U.S. Frost Dates by Weather Station

NOAA station USC00249240 · Montana

Frost Dates for Yellowtail Dam, MT

NOAA weather station Yellowtail Dam (USC00249240), Montana · elevation 3305 ft (1007 m) · hardiness zone 5b*

  • Lower-risk planting dateafter May 24only 1 spring in 10 still has a frost later
  • Typical last frostMay 750% chance of a frost after it
  • Typical first fall frostOctober 550% chance of a frost before it
  • Frost-free season151 daystypical year, 32°F
  • Hardiness zone5bcalculated*

The year at a glance

Frost season at Yellowtail Dam, MTSpring: a frost after April 20 in 9 years out of 10, after May 7 in half of the years, after May 24 in 1 year out of 10. Fall: the first frost comes before September 20 in 1 year out of 10, before October 5 in half of the years, before October 22 in 9 years out of 10. Typical frost-free season: 151 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 7First frost ~Oct 5Frost season at Yellowtail Dam, MTSpring: a frost after April 20 in 9 years out of 10, after May 7 in half of the years, after May 24 in 1 year out of 10. Fall: the first frost comes before September 20 in 1 year out of 10, before October 5 in half of the years, before October 22 in 9 years out of 10. Typical frost-free season: 151 days.JFMAMJJASONDLast frost ~May 7First frost ~Oct 5
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 28 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 24September 20
50% (typical)May 7October 5
90% (likely)April 20October 22
Frost-free season (32°F, 50% chance)
151 days
Hard frost (28°F), last in spring, 50% chance after
April 24
Hard frost (28°F), first in fall, 50% chance before
October 14

The last 15 years (2006–2020 normals)

Typical last frost May 4 (3 days earlier than the 30-year normal), typical first fall frost October 8 (3 days later). Frost-free season 156 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 Yellowtail Dam: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 24, 20% May 18, 30% May 14, 40% May 10, 50% May 7, 60% May 4, 70% April 30, 80% April 27, 90% April 20.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 24May 7Apr 20Fall at Yellowtail Dam: chance of a frost before this dateFall, chance of a frost before the date: 10% September 20, 20% September 26, 30% September 30, 40% October 2, 50% October 5, 60% October 8, 70% October 12, 80% October 15, 90% October 22. Observed first fall frost in recent years: 2021 October 30, 2022 October 2.Fall: first frostChance of a frost before this date10%50%90%Oct 1Nov 1Sep 20Oct 5Oct 222021: October 302022: October 2
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 recordOctober 30
2022incomplete recordOctober 2
2023no datano data
2024no datano data
2025no datano data
2026no dataseason not over yet

What these dates mean

The 151 days between the 50% light-frost dates of May 7 and October 5 at Yellowtail Dam frame the main growing window. Within this span a gardener can expect an even chance of a light frost on either side of the period.

For a cautious approach to tender crops the spring LATE date of May 24 is used, because it carries only a 10 % chance of a frost later. In the fall the early cautious date of September 20 is chosen, as it has only a 10 % chance of a frost earlier. The 50% dates themselves (May 7 and October 5) simply mark the midpoint of frost probability, while the 90 % dates—April 20 in spring and October 22 in fall—represent gambles.

Hard-frost dates of April 24 and October 14, when a 28 °F frost has a 50 % chance, matter more than the 32 °F dates because a hard frost can damage or kill plants that survive a 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 May 24" 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 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 Yellowtail Dam, MTNumbered nearest stations: 1 — Hardin, MT — 33 mi; 2 — Lovell, WY — 40 mi; 3 — Billings, MT — 41 mi; 4 — Deaver, WY — 43 mi; 5 — Billings (Logan Airport), MT — 45 mi.MTWY25 mi · 40 km1. Hardin, MT — 33 mi12. Lovell, WY — 40 mi23. Billings, MT — 41 mi34. Deaver, WY — 43 mi45. Billings (Logan Airport), MT — 45 mi5
  1. Hardin, MT 33 mi (53 km)
  2. Lovell, WY 40 mi (64 km)
  3. Billings, MT 41 mi (66 km)
  4. Deaver, WY 43 mi (69 km)
  5. Billings (Logan Airport), MT 45 mi (72 km)

Sources

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

Station coordinates: 45.3128, -107.9383.