U.S. Frost Dates by Weather Station

NOAA station USC00489205 · Wyoming

Frost Dates for Upton, WY

NOAA weather station Upton (USC00489205), Wyoming · elevation 4320 ft (1317 m) · hardiness zone 4b*

  • Lower-risk planting dateafter June 4only 1 spring in 10 still has a frost later
  • Typical last frostMay 2050% chance of a frost after it
  • Typical first fall frostSeptember 2350% chance of a frost before it
  • Frost-free season123 daystypical year, 32°F
  • Hardiness zone4bcalculated*

The year at a glance

Frost season at Upton, WYSpring: a frost after May 8 in 9 years out of 10, after May 20 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 23 in half of the years, before October 5 in 9 years out of 10. Typical frost-free season: 123 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 20First frost ~Sep 23Frost season at Upton, WYSpring: a frost after May 8 in 9 years out of 10, after May 20 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 23 in half of the years, before October 5 in 9 years out of 10. Typical frost-free season: 123 days.JFMAMJJASONDLast frost ~May 20First frost ~Sep 23
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 27 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)June 4September 7
50% (typical)May 20September 23
90% (likely)May 8October 5
Frost-free season (32°F, 50% chance)
123 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 20 (same as the 30-year normal), typical first fall frost September 25 (2 days later). Frost-free season 125 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 Upton: chance of a frost after this dateSpring, chance of a frost after the date: 10% June 4, 20% May 31, 30% May 27, 40% May 23, 50% May 20, 60% May 18, 70% May 15, 80% May 12, 90% May 8. Observed last spring frost in recent years: 2021 May 24, 2022 May 23, 2023 May 8, 2024 June 19, 2025 May 22, 2026 May 19.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1Jun 4May 20May 82021: May 242022: May 232023: May 82024: June 192025: May 222026: May 19Fall at Upton: chance of a frost before this dateFall, chance of a frost before the date: 10% September 7, 20% September 13, 30% September 16, 40% September 20, 50% September 23, 60% September 26, 70% September 28, 80% October 1, 90% October 5. Observed first fall frost in recent years: 2021 October 12, 2022 October 8, 2023 September 21, 2024 October 23, 2025 September 5.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 7Sep 23Oct 52021: October 122022: October 82023: September 212024: October 232025: September 5
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 24October 12
2022May 23October 8
2023May 8September 21
2024June 19October 23
2025May 22September 5
2026May 19season not over yet

What these dates mean

June 4 is the cautious spring date, with only a ten percent chance that a light frost will occur later; this allows a gardener to set out tender crops with minimal risk. The early-spring gamble falls on May 8, when a ninety percent chance of frost still exists.

The 50% spring date is May 20, indicating a fifty percent chance of a light frost later than this day, while the 50% fall date is September 23, indicating a fifty percent chance of a light frost earlier than this day. The cautious fall date is September 7, when there is only a ten percent chance that a light frost has already arrived, marking the latest realistic point to finish harvesting tender crops. Hard-frost median dates are May 9 in spring and October 2 in fall, and a hard frost at 28 °F can damage plants that survive a 32 °F light frost.

The record spans twenty-seven years, and the interval between the two 32 °F 50% dates is 123 days.

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 Upton, WYNumbered nearest stations: 1 — Dillinger, WY — 25 mi; 2 — Newcastle, WY — 27 mi; 3 — Devils Tower #2, WY — 34 mi; 4 — Spearfish, SD — 46 mi; 5 — Lead, SD — 47 mi.WYSD25 mi · 40 km1. Dillinger, WY — 25 mi12. Newcastle, WY — 27 mi23. Devils Tower #2, WY — 34 mi34. Spearfish, SD — 46 mi45. Lead, SD — 47 mi5
  1. Dillinger, WY 25 mi (40 km)
  2. Newcastle, WY 27 mi (43 km)
  3. Devils Tower #2, WY 34 mi (55 km)
  4. Spearfish, SD 46 mi (74 km)
  5. Lead, SD 47 mi (75 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: 44.0928, -104.6114.