U.S. Frost Dates by Weather Station

NOAA station USC00247265 · Montana

Frost Dates for Saco, MT

NOAA weather station Saco 1 NNW (USC00247265), Montana · elevation 2185 ft (666 m) · hardiness zone 3b*

  • Lower-risk planting dateafter May 26only 1 spring in 10 still has a frost later
  • Typical last frostMay 1550% chance of a frost after it
  • Typical first fall frostSeptember 2350% chance of a frost before it
  • Frost-free season130 daystypical year, 32°F
  • Hardiness zone3bcalculated*

The year at a glance

Frost season at Saco, MTSpring: a frost after May 3 in 9 years out of 10, after May 15 in half of the years, after May 26 in 1 year out of 10. Fall: the first frost comes before September 11 in 1 year out of 10, before September 23 in half of the years, before October 4 in 9 years out of 10. Typical frost-free season: 130 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 15First frost ~Sep 23Frost season at Saco, MTSpring: a frost after May 3 in 9 years out of 10, after May 15 in half of the years, after May 26 in 1 year out of 10. Fall: the first frost comes before September 11 in 1 year out of 10, before September 23 in half of the years, before October 4 in 9 years out of 10. Typical frost-free season: 130 days.JFMAMJJASONDLast frost ~May 15First frost ~Sep 23
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 26September 11
50% (typical)May 15September 23
90% (likely)May 3October 4
Frost-free season (32°F, 50% chance)
130 days
Hard frost (28°F), last in spring, 50% chance after
May 6
Hard frost (28°F), first in fall, 50% chance before
October 1

The last 15 years (2006–2020 normals)

Typical last frost May 15 (same as the 30-year normal), typical first fall frost September 26 (3 days later). Frost-free season 133 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 Saco: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 26, 20% May 22, 30% May 20, 40% May 17, 50% May 15, 60% May 13, 70% May 11, 80% May 8, 90% May 3. Observed last spring frost in recent years: 2021 May 13, 2022 May 23, 2023 April 30, 2024 April 30, 2025 May 2, 2026 May 19.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 26May 15May 32021: May 132022: May 232023: April 302024: April 302025: May 22026: May 19Fall at Saco: chance of a frost before this dateFall, chance of a frost before the date: 10% September 11, 20% September 15, 30% September 19, 40% September 21, 50% September 23, 60% September 25, 70% September 28, 80% October 1, 90% October 4. Observed first fall frost in recent years: 2021 October 1, 2022 September 21, 2023 October 6, 2024 October 3, 2025 October 6.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 11Sep 23Oct 42021: October 12022: September 212023: October 62024: 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 13October 1
2022May 23September 21
2023April 30October 6
2024April 30October 3
2025May 2October 6
2026May 19season not over yet

What these dates mean

May 26 is the cautious spring date for light frost (32 °F) in Saco; it marks the point when a gardener can set out tender crops with only a 10 % chance of a later frost. The 50% date is May 15, meaning there is a 50 % chance of a frost later than this date, while May 3 carries a 90 % chance and represents a gamble for early planting.

For the autumn season, September 11 is the cautious light-frost date, giving a 10 % chance that a frost has already arrived and allowing the gardener to finish harvesting tender crops. The 50% date is September 23, with a 50 % chance of a frost earlier than this date, and October 4 is a gamble because of a 90 % chance of an earlier frost. The hard-frost median dates are May 6 and October 1; a hard frost (28 °F) can damage plants more severely than a light frost (32 °F).

How to read the probabilities

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

  • Spring: "10% after May 26" 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 11" 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 Saco, MTNumbered nearest stations: 1 — Content, MT — 28 mi; 2 — Glasgow, MT — 38 mi; 3 — Glasgow #2, MT — 38 mi; 4 — Opheim, MT — 50 mi; 5 — Malta, MT — 51 mi.MT25 mi · 40 km1. Content, MT — 28 mi12. Glasgow, MT — 38 mi23. Glasgow #2, MT — 38 mi34. Opheim, MT — 50 mi45. Malta, MT — 51 mi5
  1. Content, MT 28 mi (45 km)
  2. Glasgow, MT 38 mi (61 km)
  3. Glasgow #2, MT 38 mi (61 km)
  4. Opheim, MT 50 mi (80 km)
  5. Malta, MT 51 mi (83 km)

Sources

* Hardiness zone 3b: zone derived from PRISM 2023 grid at the station location (not the official USDA map). Source data: PRISM Group, Oregon State University.

Station coordinates: 48.4661, -107.3528.