U.S. Frost Dates by Weather Station

NOAA station USC00050214 · Colorado

Frost Dates for Altenbern, CO

NOAA weather station Altenbern (USC00050214), Colorado · elevation 5637 ft (1718 m) · hardiness zone 6a*

  • Lower-risk planting dateafter June 13only 1 spring in 10 still has a frost later
  • Typical last frostMay 2550% chance of a frost after it
  • Typical first fall frostSeptember 2550% chance of a frost before it
  • Frost-free season120 daystypical year, 32°F
  • Hardiness zone6acalculated*

The year at a glance

Frost season at Altenbern, COSpring: a frost after May 10 in 9 years out of 10, after May 25 in half of the years, after June 13 in 1 year out of 10. Fall: the first frost comes before September 10 in 1 year out of 10, before September 25 in half of the years, before October 8 in 9 years out of 10. Typical frost-free season: 120 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 25First frost ~Sep 25Frost season at Altenbern, COSpring: a frost after May 10 in 9 years out of 10, after May 25 in half of the years, after June 13 in 1 year out of 10. Fall: the first frost comes before September 10 in 1 year out of 10, before September 25 in half of the years, before October 8 in 9 years out of 10. Typical frost-free season: 120 days.JFMAMJJASONDLast frost ~May 25First frost ~Sep 25
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 13September 10
50% (typical)May 25September 25
90% (likely)May 10October 8
Frost-free season (32°F, 50% chance)
120 days
Hard frost (28°F), last in spring, 50% chance after
May 8
Hard frost (28°F), first in fall, 50% chance before
October 4

The last 15 years (2006–2020 normals)

Typical last frost May 20 (5 days earlier than the 30-year normal), typical first fall frost September 30 (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 Altenbern: chance of a frost after this dateSpring, chance of a frost after the date: 10% June 13, 20% June 6, 30% June 2, 40% May 30, 50% May 25, 60% May 22, 70% May 18, 80% May 15, 90% May 10. Observed last spring frost in recent years: 2021 May 24, 2022 May 22, 2023 April 29, 2024 May 24, 2026 May 19.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1Jun 13May 25May 102021: May 242022: May 222023: April 292024: May 242026: May 19Fall at Altenbern: chance of a frost before this dateFall, chance of a frost before the date: 10% September 10, 20% September 16, 30% September 20, 40% September 23, 50% September 25, 60% September 27, 70% September 30, 80% October 3, 90% October 8. Observed first fall frost in recent years: 2021 September 22, 2022 October 14, 2023 October 10, 2024 October 22.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 10Sep 25Oct 82021: September 222022: October 142023: October 102024: October 22
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 24September 22
2022May 22October 14
2023April 29October 10
2024May 24October 22
2025incomplete recordincomplete record
2026May 19season not over yet

What these dates mean

In Altenbern the cautious fall date for finishing the harvest is September 10, when there is only a 10 % chance that a light frost has arrived earlier.

The 50% light-frost date in spring is May 25, and the 50% light-frost date in fall is September 25; at each of these dates the chance of a light frost is 50 %.

For tender crops the cautious spring date is June 13, which carries a 10 % chance of a frost later, while the cautious fall date is September 10, which carries a 10 % chance of a frost earlier. Hard-frost median dates are May 8 and October 4, and a hard frost at 28 °F can damage plants that survive a light frost at 32 °F, making it generally a more severe risk.

How to read the probabilities

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

  • Spring: "10% after June 13" 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 10" 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 Altenbern, CONumbered nearest stations: 1 — Grand Junction (Walker Field), CO — 27 mi; 2 — Palisade, CO — 27 mi; 3 — Grand Junction, CO — 32 mi; 4 — Meeker, CO — 45 mi; 5 — Rangely, CO — 46 mi.COUT25 mi · 40 km1. Grand Junction (Walker Field), CO — 27 mi12. Palisade, CO — 27 mi23. Grand Junction, CO — 32 mi34. Meeker, CO — 45 mi45. Rangely, CO — 46 mi5
  1. Grand Junction (Walker Field), CO 27 mi (43 km)
  2. Palisade, CO 27 mi (43 km)
  3. Grand Junction, CO 32 mi (51 km)
  4. Meeker, CO 45 mi (72 km)
  5. Rangely, CO 46 mi (74 km)

Sources

* Hardiness zone 6a: zone derived from PRISM 2023 grid at the station location (not the official USDA map). Source data: PRISM Group, Oregon State University.

Station coordinates: 39.4992, -108.3808.