U.S. Frost Dates by Weather Station

NOAA station USC00487388 · Wyoming

Frost Dates for Powell, WY

NOAA weather station Powell Field Station (USC00487388), Wyoming · elevation 4372 ft (1333 m) · hardiness zone 5a*

  • Lower-risk planting dateafter June 4only 1 spring in 10 still has a frost later
  • Typical last frostMay 1950% chance of a frost after it
  • Typical first fall frostSeptember 2050% chance of a frost before it
  • Frost-free season123 daystypical year, 32°F
  • Hardiness zone5acalculated*

The year at a glance

Frost season at Powell, WYSpring: a frost after May 5 in 9 years out of 10, after May 19 in half of the years, after June 4 in 1 year out of 10. Fall: the first frost comes before September 7 in 1 year out of 10, before September 20 in half of the years, before October 1 in 9 years out of 10. Typical frost-free season: 123 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 19First frost ~Sep 20Frost season at Powell, WYSpring: a frost after May 5 in 9 years out of 10, after May 19 in half of the years, after June 4 in 1 year out of 10. Fall: the first frost comes before September 7 in 1 year out of 10, before September 20 in half of the years, before October 1 in 9 years out of 10. Typical frost-free season: 123 days.JFMAMJJASONDLast frost ~May 19First frost ~Sep 20
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 4September 7
50% (typical)May 19September 20
90% (likely)May 5October 1
Frost-free season (32°F, 50% chance)
123 days
Hard frost (28°F), last in spring, 50% chance after
May 5
Hard frost (28°F), first in fall, 50% chance before
September 29

The last 15 years (2006–2020 normals)

Typical last frost May 20 (1 day later than the 30-year normal), typical first fall frost September 22 (2 days later). Frost-free season 123 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 Powell: chance of a frost after this dateSpring, chance of a frost after the date: 10% June 4, 20% May 29, 30% May 26, 40% May 23, 50% May 19, 60% May 16, 70% May 13, 80% May 9, 90% May 5. Observed last spring frost in recent years: 2021 May 21, 2022 May 22, 2023 May 20.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1Jun 4May 19May 52021: May 212022: May 222023: May 20Fall at Powell: chance of a frost before this dateFall, chance of a frost before the date: 10% September 7, 20% September 12, 30% September 15, 40% September 17, 50% September 20, 60% September 22, 70% September 24, 80% September 27, 90% October 1. Observed first fall frost in recent years: 2021 September 17, 2022 October 6, 2023 September 15.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 7Sep 20Oct 12021: September 172022: October 62023: September 15
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 21September 17
2022May 22October 6
2023May 20September 15
2024incomplete recordincomplete record
2025incomplete recordincomplete record
2026incomplete recordseason not over yet

What these dates mean

The cautious fall date in Powell for finishing the harvest is September 7, when there is only a 10 % chance a light frost has arrived. In spring the cautious date for setting out tender crops is June 4, with a 10 % chance of a later frost, while the 50 % date of May 19 marks the point where a light frost is equally likely to have occurred or not. The fall 50 % date of September 20 serves the same purpose for the end of the season, indicating an even odds chance of a light frost by that time.

For hard frost, the 50 % dates are May 5 in spring and September 29 in fall; a hard frost at 28 °F can damage plants more severely than a light frost at 32 °F because it reaches a lower temperature threshold. These dates, drawn from 26 years of NOAA records, help gardeners gauge the risk of frost for tender crops at Powell.

How to read the probabilities

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

  • Spring: "10% after June 4" 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 7" 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 Powell, WYNumbered nearest stations: 1 — Deaver, WY — 11 mi; 2 — Lovell, WY — 18 mi; 3 — Cody, WY — 26 mi; 4 — Emblem, WY — 26 mi; 5 — Basin, WY — 45 mi.WYMT25 mi · 40 km1. Deaver, WY — 11 mi12. Lovell, WY — 18 mi23. Cody, WY — 26 mi34. Emblem, WY — 26 mi45. Basin, WY — 45 mi5
  1. Deaver, WY 11 mi (18 km)
  2. Lovell, WY 18 mi (29 km)
  3. Cody, WY 26 mi (41 km)
  4. Emblem, WY 26 mi (43 km)
  5. Basin, WY 45 mi (72 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: 44.7764, -108.7592.