U.S. Frost Dates by Weather Station

NOAA station USC00242793 · Montana

Frost Dates for Ennis, MT

NOAA weather station Ennis (USC00242793), Montana · elevation 4953 ft (1510 m) · hardiness zone 5a*

  • Lower-risk planting dateafter July 6only 1 spring in 10 still has a frost later
  • Typical last frostJune 1650% chance of a frost after it
  • Typical first fall frostSeptember 450% chance of a frost before it
  • Frost-free season78 daystypical year, 32°F
  • Hardiness zone5acalculated*

The year at a glance

Frost season at Ennis, MTSpring: a frost after May 30 in 9 years out of 10, after June 16 in half of the years, after July 6 in 1 year out of 10. Fall: the first frost comes before August 20 in 1 year out of 10, before September 4 in half of the years, before September 19 in 9 years out of 10. Typical frost-free season: 78 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~Jun 16First frost ~Sep 4Frost season at Ennis, MTSpring: a frost after May 30 in 9 years out of 10, after June 16 in half of the years, after July 6 in 1 year out of 10. Fall: the first frost comes before August 20 in 1 year out of 10, before September 4 in half of the years, before September 19 in 9 years out of 10. Typical frost-free season: 78 days.JFMAMJJASONDLast frost ~Jun 16First frost ~Sep 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 27 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)July 6August 20
50% (typical)June 16September 4
90% (likely)May 30September 19
Frost-free season (32°F, 50% chance)
78 days
Hard frost (28°F), last in spring, 50% chance after
May 23
Hard frost (28°F), first in fall, 50% chance before
September 18

The last 15 years (2006–2020 normals)

Typical last frost June 18 (2 days later than the 30-year normal), typical first fall frost September 4. Frost-free season 79 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 Ennis: chance of a frost after this dateSpring, chance of a frost after the date: 10% July 6, 20% June 29, 30% June 23, 40% June 20, 50% June 16, 60% June 12, 70% June 8, 80% June 4, 90% May 30. Observed last spring frost in recent years: 2021 June 11, 2022 May 23, 2023 June 22, 2024 June 19, 2025 June 24, 2026 June 15.Spring: last frostChance of a frost after this date10%50%90%Jun 1Jul 1Jul 6Jun 16May 302021: June 112022: May 232023: June 222024: June 192025: June 242026: June 15Fall at Ennis: chance of a frost before this dateFall, chance of a frost before the date: 10% August 20, 20% August 25, 30% August 30, 40% September 1, 50% September 4, 60% September 6, 70% September 8, 80% September 13, 90% September 19. Observed first fall frost in recent years: 2021 September 24, 2022 September 10, 2023 September 15, 2024 September 21, 2025 September 23.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Aug 20Sep 4Sep 192021: September 242022: September 102023: September 152024: September 212025: September 23
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
2021June 11September 24
2022May 23September 10
2023June 22September 15
2024June 19September 21
2025June 24September 23
2026June 15season not over yet

What these dates mean

July 6 is the cautious spring date for Ennis, marking the latest point at which a 10 % chance of a light frost remains; after that date a gardener can set out tender crops with only a small risk of frost damage. The 50 % spring date, June 16, indicates an even chance that a light frost will occur later, while the 50 % fall date, September 4, signals an even chance that a light frost will arrive earlier; the cautious fall date is August 20, when only a 10 % chance of a light frost remains, allowing a gardener to finish harvesting tender crops. These median dates help gauge the midpoint of frost risk. For hard frost, the 50 % dates are May 23 in spring and September 18 in fall, showing when a 28 °F freeze is as likely as not. A hard frost matters more than a light frost because temperatures at 28 °F can cause more severe tissue freezing than the milder 32 °F light frost.

How to read the probabilities

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

  • Spring: "10% after July 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 August 20" 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 Ennis, MTNumbered nearest stations: 1 — Norris Madison Power House, MT — 11 mi; 2 — Twin Bridges, MT — 33 mi; 3 — near Four Corners, MT — 35 mi; 4 — near Bozeman, MT — 39 mi; 5 — Bozeman (Gallatin Field), MT — 41 mi.25 mi · 40 km1. Norris Madison Power House, MT — 11 mi12. Twin Bridges, MT — 33 mi23. near Four Corners, MT — 35 mi34. near Bozeman, MT — 39 mi45. Bozeman (Gallatin Field), MT — 41 mi5
  1. Norris Madison Power House, MT 11 mi (18 km)
  2. Twin Bridges, MT 33 mi (53 km)
  3. near Four Corners, MT 35 mi (57 km)
  4. near Bozeman, MT 39 mi (63 km)
  5. Bozeman (Gallatin Field), MT 41 mi (66 km)

Sources

* Hardiness zone 5a: zone derived from PRISM 2023 grid at the station location (not the official USDA map). Source data: PRISM Group, Oregon State University.

Station coordinates: 45.3394, -111.7122.