U.S. Frost Dates by Weather Station

NOAA station USC00489905 · Wyoming

Frost Dates for Yellowstone Park Mammoth, WY

NOAA weather station Yellowstone Park Mammoth (USC00489905), Wyoming · elevation 6195 ft (1888 m) · hardiness zone 5a*

  • Lower-risk planting dateafter June 19only 1 spring in 10 still has a frost later
  • Typical last frostJune 350% chance of a frost after it
  • Typical first fall frostSeptember 1450% chance of a frost before it
  • Frost-free season102 daystypical year, 32°F
  • Hardiness zone5acalculated*

The year at a glance

Frost season at Yellowstone Park Mammoth, WYSpring: a frost after May 18 in 9 years out of 10, after June 3 in half of the years, after June 19 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 30 in 9 years out of 10. Typical frost-free season: 102 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~Jun 3First frost ~Sep 14Frost season at Yellowstone Park Mammoth, WYSpring: a frost after May 18 in 9 years out of 10, after June 3 in half of the years, after June 19 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 30 in 9 years out of 10. Typical frost-free season: 102 days.JFMAMJJASONDLast frost ~Jun 3First 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 19August 31
50% (typical)June 3September 14
90% (likely)May 18September 30
Frost-free season (32°F, 50% chance)
102 days
Hard frost (28°F), last in spring, 50% chance after
May 16
Hard frost (28°F), first in fall, 50% chance before
September 29

The last 15 years (2006–2020 normals)

Typical last frost June 2 (1 day earlier than the 30-year normal), typical first fall frost September 18 (4 days later). Frost-free season 106 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 Yellowstone Park Mammoth: chance of a frost after this dateSpring, chance of a frost after the date: 10% June 19, 20% June 14, 30% June 10, 40% June 6, 50% June 3, 60% May 31, 70% May 27, 80% May 23, 90% May 18. Observed last spring frost in recent years: 2021 May 23, 2022 July 2, 2023 June 24, 2024 June 19, 2025 June 24, 2026 June 23.Spring: last frostChance of a frost after this date10%50%90%Jun 1Jul 1Jun 19Jun 3May 182021: May 232022: July 22023: June 242024: June 192025: June 242026: June 23Fall at Yellowstone Park Mammoth: chance of a frost before this dateFall, chance of a frost before the date: 10% August 31, 20% September 4, 30% September 8, 40% September 11, 50% September 14, 60% September 18, 70% September 21, 80% September 25, 90% September 30. Observed first fall frost in recent years: 2021 September 17, 2022 September 10, 2023 September 24, 2024 August 30, 2025 September 23.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Aug 31Sep 14Sep 302021: September 172022: September 102023: September 242024: August 302025: 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
2021May 23September 17
2022July 2September 10
2023June 24September 24
2024June 19August 30
2025June 24September 23
2026June 23season not over yet

What these dates mean

June 19 is the cautious spring light-frost date in Yellowstone Park Mammoth; with only a 10 % chance that a frost will occur later, it gives a gardener the confidence to set out tender crops at that time. The earlier spring date, May 18, carries a 90 % chance and is a gamble.

In autumn, August 31 is the cautious fall light-frost date, providing a 10 % chance that a frost has already arrived and thus marking a reference point to finish harvesting tender crops. The median spring date, June 3, represents a 50 % chance of a frost later than this date, while the median fall date, September 14, carries a 50 % chance of a frost earlier than this date. The hard-frost median dates are May 16 and September 29; hard frost at 28 °F can damage plants more severely than a light frost at 32 °F.

These dates are based on 27 years of NOAA records for this station.

How to read the probabilities

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

  • Spring: "10% after June 19" 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 Yellowstone Park Mammoth, WYNumbered nearest stations: 1 — near Bozeman, MT — 50 mi; 2 — Mystic Lake, MT — 51 mi; 3 — near Four Corners, MT — 53 mi; 4 — Nye #2, MT — 54 mi; 5 — Ennis, MT — 55 mi.MTWY25 mi · 40 km1. near Bozeman, MT — 50 mi12. Mystic Lake, MT — 51 mi23. near Four Corners, MT — 53 mi34. Nye #2, MT — 54 mi45. Ennis, MT — 55 mi5
  1. near Bozeman, MT 50 mi (81 km)
  2. Mystic Lake, MT 51 mi (81 km)
  3. near Four Corners, MT 53 mi (85 km)
  4. Nye #2, MT 54 mi (86 km)
  5. Ennis, 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: 44.9767, -110.6964.