U.S. Frost Dates by Weather Station

NOAA station USC00488875 · Wyoming

Frost Dates for Thermopolis, WY

NOAA weather station Thermopolis (USC00488875), Wyoming · elevation 4321 ft (1317 m) · hardiness zone 5a*

  • Lower-risk planting dateafter May 19only 1 spring in 10 still has a frost later
  • Typical last frostMay 550% chance of a frost after it
  • Typical first fall frostOctober 550% chance of a frost before it
  • Frost-free season152 daystypical year, 32°F
  • Hardiness zone5acalculated*

The year at a glance

Frost season at Thermopolis, WYSpring: a frost after April 23 in 9 years out of 10, after May 5 in half of the years, after May 19 in 1 year out of 10. Fall: the first frost comes before September 23 in 1 year out of 10, before October 5 in half of the years, before October 18 in 9 years out of 10. Typical frost-free season: 152 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 5First frost ~Oct 5Frost season at Thermopolis, WYSpring: a frost after April 23 in 9 years out of 10, after May 5 in half of the years, after May 19 in 1 year out of 10. Fall: the first frost comes before September 23 in 1 year out of 10, before October 5 in half of the years, before October 18 in 9 years out of 10. Typical frost-free season: 152 days.JFMAMJJASONDLast frost ~May 5First frost ~Oct 5
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 30 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 19September 23
50% (typical)May 5October 5
90% (likely)April 23October 18
Frost-free season (32°F, 50% chance)
152 days
Hard frost (28°F), last in spring, 50% chance after
April 23
Hard frost (28°F), first in fall, 50% chance before
October 15

The last 15 years (2006–2020 normals)

Typical last frost May 5 (same as the 30-year normal), typical first fall frost October 8 (3 days later). Frost-free season 155 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 Thermopolis: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 19, 20% May 14, 30% May 11, 40% May 8, 50% May 5, 60% May 3, 70% April 30, 80% April 28, 90% April 23. Observed last spring frost in recent years: 2021 May 9, 2022 May 22, 2023 April 29, 2024 May 4, 2025 May 2, 2026 May 19.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 19May 5Apr 232021: May 92022: May 222023: April 292024: May 42025: May 22026: May 19Fall at Thermopolis: chance of a frost before this dateFall, chance of a frost before the date: 10% September 23, 20% September 27, 30% September 30, 40% October 2, 50% October 5, 60% October 7, 70% October 10, 80% October 14, 90% October 18. Observed first fall frost in recent years: 2021 September 30, 2022 October 14, 2023 October 25, 2024 October 1, 2025 October 13.Fall: first frostChance of a frost before this date10%50%90%Oct 1Nov 1Sep 23Oct 5Oct 182021: September 302022: October 142023: October 252024: October 12025: October 13
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 9September 30
2022May 22October 14
2023April 29October 25
2024May 4October 1
2025May 2October 13
2026May 19season not over yet

What these dates mean

May 19 is the cautious spring date for tender crops in Thermopolis; with only a 10 % chance of a light frost later, gardeners can plan to set out sensitive seedlings around that time.

The 50% light-frost dates are May 5 in spring and October 5 in fall, each marking an even-odds point for a frost later (spring) or earlier (fall). The hard-frost 50% dates are April 23 and October 15, indicating a 50 % chance of a 28 °F frost later in spring or earlier in fall. These median dates help gardeners gauge the midpoint of frost risk over the 30-year record.

September 23 is the cautious fall date for finishing tender crops, carrying only a 10 % chance that a light frost has arrived earlier. Hard frost at 28 °F can damage plants more severely than a light frost at 32 °F, so its timing is especially important.

How to read the probabilities

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

  • Spring: "10% after May 19" 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 23" 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 Thermopolis, WYNumbered nearest stations: 1 — Worland (USW00024062), WY — 25 mi; 2 — Worland (USC00489770), WY — 28 mi; 3 — Tensleep 16 SSE, WY — 43 mi; 4 — Tensleep 4 NE, WY — 50 mi; 5 — Basin, WY — 51 mi.25 mi · 40 km1. Worland (USW00024062), WY — 25 mi12. Worland (USC00489770), WY — 28 mi23. Tensleep 16 SSE, WY — 43 mi34. Tensleep 4 NE, WY — 50 mi45. Basin, WY — 51 mi5
  1. Worland (USW00024062), WY 25 mi (41 km)
  2. Worland (USC00489770), WY 28 mi (45 km)
  3. Tensleep 16 SSE, WY 43 mi (70 km)
  4. Tensleep 4 NE, WY 50 mi (81 km)
  5. Basin, WY 51 mi (82 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: 43.6483, -108.2036.