U.S. Frost Dates by Weather Station

NOAA station USC00487555 · Wyoming

Frost Dates for Redbird, WY

NOAA weather station Redbird (USC00487555), Wyoming · elevation 3890 ft (1186 m) · hardiness zone 4a*

  • Lower-risk planting dateafter May 31only 1 spring in 10 still has a frost later
  • Typical last frostMay 1650% chance of a frost after it
  • Typical first fall frostSeptember 2450% chance of a frost before it
  • Frost-free season130 daystypical year, 32°F
  • Hardiness zone4acalculated*

The year at a glance

Frost season at Redbird, WYSpring: a frost after May 4 in 9 years out of 10, after May 16 in half of the years, after May 31 in 1 year out of 10. Fall: the first frost comes before September 10 in 1 year out of 10, before September 24 in half of the years, before October 5 in 9 years out of 10. Typical frost-free season: 130 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 16First frost ~Sep 24Frost season at Redbird, WYSpring: a frost after May 4 in 9 years out of 10, after May 16 in half of the years, after May 31 in 1 year out of 10. Fall: the first frost comes before September 10 in 1 year out of 10, before September 24 in half of the years, before October 5 in 9 years out of 10. Typical frost-free season: 130 days.JFMAMJJASONDLast frost ~May 16First frost ~Sep 24
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 29 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 31September 10
50% (typical)May 16September 24
90% (likely)May 4October 5
Frost-free season (32°F, 50% chance)
130 days
Hard frost (28°F), last in spring, 50% chance after
May 6
Hard frost (28°F), first in fall, 50% chance before
October 2

The last 15 years (2006–2020 normals)

Typical last frost May 16 (same as the 30-year normal), typical first fall frost September 26 (2 days later). Frost-free season 131 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 Redbird: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 31, 20% May 26, 30% May 22, 40% May 19, 50% May 16, 60% May 13, 70% May 11, 80% May 8, 90% May 4. Observed last spring frost in recent years: 2021 May 14, 2023 May 1, 2024 May 4, 2025 May 2, 2026 May 19.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 31May 16May 42021: May 142023: May 12024: May 42025: May 22026: May 19Fall at Redbird: chance of a frost before this dateFall, chance of a frost before the date: 10% September 10, 20% September 16, 30% September 19, 40% September 22, 50% September 24, 60% September 27, 70% September 30, 80% October 2, 90% October 5. Observed first fall frost in recent years: 2021 October 2, 2022 October 8, 2023 October 7, 2024 October 1, 2025 October 19.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 10Sep 24Oct 52021: October 22022: October 82023: October 72024: October 12025: October 19
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 14October 2
2022incomplete recordOctober 8
2023May 1October 7
2024May 4October 1
2025May 2October 19
2026May 19season not over yet

What these dates mean

The 130 days between the 50% light-frost dates in Redbird frame a middle-ground period when a gardener faces an even chance of frost—May 16 marks the median spring light-frost point and September 24 marks the median fall light-frost point. Those dates are not frost dates themselves, but they indicate the midpoint of the historical frost probability curve for this station.

A cautious planting plan in spring waits until May 31, when there is only a 10 % chance of a light frost occurring later, while a cautious harvest-finish date in fall is September 10, when there is only a 10 % chance that a light frost has already arrived. The hard-frost median dates, May 6 and October 2, show when a 28 °F frost is equally likely to have happened or not, and a hard frost matters more than a light frost because the lower temperature can damage or kill plants that survive the milder 32 °F event.

How to read the probabilities

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

  • Spring: "10% after May 31" 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 10" 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 Redbird, WYNumbered nearest stations: 1 — Dull Center, WY — 36 mi; 2 — Newcastle, WY — 42 mi; 3 — Hot Springs, SD — 43 mi; 4 — Harrison, NE — 44 mi; 5 — Wind Cave, SD — 46 mi.WYNE25 mi · 40 km1. Dull Center, WY — 36 mi12. Newcastle, WY — 42 mi23. Hot Springs, SD — 43 mi34. Harrison, NE — 44 mi45. Wind Cave, SD — 46 mi5
  1. Dull Center, WY 36 mi (58 km)
  2. Newcastle, WY 42 mi (68 km)
  3. Hot Springs, SD 43 mi (69 km)
  4. Harrison, NE 44 mi (70 km)
  5. Wind Cave, SD 46 mi (74 km)

Sources

* Hardiness zone 4a: zone derived from PRISM 2023 grid at the station location (not the official USDA map). Source data: PRISM Group, Oregon State University.

Station coordinates: 43.245, -104.2881.