U.S. Frost Dates by Weather Station

NOAA station USC00241552 · Montana

Frost Dates for Cascade, MT

NOAA weather station Cascade 5 S (USC00241552), Montana · elevation 3360 ft (1024 m) · hardiness zone 4b*

  • Lower-risk planting dateafter June 6only 1 spring in 10 still has a frost later
  • Typical last frostMay 2150% chance of a frost after it
  • Typical first fall frostSeptember 1950% chance of a frost before it
  • Frost-free season120 daystypical year, 32°F
  • Hardiness zone4bcalculated*

The year at a glance

Frost season at Cascade, MTSpring: a frost after May 5 in 9 years out of 10, after May 21 in half of the years, after June 6 in 1 year out of 10. Fall: the first frost comes before September 2 in 1 year out of 10, before September 19 in half of the years, before October 2 in 9 years out of 10. Typical frost-free season: 120 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 21First frost ~Sep 19Frost season at Cascade, MTSpring: a frost after May 5 in 9 years out of 10, after May 21 in half of the years, after June 6 in 1 year out of 10. Fall: the first frost comes before September 2 in 1 year out of 10, before September 19 in half of the years, before October 2 in 9 years out of 10. Typical frost-free season: 120 days.JFMAMJJASONDLast frost ~May 21First 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.Today

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)June 6September 2
50% (typical)May 21September 19
90% (likely)May 5October 2
Frost-free season (32°F, 50% chance)
120 days
Hard frost (28°F), last in spring, 50% chance after
May 7
Hard frost (28°F), first in fall, 50% chance before
October 1

The last 15 years (2006–2020 normals)

Typical last frost May 21 (same as the 30-year normal), typical first fall frost September 18 (1 day earlier). Frost-free season 120 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 Cascade: chance of a frost after this dateSpring, chance of a frost after the date: 10% June 6, 20% June 1, 30% May 27, 40% May 23, 50% May 21, 60% May 17, 70% May 14, 80% May 10, 90% May 5. Observed last spring frost in recent years: 2022 May 22.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1Jun 6May 21May 52022: May 22Fall at Cascade: chance of a frost before this dateFall, chance of a frost before the date: 10% September 2, 20% September 8, 30% September 12, 40% September 16, 50% September 19, 60% September 22, 70% September 25, 80% September 29, 90% October 2. Observed first fall frost in recent years: 2021 September 17.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 2Sep 19Oct 22021: September 17
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 recordSeptember 17
2022May 22incomplete record
2023incomplete recordno data
2024no datano data
2025no datano data
2026no dataseason not over yet

What these dates mean

Setting out tender crops on the early spring date, May 5, carries a 90 % chance of a frost and therefore is a gamble for growers in Cascade. The cautious spring date for planting tender crops is June 6, when only a 10 % chance of frost remains.

The median spring date, May 21, indicates a 50 % chance of a frost later than this day, while the median fall date, September 19, denotes a 50 % chance of a frost earlier than that day. In the fall the cautious date to finish harvesting tender crops is September 2, with only a 10 % chance that frost has already arrived, and the gamble date is October 2, when a frost is 90 % likely.

A hard frost at 28 °F can cause far more severe damage to plants than a light frost at 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 June 6" 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 2" 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 Cascade, MTNumbered nearest stations: 1 — Sun River, MT — 18 mi; 2 — Holter Dam, MT — 21 mi; 3 — Great Falls, MT — 23 mi; 4 — Fairfield, MT — 30 mi; 5 — Millegan, MT — 35 mi.25 mi · 40 km1. Sun River, MT — 18 mi12. Holter Dam, MT — 21 mi23. Great Falls, MT — 23 mi34. Fairfield, MT — 30 mi45. Millegan, MT — 35 mi5
  1. Sun River, MT 18 mi (29 km)
  2. Holter Dam, MT 21 mi (34 km)
  3. Great Falls, MT 23 mi (38 km)
  4. Fairfield, MT 30 mi (49 km)
  5. Millegan, MT 35 mi (56 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: 47.2192, -111.7106.