U.S. Frost Dates by Weather Station

NOAA station USC00480540 · Wyoming

Frost Dates for Basin, WY

NOAA weather station Basin (USC00480540), Wyoming · elevation 3840 ft (1170 m) · hardiness zone 4b*

  • Lower-risk planting dateafter May 11only 1 spring in 10 still has a frost later
  • Typical last frostApril 2850% chance of a frost after it
  • Typical first fall frostOctober 1050% chance of a frost before it
  • Frost-free season164 daystypical year, 32°F
  • Hardiness zone4bcalculated*

The year at a glance

Frost season at Basin, WYSpring: a frost after April 15 in 9 years out of 10, after April 28 in half of the years, after May 11 in 1 year out of 10. Fall: the first frost comes before September 26 in 1 year out of 10, before October 10 in half of the years, before October 22 in 9 years out of 10. Typical frost-free season: 164 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~Apr 28First frost ~Oct 10Frost season at Basin, WYSpring: a frost after April 15 in 9 years out of 10, after April 28 in half of the years, after May 11 in 1 year out of 10. Fall: the first frost comes before September 26 in 1 year out of 10, before October 10 in half of the years, before October 22 in 9 years out of 10. Typical frost-free season: 164 days.JFMAMJJASONDLast frost ~Apr 28First frost ~Oct 10
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)May 11September 26
50% (typical)April 28October 10
90% (likely)April 15October 22
Frost-free season (32°F, 50% chance)
164 days
Hard frost (28°F), last in spring, 50% chance after
April 17
Hard frost (28°F), first in fall, 50% chance before
October 20

The last 15 years (2006–2020 normals)

Typical last frost April 29 (1 day later than the 30-year normal), typical first fall frost October 13 (3 days later). Frost-free season 165 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 Basin: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 11, 20% May 6, 30% May 3, 40% April 30, 50% April 28, 60% April 26, 70% April 23, 80% April 20, 90% April 15. Observed last spring frost in recent years: 2021 May 10, 2022 April 30, 2023 April 28, 2024 May 24, 2026 May 19.Spring: last frostChance of a frost after this date10%50%90%Apr 1May 1May 11Apr 28Apr 152021: May 102022: April 302023: April 282024: May 242026: May 19Fall at Basin: chance of a frost before this dateFall, chance of a frost before the date: 10% September 26, 20% October 1, 30% October 4, 40% October 7, 50% October 10, 60% October 13, 70% October 15, 80% October 18, 90% October 22. Observed first fall frost in recent years: 2021 October 12, 2022 October 17, 2023 October 14, 2024 October 1, 2025 October 6.Fall: first frostChance of a frost before this date10%50%90%Oct 1Nov 1Sep 26Oct 10Oct 222021: October 122022: October 172023: October 142024: October 12025: October 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 10October 12
2022April 30October 17
2023April 28October 14
2024May 24October 1
2025incomplete recordOctober 6
2026May 19season not over yet

What these dates mean

September 26 is the cautious fall date for Basin, carrying only a 10 % chance that a light frost has arrived; it marks the point by which tender crops should be harvested to avoid a likely frost later in the season. The median light-frost date in fall is October 10, which carries a 50 % chance of a frost earlier than this date, while the hard-frost 50 % date is October 20.

In spring the cautious date for setting out tender crops is May 11, when the chance of a light frost later is just 10 %; the 50 % light-frost date is April 28, which carries a 50 % chance of a frost later than this date, and the hard-frost 50 % date is April 17.

A hard frost at 28 °F poses a greater threat than a light frost at 32 °F because it can damage plant tissue that would survive the milder temperature.

How to read the probabilities

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

  • Spring: "10% after May 11" 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 26" 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 Basin, WYNumbered nearest stations: 1 — Emblem, WY — 19 mi; 2 — Worland (USC00489770), WY — 26 mi; 3 — Worland (USW00024062), WY — 29 mi; 4 — Lovell, WY — 36 mi; 5 — Tensleep 4 NE, WY — 39 mi.MT25 mi · 40 km1. Emblem, WY — 19 mi12. Worland (USC00489770), WY — 26 mi23. Worland (USW00024062), WY — 29 mi34. Lovell, WY — 36 mi45. Tensleep 4 NE, WY — 39 mi5
  1. Emblem, WY 19 mi (31 km)
  2. Worland (USC00489770), WY 26 mi (41 km)
  3. Worland (USW00024062), WY 29 mi (47 km)
  4. Lovell, WY 36 mi (59 km)
  5. Tensleep 4 NE, WY 39 mi (63 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: 44.3797, -108.035.