U.S. Frost Dates by Weather Station

NOAA station USW00024089 · Wyoming

Frost Dates for Casper, WY

NOAA weather station Casper Natrona County Airport (USW00024089), Wyoming · elevation 5338 ft (1627 m) · hardiness zone 5a*

  • Lower-risk planting dateafter June 10only 1 spring in 10 still has a frost later
  • Typical last frostMay 2650% chance of a frost after it
  • Typical first fall frostSeptember 2250% chance of a frost before it
  • Frost-free season117 daystypical year, 32°F
  • Hardiness zone5acalculated*

The year at a glance

Frost season at Casper, WYSpring: a frost after May 12 in 9 years out of 10, after May 26 in half of the years, after June 10 in 1 year out of 10. Fall: the first frost comes before September 5 in 1 year out of 10, before September 22 in half of the years, before October 4 in 9 years out of 10. Typical frost-free season: 117 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 26First frost ~Sep 22Frost season at Casper, WYSpring: a frost after May 12 in 9 years out of 10, after May 26 in half of the years, after June 10 in 1 year out of 10. Fall: the first frost comes before September 5 in 1 year out of 10, before September 22 in half of the years, before October 4 in 9 years out of 10. Typical frost-free season: 117 days.JFMAMJJASONDLast frost ~May 26First frost ~Sep 22
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)June 10September 5
50% (typical)May 26September 22
90% (likely)May 12October 4
Frost-free season (32°F, 50% chance)
117 days
Hard frost (28°F), last in spring, 50% chance after
May 11
Hard frost (28°F), first in fall, 50% chance before
October 2

The last 15 years (2006–2020 normals)

Typical last frost May 26 (same as the 30-year normal), typical first fall frost September 19 (3 days earlier). Frost-free season 112 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 Casper: chance of a frost after this dateSpring, chance of a frost after the date: 10% June 10, 20% June 4, 30% June 1, 40% May 30, 50% May 26, 60% May 23, 70% May 20, 80% May 17, 90% May 12. Observed last spring frost in recent years: 2021 May 27, 2022 May 25, 2023 May 10, 2024 May 27, 2025 June 23, 2026 June 15.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1Jun 10May 26May 122021: May 272022: May 252023: May 102024: May 272025: June 232026: June 15Fall at Casper: chance of a frost before this dateFall, chance of a frost before the date: 10% September 5, 20% September 11, 30% September 15, 40% September 19, 50% September 22, 60% September 24, 70% September 28, 80% October 1, 90% October 4. Observed first fall frost in recent years: 2021 September 17, 2022 September 25, 2023 October 6, 2024 September 14, 2025 September 6.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 5Sep 22Oct 42021: September 172022: September 252023: October 62024: September 142025: September 6
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 27September 17
2022May 25September 25
2023May 10October 6
2024May 27September 14
2025June 23September 6
2026June 15season not over yet

What these dates mean

June 10 is the cautious spring light-frost date for Casper, carrying only a 10 % chance of a frost later and therefore allowing a gardener to set out tender crops with confidence. The median light-frost date, May 26, marks the point at which a frost is equally likely to occur later or not, while the early spring date, May 12, still carries a 90 % chance of frost and should be treated as a gamble.

In the fall, September 5 is the cautious light-frost date, giving a 10 % chance that a frost has already arrived and signalling when tender crops should be finished harvesting. The median fall light-frost date, September 22, again represents a 50 % chance of frost earlier. The hard-frost 50 % dates, May 11 in spring and October 2 in fall, are noted because a 28 °F frost can damage plants more severely than a 32 °F light frost. The interval between the two 32 °F 50 % dates is 117 days, based on 29 years of record.

How to read the probabilities

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

  • Spring: "10% after June 10" 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 5" 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 Casper, WYNumbered nearest stations: 1 — Bates Creek #2, WY — 20 mi; 2 — Midwest, WY — 37 mi; 3 — Kaycee, WY — 57 mi; 4 — Wright, WY — 69 mi; 5 — Jeffrey City, WY — 75 mi.WY25 mi · 40 km1. Bates Creek #2, WY — 20 mi12. Midwest, WY — 37 mi23. Kaycee, WY — 57 mi34. Wright, WY — 69 mi45. Jeffrey City, WY — 75 mi5
  1. Bates Creek #2, WY 20 mi (32 km)
  2. Midwest, WY 37 mi (59 km)
  3. Kaycee, WY 57 mi (92 km)
  4. Wright, WY 69 mi (111 km)
  5. Jeffrey City, WY 75 mi (120 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: 42.8975, -106.4636.