U.S. Frost Dates by Weather Station

NOAA station USC00053500 · Colorado

Frost Dates near Granby, CO

NOAA weather station Grand Lake 6 SSW (USC00053500), Colorado · elevation 8288 ft (2526 m) · hardiness zone 4a*

Granby is the nearest town to the station (7.8 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 28only 1 spring in 10 still has a frost later
  • Typical last frostJune 1350% chance of a frost after it
  • Typical first fall frostSeptember 750% chance of a frost before it
  • Frost-free season84 daystypical year, 32°F
  • Hardiness zone4acalculated*

The year at a glance

Frost season near Granby, COSpring: a frost after June 1 in 9 years out of 10, after June 13 in half of the years, after June 28 in 1 year out of 10. Fall: the first frost comes before August 21 in 1 year out of 10, before September 7 in half of the years, before September 21 in 9 years out of 10. Typical frost-free season: 84 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~Jun 13First frost ~Sep 7Frost season near Granby, COSpring: a frost after June 1 in 9 years out of 10, after June 13 in half of the years, after June 28 in 1 year out of 10. Fall: the first frost comes before August 21 in 1 year out of 10, before September 7 in half of the years, before September 21 in 9 years out of 10. Typical frost-free season: 84 days.JFMAMJJASONDLast frost ~Jun 13First frost ~Sep 7
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 28August 21
50% (typical)June 13September 7
90% (likely)June 1September 21
Frost-free season (32°F, 50% chance)
84 days
Hard frost (28°F), last in spring, 50% chance after
May 26
Hard frost (28°F), first in fall, 50% chance before
September 22

The last 15 years (2006–2020 normals)

Typical last frost June 11 (2 days earlier than the 30-year normal), typical first fall frost September 10 (3 days later). Frost-free season 90 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 Granby: chance of a frost after this dateSpring, chance of a frost after the date: 10% June 28, 20% June 23, 30% June 19, 40% June 17, 50% June 13, 60% June 10, 70% June 8, 80% June 5, 90% June 1. Observed last spring frost in recent years: 2021 June 1, 2022 June 21, 2024 May 31, 2025 May 27.Spring: last frostChance of a frost after this date10%50%90%Jun 1Jul 1Jun 28Jun 13Jun 12021: June 12022: June 212024: May 312025: May 27Fall near Granby: chance of a frost before this dateFall, chance of a frost before the date: 10% August 21, 20% August 29, 30% September 2, 40% September 5, 50% September 7, 60% September 10, 70% September 13, 80% September 16, 90% September 21. Observed first fall frost in recent years: 2021 September 21, 2022 September 11, 2023 September 16, 2024 September 19.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Aug 21Sep 7Sep 212021: September 212022: September 112023: September 162024: 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
2021June 1September 21
2022June 21September 11
2023incomplete recordSeptember 16
2024May 31September 19
2025May 27incomplete record
2026incomplete recordseason not over yet

What these dates mean

August 21 is the cautious fall date for the Granby area, offering only a 10 % chance that a light frost has arrived earlier and therefore marking a reliable point to finish harvesting tender crops.

In spring the cautious date for setting out tender crops is June 28, when the probability of a light frost later than that day has dropped to 10 %; planting on June 1 carries a 90 % chance of frost and is a gamble. The 50 % date for light frost is June 13, indicating an even odds of a frost occurring later than that day, while the 50 % fall date is September 7, indicating an even odds of a frost occurring earlier than that day. Hard-frost median dates are May 26 in spring and September 22 in fall.

A hard frost at 28 °F 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 28" 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 August 21" 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 Granby, CONumbered nearest stations: 1 — Kremmling, CO — 28 mi; 2 — near Coal Creek, CO — 32 mi; 3 — Georgetown, CO — 34 mi; 4 — Boulder, CO — 34 mi; 5 — Rustic, CO — 37 mi.WY25 mi · 40 km1. Kremmling, CO — 28 mi12. near Coal Creek, CO — 32 mi23. Georgetown, CO — 34 mi34. Boulder, CO — 34 mi45. Rustic, CO — 37 mi5
  1. Kremmling, CO 28 mi (45 km)
  2. near Coal Creek, CO 32 mi (52 km)
  3. Georgetown, CO 34 mi (55 km)
  4. Boulder, CO 34 mi (55 km)
  5. Rustic, CO 37 mi (59 km)

Sources

* Hardiness zone 4a: zone derived from PRISM 2023 grid at the station location (not the official USDA map). Source data: PRISM Group, Oregon State University.

Station coordinates: 40.185, -105.8667.