U.S. Frost Dates by Weather Station

NOAA station USC00245011 · Montana

Frost Dates for Libby Dam (Base), MT

NOAA weather station Libby Dam (Base) (USC00245011), Montana · elevation 2110 ft (643 m) · hardiness zone 6a*

  • Lower-risk planting dateafter May 27only 1 spring in 10 still has a frost later
  • Typical last frostMay 1150% chance of a frost after it
  • Typical first fall frostOctober 150% chance of a frost before it
  • Frost-free season142 daystypical year, 32°F
  • Hardiness zone6acalculated*

The year at a glance

Frost season at Libby Dam (Base), MTSpring: a frost after April 30 in 9 years out of 10, after May 11 in half of the years, after May 27 in 1 year out of 10. Fall: the first frost comes before September 14 in 1 year out of 10, before October 1 in half of the years, before October 18 in 9 years out of 10. Typical frost-free season: 142 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 11First frost ~Oct 1Frost season at Libby Dam (Base), MTSpring: a frost after April 30 in 9 years out of 10, after May 11 in half of the years, after May 27 in 1 year out of 10. Fall: the first frost comes before September 14 in 1 year out of 10, before October 1 in half of the years, before October 18 in 9 years out of 10. Typical frost-free season: 142 days.JFMAMJJASONDLast frost ~May 11First frost ~Oct 1
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 27 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 27September 14
50% (typical)May 11October 1
90% (likely)April 30October 18
Frost-free season (32°F, 50% chance)
142 days
Hard frost (28°F), last in spring, 50% chance after
April 28
Hard frost (28°F), first in fall, 50% chance before
October 16

The last 15 years (2006–2020 normals)

Typical last frost May 9 (2 days earlier than the 30-year normal), typical first fall frost October 4 (3 days later). Frost-free season 146 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 Libby Dam (Base): chance of a frost after this dateSpring, chance of a frost after the date: 10% May 27, 20% May 21, 30% May 17, 40% May 14, 50% May 11, 60% May 9, 70% May 6, 80% May 4, 90% April 30. Observed last spring frost in recent years: 2022 May 22, 2023 April 23, 2024 June 20, 2025 May 6, 2026 April 29.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 27May 11Apr 302022: May 222023: April 232024: June 202025: May 62026: April 29Fall at Libby Dam (Base): chance of a frost before this dateFall, chance of a frost before the date: 10% September 14, 20% September 23, 30% September 26, 40% September 29, 50% October 1, 60% October 4, 70% October 7, 80% October 12, 90% October 18. Observed first fall frost in recent years: 2021 October 7, 2022 November 3, 2023 October 25, 2024 October 3, 2025 October 6.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Nov 1Sep 14Oct 1Oct 182021: October 72022: November 32023: October 252024: 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
2021incomplete recordOctober 7
2022May 22November 3
2023April 23October 25
2024June 20October 3
2025May 6October 6
2026April 29season not over yet

What these dates mean

The 142 days between the 50% dates in Libby Dam (Base)—May 11 and October 1—mark the interval when a light frost (32 °F) is less likely than not, giving a gardener a median window for tender-crop activity. In spring the 50% date means there is a 50 % chance of a frost later than May 11, and in fall it means a 50 % chance of a frost earlier than October 1.

For a cautious plan, the gardener would wait until the May 27 date, which carries only a 10 % chance of a frost later, before setting out tender crops, and would aim to finish harvesting tender crops by September 14, the fall cautious date with only a 10 % chance that a frost has arrived earlier. A hard frost at 28 °F poses a greater risk than a light frost, so the median hard-frost dates—April 28 in spring and October 16 in fall—signal when severe damage becomes as likely as not.

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 14" 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 Libby Dam (Base), MTNumbered nearest stations: 1 — Cabinet Gorge, ID — 41 mi; 2 — near Kalispell, MT — 49 mi; 3 — Bonners Ferry, ID — 49 mi; 4 — near Thompson Falls, MT — 56 mi; 5 — Creston, MT — 57 mi.MTID25 mi · 40 km1. Cabinet Gorge, ID — 41 mi12. near Kalispell, MT — 49 mi23. Bonners Ferry, ID — 49 mi34. near Thompson Falls, MT — 56 mi45. Creston, MT — 57 mi5
  1. Cabinet Gorge, ID 41 mi (65 km)
  2. near Kalispell, MT 49 mi (79 km)
  3. Bonners Ferry, ID 49 mi (80 km)
  4. near Thompson Falls, MT 56 mi (91 km)
  5. Creston, MT 57 mi (91 km)

Sources

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

Station coordinates: 48.41, -115.3181.