U.S. Frost Dates by Weather Station

NOAA station USC00248211 · Montana

Frost Dates near Thompson Falls, MT

NOAA weather station Thompson Falls Powerhouse (USC00248211), Montana · elevation 2380 ft (725 m) · hardiness zone 6b*

Thompson Falls is the nearest town to the station (1.0 mi away) in GeoNames (CC BY 4.0). The dates below are measured at the station, not in the town itself.

  • Lower-risk planting dateafter May 19only 1 spring in 10 still has a frost later
  • Typical last frostMay 750% chance of a frost after it
  • Typical first fall frostOctober 450% chance of a frost before it
  • Frost-free season150 daystypical year, 32°F
  • Hardiness zone6bcalculated*

The year at a glance

Frost season near Thompson Falls, MTSpring: a frost after April 25 in 9 years out of 10, after May 7 in half of the years, after May 19 in 1 year out of 10. Fall: the first frost comes before September 21 in 1 year out of 10, before October 4 in half of the years, before October 19 in 9 years out of 10. Typical frost-free season: 150 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 7First frost ~Oct 4Frost season near Thompson Falls, MTSpring: a frost after April 25 in 9 years out of 10, after May 7 in half of the years, after May 19 in 1 year out of 10. Fall: the first frost comes before September 21 in 1 year out of 10, before October 4 in half of the years, before October 19 in 9 years out of 10. Typical frost-free season: 150 days.JFMAMJJASONDLast frost ~May 7First frost ~Oct 4
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 19September 21
50% (typical)May 7October 4
90% (likely)April 25October 19
Frost-free season (32°F, 50% chance)
150 days
Hard frost (28°F), last in spring, 50% chance after
April 22
Hard frost (28°F), first in fall, 50% chance before
October 18

The last 15 years (2006–2020 normals)

Typical last frost May 5 (2 days earlier than the 30-year normal), typical first fall frost October 5 (1 day later). Frost-free season 151 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 near Thompson Falls: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 19, 20% May 14, 30% May 11, 40% May 9, 50% May 7, 60% May 4, 70% May 2, 80% April 29, 90% April 25. Observed last spring frost in recent years: 2021 May 9, 2022 May 11, 2023 April 23, 2024 May 2, 2025 May 6, 2026 April 26.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 19May 7Apr 252021: May 92022: May 112023: April 232024: May 22025: May 62026: April 26Fall near Thompson Falls: chance of a frost before this dateFall, chance of a frost before the date: 10% September 21, 20% September 26, 30% September 29, 40% October 1, 50% October 4, 60% October 6, 70% October 10, 80% October 14, 90% October 19. Observed first fall frost in recent years: 2021 October 7, 2022 November 3, 2023 October 12, 2024 October 3, 2025 October 6.Fall: first frostChance of a frost before this date10%50%90%Oct 1Nov 1Sep 21Oct 4Oct 192021: October 72022: November 32023: October 122024: October 32025: 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 9October 7
2022May 11November 3
2023April 23October 12
2024May 2October 3
2025May 6October 6
2026April 26season not over yet

What these dates mean

Setting out tender crops on the early spring date of April 25 in the Thompson Falls area carries a 90 % chance of a frost, making it a gamble for growers. The cautious approach uses the later May 19 date, which has only a 10 % chance of a frost after it.

In autumn the cautious date to finish harvesting tender crops is September 21, where there is a 10 % chance of a frost earlier, while October 19 would be a gamble with a 90 % chance. The 50 % hard-frost dates are April 22 and October 18; each represents a point where there is a 50 % chance of a hard frost later (April 22) or earlier (October 18). A hard frost at 28 °F can damage plants more severely than a light frost at 32 °F, so those dates are especially relevant for protecting crops.

How to read the probabilities

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

  • Spring: "10% after May 19" 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 21" 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 Thompson Falls, MTNumbered nearest stations: 1 — Kellogg, ID — 36 mi; 2 — Cabinet Gorge, ID — 47 mi; 3 — Libby Dam (Base), MT — 56 mi; 4 — Saint Maries, ID — 60 mi; 5 — Creston, MT — 70 mi.MTID25 mi · 40 km1. Kellogg, ID — 36 mi12. Cabinet Gorge, ID — 47 mi23. Libby Dam (Base), MT — 56 mi34. Saint Maries, ID — 60 mi45. Creston, MT — 70 mi5
  1. Kellogg, ID 36 mi (58 km)
  2. Cabinet Gorge, ID 47 mi (76 km)
  3. Libby Dam (Base), MT 56 mi (91 km)
  4. Saint Maries, ID 60 mi (97 km)
  5. Creston, MT 70 mi (113 km)

Sources

* Hardiness zone 6b: zone derived from PRISM 2023 grid at the station location (not the official USDA map). Source data: PRISM Group, Oregon State University.

Station coordinates: 47.5933, -115.3594.