U.S. Frost Dates by Weather Station

NOAA station USC00486195 · Wyoming

Frost Dates for Midwest, WY

NOAA weather station Midwest (USC00486195), Wyoming · elevation 4860 ft (1481 m) · hardiness zone 5a*

  • Lower-risk planting dateafter June 6only 1 spring in 10 still has a frost later
  • Typical last frostMay 2250% chance of a frost after it
  • Typical first fall frostSeptember 2250% chance of a frost before it
  • Frost-free season121 daystypical year, 32°F
  • Hardiness zone5acalculated*

The year at a glance

Frost season at Midwest, WYSpring: a frost after May 8 in 9 years out of 10, after May 22 in half of the years, after June 6 in 1 year out of 10. Fall: the first frost comes before September 8 in 1 year out of 10, before September 22 in half of the years, before October 5 in 9 years out of 10. Typical frost-free season: 121 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 22First frost ~Sep 22Frost season at Midwest, WYSpring: a frost after May 8 in 9 years out of 10, after May 22 in half of the years, after June 6 in 1 year out of 10. Fall: the first frost comes before September 8 in 1 year out of 10, before September 22 in half of the years, before October 5 in 9 years out of 10. Typical frost-free season: 121 days.JFMAMJJASONDLast frost ~May 22First 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 26 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)June 6September 8
50% (typical)May 22September 22
90% (likely)May 8October 5
Frost-free season (32°F, 50% chance)
121 days
Hard frost (28°F), last in spring, 50% chance after
May 9
Hard frost (28°F), first in fall, 50% chance before
October 2

The last 15 years (2006–2020 normals)

Typical last frost May 23 (1 day later than the 30-year normal), typical first fall frost September 23 (1 day later). Frost-free season 122 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 Midwest: chance of a frost after this dateSpring, chance of a frost after the date: 10% June 6, 20% June 1, 30% May 29, 40% May 25, 50% May 22, 60% May 19, 70% May 16, 80% May 13, 90% May 8. Observed last spring frost in recent years: 2021 May 25, 2022 May 24, 2024 May 22.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1Jun 6May 22May 82021: May 252022: May 242024: May 22Fall at Midwest: chance of a frost before this dateFall, chance of a frost before the date: 10% September 8, 20% September 12, 30% September 16, 40% September 19, 50% September 22, 60% September 24, 70% September 27, 80% October 1, 90% October 5. Observed first fall frost in recent years: 2021 September 30, 2022 October 9, 2024 October 1.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 8Sep 22Oct 52021: September 302022: October 92024: October 1
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 25September 30
2022May 24October 9
2023incomplete recordincomplete record
2024May 22October 1
2025incomplete recordincomplete record
2026incomplete recordseason not over yet

What these dates mean

Setting out tender crops on the early spring date, May 8, is a gamble because there is a 90 % chance a light frost will follow later in the season. The cautious alternative for planting is June 6, when only a 10 % chance of a later frost remains.

The 50 % date in spring, May 22, indicates a 50 % chance of a light frost later than that date, while the 50 % date in fall, September 22, indicates a 50 % chance of a light frost earlier than that date. Gardeners who prefer a lower risk finish harvesting tender crops by the cautious fall date, September 8, which carries only a 10 % chance that a frost has already arrived.

Hard frost at 28 °F (the May 9 and October 2 50 % dates) can injure plants that survive a 32 °F light frost, so it represents a more critical temperature threshold.

How to read the probabilities

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

  • Spring: "10% after June 6" 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 8" 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 Midwest, WYNumbered nearest stations: 1 — Kaycee, WY — 28 mi; 2 — Wright, WY — 36 mi; 3 — Casper, WY — 37 mi; 4 — Bates Creek #2, WY — 55 mi; 5 — Tensleep 16 SSE, WY — 61 mi.WY25 mi · 40 km1. Kaycee, WY — 28 mi12. Wright, WY — 36 mi23. Casper, WY — 37 mi34. Bates Creek #2, WY — 55 mi45. Tensleep 16 SSE, WY — 61 mi5
  1. Kaycee, WY 28 mi (44 km)
  2. Wright, WY 36 mi (57 km)
  3. Casper, WY 37 mi (59 km)
  4. Bates Creek #2, WY 55 mi (88 km)
  5. Tensleep 16 SSE, WY 61 mi (98 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: 43.4131, -106.2772.