U.S. Frost Dates by Weather Station

NOAA station USC00245961 · Montana

Frost Dates for Mystic Lake, MT

NOAA weather station Mystic Lake (USC00245961), Montana · elevation 6544 ft (1995 m) · hardiness zone 5a*

  • Lower-risk planting dateafter June 21only 1 spring in 10 still has a frost later
  • Typical last frostJune 550% chance of a frost after it
  • Typical first fall frostSeptember 1450% chance of a frost before it
  • Frost-free season99 daystypical year, 32°F
  • Hardiness zone5acalculated*

The year at a glance

Frost season at Mystic Lake, MTSpring: a frost after May 22 in 9 years out of 10, after June 5 in half of the years, after June 21 in 1 year out of 10. Fall: the first frost comes before August 31 in 1 year out of 10, before September 14 in half of the years, before September 28 in 9 years out of 10. Typical frost-free season: 99 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~Jun 5First frost ~Sep 14Frost season at Mystic Lake, MTSpring: a frost after May 22 in 9 years out of 10, after June 5 in half of the years, after June 21 in 1 year out of 10. Fall: the first frost comes before August 31 in 1 year out of 10, before September 14 in half of the years, before September 28 in 9 years out of 10. Typical frost-free season: 99 days.JFMAMJJASONDLast frost ~Jun 5First frost ~Sep 14
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)June 21August 31
50% (typical)June 5September 14
90% (likely)May 22September 28
Frost-free season (32°F, 50% chance)
99 days
Hard frost (28°F), last in spring, 50% chance after
May 24
Hard frost (28°F), first in fall, 50% chance before
September 25

The last 15 years (2006–2020 normals)

Typical last frost June 6 (1 day later than the 30-year normal), typical first fall frost September 15 (1 day later). Frost-free season 101 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 Mystic Lake: chance of a frost after this dateSpring, chance of a frost after the date: 10% June 21, 20% June 16, 30% June 12, 40% June 8, 50% June 5, 60% June 3, 70% May 31, 80% May 27, 90% May 22. Observed last spring frost in recent years: 2021 May 23, 2022 June 14, 2023 May 10, 2024 June 19, 2025 June 22.Spring: last frostChance of a frost after this date10%50%90%Jun 1Jul 1Jun 21Jun 5May 222021: May 232022: June 142023: May 102024: June 192025: June 22Fall at Mystic Lake: chance of a frost before this dateFall, chance of a frost before the date: 10% August 31, 20% September 5, 30% September 8, 40% September 11, 50% September 14, 60% September 18, 70% September 21, 80% September 24, 90% September 28. Observed first fall frost in recent years: 2021 October 11, 2022 October 12, 2023 October 6, 2024 October 18, 2025 October 5.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Aug 31Sep 14Sep 282021: October 112022: October 122023: October 62024: October 182025: October 5
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 23October 11
2022June 14October 12
2023May 10October 6
2024June 19October 18
2025June 22October 5
2026incomplete recordseason not over yet

What these dates mean

The 99 days between the 50% dates in Mystic Lake, June 5 and September 14, frame the window when a light frost is equally likely to have occurred or not. Gardeners can treat the June 21 date as the cautious spring point for setting out tender crops, because only a 10 % chance of a frost remains later, while the May 22 date carries a 90 % chance and is a gamble. In the fall the cautious date to finish harvesting tender crops is August 31, when there is only a 10 % chance a frost has arrived earlier, whereas September 28 is a gamble with a 90 % chance of an earlier frost. Hard frost at 28 °F, such as the 50% dates May 24 and September 25, can injure plants more severely 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 21" 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 31" 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 Mystic Lake, MTNumbered nearest stations: 1 — Nye #2, MT — 14 mi; 2 — Columbus, MT — 36 mi; 3 — Big Timber, MT — 42 mi; 4 — Yellowstone Park Mammoth, WY — 51 mi; 5 — Rapelje, MT — 55 mi.MTWY25 mi · 40 km1. Nye #2, MT — 14 mi12. Columbus, MT — 36 mi23. Big Timber, MT — 42 mi34. Yellowstone Park Mammoth, WY — 51 mi45. Rapelje, MT — 55 mi5
  1. Nye #2, MT 14 mi (22 km)
  2. Columbus, MT 36 mi (57 km)
  3. Big Timber, MT 42 mi (68 km)
  4. Yellowstone Park Mammoth, WY 51 mi (81 km)
  5. Rapelje, MT 55 mi (89 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: 45.2436, -109.7311.