U.S. Frost Dates by Weather Station

NOAA station USW00023061 · Colorado

Frost Dates near Alamosa, CO

NOAA weather station Alamosa San Luis Airport (USW00023061), Colorado · elevation 7533 ft (2296 m) · hardiness zone 4b*

Alamosa is the nearest town to the station (2.2 mi away) in GeoNames (CC BY 4.0). The dates below are measured at the station, not in the town itself.

  • Lower-risk planting dateafter June 24only 1 spring in 10 still has a frost later
  • Typical last frostJune 750% chance of a frost after it
  • Typical first fall frostSeptember 1550% chance of a frost before it
  • Frost-free season98 daystypical year, 32°F
  • Hardiness zone4bcalculated*

The year at a glance

Frost season near Alamosa, COSpring: a frost after May 24 in 9 years out of 10, after June 7 in half of the years, after June 24 in 1 year out of 10. Fall: the first frost comes before September 4 in 1 year out of 10, before September 15 in half of the years, before September 26 in 9 years out of 10. Typical frost-free season: 98 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~Jun 7First frost ~Sep 15Frost season near Alamosa, COSpring: a frost after May 24 in 9 years out of 10, after June 7 in half of the years, after June 24 in 1 year out of 10. Fall: the first frost comes before September 4 in 1 year out of 10, before September 15 in half of the years, before September 26 in 9 years out of 10. Typical frost-free season: 98 days.JFMAMJJASONDLast frost ~Jun 7First frost ~Sep 15
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 29 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)June 24September 4
50% (typical)June 7September 15
90% (likely)May 24September 26
Frost-free season (32°F, 50% chance)
98 days
Hard frost (28°F), last in spring, 50% chance after
May 23
Hard frost (28°F), first in fall, 50% chance before
September 25

The last 15 years (2006–2020 normals)

Typical last frost June 5 (2 days earlier than the 30-year normal), typical first fall frost September 17 (2 days later). Frost-free season 101 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 near Alamosa: chance of a frost after this dateSpring, chance of a frost after the date: 10% June 24, 20% June 18, 30% June 13, 40% June 10, 50% June 7, 60% June 4, 70% June 2, 80% May 30, 90% May 24. Observed last spring frost in recent years: 2021 May 28, 2022 June 2, 2023 June 24, 2024 May 30, 2025 May 27, 2026 June 1.Spring: last frostChance of a frost after this date10%50%90%Jun 1Jul 1Jun 24Jun 7May 242021: May 282022: June 22023: June 242024: May 302025: May 272026: June 1Fall near Alamosa: chance of a frost before this dateFall, chance of a frost before the date: 10% September 4, 20% September 7, 30% September 11, 40% September 13, 50% September 15, 60% September 17, 70% September 20, 80% September 22, 90% September 26. Observed first fall frost in recent years: 2021 September 17, 2022 September 12, 2023 September 20, 2024 September 12, 2025 September 19.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 4Sep 15Sep 262021: September 172022: September 122023: September 202024: September 122025: September 19
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 28September 17
2022June 2September 12
2023June 24September 20
2024May 30September 12
2025May 27September 19
2026June 1season not over yet

What these dates mean

Setting out tender crops on May 24 in the Alamosa area carries a 90% chance of a light frost and is therefore a gamble for gardeners. The cautious spring date, June 24, has only a 10% chance of a frost later than that day, offering a much lower risk for planting.

June 7 is the 50% date for light frost in spring and September 15 is the 50% date in fall; each marks the point where the chance of a frost is even, but they are not frost dates themselves. For a careful schedule, gardeners use the cautious June 24 date to set out tender crops in spring and the cautious September 4 date, which has a 10% chance of a frost earlier, to finish harvesting in fall.

A hard frost at 28 °F, noted by the median dates May 23 and September 25, can damage plants more severely than a light frost at 32 °F.

How to read the probabilities

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

  • Spring: "10% after June 24" 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 4" 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 Alamosa, CONumbered nearest stations: 1 — Monte Vista, CO — 20 mi; 2 — Great Sand Dunes National Park & Preserve, CO — 28 mi; 3 — Sheep Mountain, CO — 39 mi; 4 — Westcliffe, CO — 52 mi; 5 — Chama, NM — 53 mi.CONM25 mi · 40 km1. Monte Vista, CO — 20 mi12. Great Sand Dunes National Park & Preserve, CO — 28 mi23. Sheep Mountain, CO — 39 mi34. Westcliffe, CO — 52 mi45. Chama, NM — 53 mi5
  1. Monte Vista, CO 20 mi (33 km)
  2. Great Sand Dunes National Park & Preserve, CO 28 mi (45 km)
  3. Sheep Mountain, CO 39 mi (63 km)
  4. Westcliffe, CO 52 mi (84 km)
  5. Chama, NM 53 mi (86 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: 37.4389, -105.8614.