914 ft (1193 m) · hardiness zone 4b*

  • Lower-risk planting dateafter June 7only 1 spring in 10 still has a frost later
  • Typical last frostMay 2550% chance of a frost after it
  • Typical first fall frostSeptember 1950% chance of a frost before it
  • Frost-free season115 daystypical year, 32°F
  • Hardiness zone4bcalculated*

The year at a glance

Frost season at Sonnette, MTSpring: a frost after May 12 in 9 years out of 10, after May 25 in half of the years, after June 7 in 1 year out of 10. Fall: the first frost comes before September 1 in 1 year out of 10, before September 19 in half of the years, before October 1 in 9 years out of 10. Typical frost-free season: 115 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 25First frost ~Sep 19Frost season at Sonnette, MTSpring: a frost after May 12 in 9 years out of 10, after May 25 in half of the years, after June 7 in 1 year out of 10. Fall: the first frost comes before September 1 in 1 year out of 10, before September 19 in half of the years, before October 1 in 9 years out of 10. Typical frost-free season: 115 days.JFMAMJJASONDLast frost ~May 25First 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.

Frost dates (32°F)

Probability of frost, based on 26 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)June 7September 1
50% (typical)May 25September 19
90% (likely)May 12October 1
Frost-free season (32°F, 50% chance)
115 days
Hard frost (28°F), last in spring, 50% chance after
May 14
Hard frost (28°F), first in fall, 50% chance before
September 29

The last 15 years (2006–2020 normals)

Typical last frost May 24 (1 day earlier than the 30-year normal), typical first fall frost September 19. Frost-free season 116 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 Sonnette: chance of a frost after this dateSpring, chance of a frost after the date: 10% June 7, 20% June 3, 30% May 31, 40% May 27, 50% May 25, 60% May 22, 70% May 19, 80% May 16, 90% May 12. Observed last spring frost in recent years: 2021 May 22, 2022 June 1, 2023 May 19.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1Jun 7May 25May 122021: May 222022: June 12023: May 19Fall at Sonnette: chance of a frost before this dateFall, chance of a frost before the date: 10% September 1, 20% September 7, 30% September 12, 40% September 16, 50% September 19, 60% September 22, 70% September 24, 80% September 27, 90% October 1. Observed first fall frost in recent years: 2021 September 17, 2022 September 10, 2023 October 6.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 1Sep 19Oct 12021: September 172022: September 102023: 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 22September 17
2022June 1September 10
2023May 19October 6
2024incomplete recordincomplete record
2025incomplete recordno data
2026no dataseason not over yet

What these dates mean

Setting out tender crops on May 12 is a gamble, because there is a 90% chance of a light frost later than that date in Sonnette.

A cautious spring plan waits until June 7, when only a 10% chance of a light frost remains later, while a cautious fall plan finishes tender crops by September 1, when there is only a 10% chance that a light frost has already arrived. The 50% dates are May 25 and September 19; May 25 marks a 50% chance of a light frost later, and September 19 marks a 50% chance of a light frost earlier. The median hard-frost dates are May 14 and September 29.

A hard frost at 28 °F can damage plants that survive a light frost at 32 °F, so the hard-frost dates are a critical threshold.

How to read the probabilities

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

  • Spring: "10% after June 7" 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 1" 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 Sonnette, MTNumbered nearest stations: 1 — Moorhead, MT — 13 mi; 2 — Biddle, MT — 29 mi; 3 — Brandenberg, MT — 37 mi; 4 — Leiter, WY — 38 mi; 5 — Busby, MT — 53 mi.MT25 mi · 40 km1. Moorhead, MT — 13 mi12. Biddle, MT — 29 mi23. Brandenberg, MT — 37 mi34. Leiter, WY — 38 mi45. Busby, MT — 53 mi5
  1. Moorhead, MT 13 mi (21 km)
  2. Biddle, MT 29 mi (46 km)
  3. Brandenberg, MT 37 mi (60 km)
  4. Leiter, WY 38 mi (60 km)
  5. Busby, MT 53 mi (85 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: 45.3242, -105.9136.