U.S. Frost Dates by Weather Station

NOAA station USC00485055 · Wyoming

Frost Dates for Kaycee, WY

NOAA weather station Kaycee (USC00485055), Wyoming · elevation 4660 ft (1420 m) · hardiness zone 4b*

  • Lower-risk planting dateafter May 31only 1 spring in 10 still has a frost later
  • Typical last frostMay 1850% chance of a frost after it
  • Typical first fall frostSeptember 2550% chance of a frost before it
  • Frost-free season129 daystypical year, 32°F
  • Hardiness zone4bcalculated*

The year at a glance

Frost season at Kaycee, WYSpring: a frost after May 5 in 9 years out of 10, after May 18 in half of the years, after May 31 in 1 year out of 10. Fall: the first frost comes before September 11 in 1 year out of 10, before September 25 in half of the years, before October 9 in 9 years out of 10. Typical frost-free season: 129 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 18First frost ~Sep 25Frost season at Kaycee, WYSpring: a frost after May 5 in 9 years out of 10, after May 18 in half of the years, after May 31 in 1 year out of 10. Fall: the first frost comes before September 11 in 1 year out of 10, before September 25 in half of the years, before October 9 in 9 years out of 10. Typical frost-free season: 129 days.JFMAMJJASONDLast frost ~May 18First frost ~Sep 25
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 31September 11
50% (typical)May 18September 25
90% (likely)May 5October 9
Frost-free season (32°F, 50% chance)
129 days
Hard frost (28°F), last in spring, 50% chance after
May 6
Hard frost (28°F), first in fall, 50% chance before
October 5

The last 15 years (2006–2020 normals)

Typical last frost May 17 (1 day earlier than the 30-year normal), typical first fall frost September 30 (5 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 Kaycee: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 31, 20% May 26, 30% May 23, 40% May 20, 50% May 18, 60% May 15, 70% May 12, 80% May 9, 90% May 5. Observed last spring frost in recent years: 2021 May 13, 2022 May 23, 2023 May 2, 2024 May 23, 2025 May 3, 2026 May 20.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 31May 18May 52021: May 132022: May 232023: May 22024: May 232025: May 32026: May 20Fall at Kaycee: chance of a frost before this dateFall, chance of a frost before the date: 10% September 11, 20% September 16, 30% September 19, 40% September 22, 50% September 25, 60% September 28, 70% October 1, 80% October 4, 90% October 9. Observed first fall frost in recent years: 2021 September 22, 2022 October 9, 2023 October 7, 2024 October 6, 2025 October 7.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 11Sep 25Oct 92021: September 222022: October 92023: October 72024: October 62025: October 7
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 13September 22
2022May 23October 9
2023May 2October 7
2024May 23October 6
2025May 3October 7
2026May 20season not over yet

What these dates mean

The 129 days between the 50% dates in Kaycee, Wyoming (May 18 and September 25) define the period in which a light frost is equally likely to have occurred or not. For a gardener this median window marks the span with an even odds of a 32 °F frost, providing a statistical reference for planning activities that are sensitive to any frost.

In spring the cautious date for setting out tender crops is May 31, which carries a 10 % chance of a light frost later; planting on May 5 carries a 90 % chance and is a gamble. In fall the cautious date to finish harvesting tender crops is September 11, with a 10 % chance of a light frost earlier; waiting until October 9 carries a 90 % chance and is a gamble. The 50% light-frost dates themselves—May 18 and September 25—are not frost dates, and the hard-frost median dates are May 6 and October 5.

Because a hard frost reaches 28 °F it can damage plants that survive a light frost, making the hard-frost dates a more critical threshold for winter survival.

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 11" 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 Kaycee, WYNumbered nearest stations: 1 — Midwest, WY — 28 mi; 2 — Tensleep 16 SSE, WY — 37 mi; 3 — Tensleep 4 NE, WY — 44 mi; 4 — Buffalo, WY — 44 mi; 5 — Wright, WY — 45 mi.WY25 mi · 40 km1. Midwest, WY — 28 mi12. Tensleep 16 SSE, WY — 37 mi23. Tensleep 4 NE, WY — 44 mi34. Buffalo, WY — 44 mi45. Wright, WY — 45 mi5
  1. Midwest, WY 28 mi (44 km)
  2. Tensleep 16 SSE, WY 37 mi (59 km)
  3. Tensleep 4 NE, WY 44 mi (71 km)
  4. Buffalo, WY 44 mi (71 km)
  5. Wright, WY 45 mi (73 km)

Sources

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

Station coordinates: 43.7144, -106.6378.