U.S. Frost Dates by Weather Station

NOAA station USC00481730 · Wyoming

Frost Dates for Chugwater, WY

NOAA weather station Chugwater (USC00481730), Wyoming · elevation 5304 ft (1617 m) · hardiness zone 5a*

  • Lower-risk planting dateafter June 2only 1 spring in 10 still has a frost later
  • Typical last frostMay 1850% chance of a frost after it
  • Typical first fall frostSeptember 2450% chance of a frost before it
  • Frost-free season129 daystypical year, 32°F
  • Hardiness zone5acalculated*

The year at a glance

Frost season at Chugwater, WYSpring: a frost after May 5 in 9 years out of 10, after May 18 in half of the years, after June 2 in 1 year out of 10. Fall: the first frost comes before September 9 in 1 year out of 10, before September 24 in half of the years, before October 8 in 9 years out of 10. Typical frost-free season: 129 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 18First frost ~Sep 24Frost season at Chugwater, WYSpring: a frost after May 5 in 9 years out of 10, after May 18 in half of the years, after June 2 in 1 year out of 10. Fall: the first frost comes before September 9 in 1 year out of 10, before September 24 in half of the years, before October 8 in 9 years out of 10. Typical frost-free season: 129 days.JFMAMJJASONDLast frost ~May 18First frost ~Sep 24
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 28 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)June 2September 9
50% (typical)May 18September 24
90% (likely)May 5October 8
Frost-free season (32°F, 50% chance)
129 days
Hard frost (28°F), last in spring, 50% chance after
May 7
Hard frost (28°F), first in fall, 50% chance before
October 4

The last 15 years (2006–2020 normals)

Typical last frost May 17 (1 day earlier than the 30-year normal), typical first fall frost September 29 (5 days later). Frost-free season 130 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 Chugwater: chance of a frost after this dateSpring, chance of a frost after the date: 10% June 2, 20% May 29, 30% May 25, 40% May 21, 50% May 18, 60% May 16, 70% May 13, 80% May 9, 90% May 5.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1Jun 2May 18May 5Fall at Chugwater: chance of a frost before this dateFall, chance of a frost before the date: 10% September 9, 20% September 15, 30% September 18, 40% September 21, 50% September 24, 60% September 28, 70% October 1, 80% October 3, 90% October 8.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 9Sep 24Oct 8
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.

What these dates mean

Setting out tender crops on the early spring date, May 5, carries a 90 % chance of a frost later that season in Chugwater, making it a high-risk gamble. The safer alternative for early planting is the cautious date of June 2, which has only a 10 % chance of a frost occurring after it.

For a gardener, the 50 % dates—May 18 in spring and September 24 in fall—represent the point at which a light frost is equally likely to have happened or not; they are not frost dates themselves but a statistical midpoint of the record. The cautious plan therefore uses June 2 to start tender crops and September 9 to finish harvesting them, each with only a 10 % chance of a frost occurring before (fall) or after (spring) the date. The period between the two 32 °F 50 % dates is 129 days, based on 28 years of data.

Because a hard frost at 28 °F can damage plant tissue more severely than a light frost at 32 °F, the hard-frost median dates of May 7 and October 4 are important reference points.

How to read the probabilities

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

  • Spring: "10% after June 2" 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 9" 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 Chugwater, WYNumbered nearest stations: 1 — Wheatland, WY — 25 mi; 2 — Old Fort Laramie, WY — 34 mi; 3 — Cheyenne, WY — 42 mi; 4 — Harrisburg, NE — 46 mi; 5 — Laramie, WY — 54 mi.WYNE25 mi · 40 km1. Wheatland, WY — 25 mi12. Old Fort Laramie, WY — 34 mi23. Cheyenne, WY — 42 mi34. Harrisburg, NE — 46 mi45. Laramie, WY — 54 mi5
  1. Wheatland, WY 25 mi (40 km)
  2. Old Fort Laramie, WY 34 mi (54 km)
  3. Cheyenne, WY 42 mi (68 km)
  4. Harrisburg, NE 46 mi (73 km)
  5. Laramie, WY 54 mi (87 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: 41.7594, -104.8222.