U.S. Frost Dates by Weather Station

NOAA station USC00051179 · Colorado

Frost Dates for Byers, CO

NOAA weather station Byers 5 ENE (USC00051179), Colorado · elevation 5101 ft (1555 m) · hardiness zone 5b*

  • Lower-risk planting dateafter May 30only 1 spring in 10 still has a frost later
  • Typical last frostMay 1350% chance of a frost after it
  • Typical first fall frostOctober 250% chance of a frost before it
  • Frost-free season142 daystypical year, 32°F
  • Hardiness zone5bcalculated*

The year at a glance

Frost season at Byers, COSpring: a frost after April 30 in 9 years out of 10, after May 13 in half of the years, after May 30 in 1 year out of 10. Fall: the first frost comes before September 20 in 1 year out of 10, before October 2 in half of the years, before October 14 in 9 years out of 10. Typical frost-free season: 142 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 13First frost ~Oct 2Frost season at Byers, COSpring: a frost after April 30 in 9 years out of 10, after May 13 in half of the years, after May 30 in 1 year out of 10. Fall: the first frost comes before September 20 in 1 year out of 10, before October 2 in half of the years, before October 14 in 9 years out of 10. Typical frost-free season: 142 days.JFMAMJJASONDLast frost ~May 13First frost ~Oct 2
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.Today

Frost dates (32°F)

Probability of frost, based on 25 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 30September 20
50% (typical)May 13October 2
90% (likely)April 30October 14
Frost-free season (32°F, 50% chance)
142 days
Hard frost (28°F), last in spring, 50% chance after
May 2
Hard frost (28°F), first in fall, 50% chance before
October 10

The last 15 years (2006–2020 normals)

Typical last frost May 13 (same as the 30-year normal), typical first fall frost October 5 (3 days later). Frost-free season 142 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 Byers: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 30, 20% May 23, 30% May 18, 40% May 15, 50% May 13, 60% May 10, 70% May 7, 80% May 4, 90% April 30.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 30May 13Apr 30Fall at Byers: chance of a frost before this dateFall, chance of a frost before the date: 10% September 20, 20% September 25, 30% September 28, 40% October 1, 50% October 2, 60% October 4, 70% October 7, 80% October 10, 90% October 14.Fall: first frostChance of a frost before this date10%50%90%Oct 1Sep 20Oct 2Oct 14
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

The 142 days between the 50% dates in Byers, May 13 and October 2, frame the period when light frosts are less than even odds. Within that span gardeners can expect a relatively low risk of a 32 °F frost, though occasional colder events may still occur.

The 50% dates themselves are not frost dates; they simply mark the median point where a light frost has a 50 % chance of occurring later (spring) or earlier (fall). For a cautious plan a gardener would wait until the late-spring date of May 30, which carries only a 10 % chance of a frost later, and would aim to finish tender-crop harvest by the early-fall date of September 20, which carries only a 10 % chance of a frost earlier. Planting before April 30 or extending into October 14 would be a gamble with a 90 % chance of frost.

Hard frosts at 28 °F, represented by the median dates May 2 and October 10, can damage plants more severely than the lighter 32 °F frosts because the lower temperature can freeze tissue that survives a light frost.

How to read the probabilities

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

  • Spring: "10% after May 30" 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 20" 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 Byers, CONumbered nearest stations: 1 — near Aurora, CO — 39 mi; 2 — Brighton, CO — 40 mi; 3 — near Limon, CO — 44 mi; 4 — Castle Rock, CO — 47 mi; 5 — Northglenn, CO — 48 mi.CO25 mi · 40 km1. near Aurora, CO — 39 mi12. Brighton, CO — 40 mi23. near Limon, CO — 44 mi34. Castle Rock, CO — 47 mi45. Northglenn, CO — 48 mi5
  1. near Aurora, CO 39 mi (63 km)
  2. Brighton, CO 40 mi (65 km)
  3. near Limon, CO 44 mi (71 km)
  4. Castle Rock, CO 47 mi (76 km)
  5. Northglenn, CO 48 mi (78 km)

Sources

* Hardiness zone 5b: zone derived from PRISM 2023 grid at the station location (not the official USDA map). Source data: PRISM Group, Oregon State University.

Station coordinates: 39.7406, -104.1275.