U.S. Frost Dates by Weather Station

NOAA station USC00050950 · Colorado

Frost Dates for Brighton, CO

NOAA weather station Brighton 3 SE (USC00050950), Colorado · elevation 5016 ft (1529 m) · hardiness zone 5b*

  • Lower-risk planting dateafter May 16only 1 spring in 10 still has a frost later
  • Typical last frostMay 350% chance of a frost after it
  • Typical first fall frostOctober 750% chance of a frost before it
  • Frost-free season155 daystypical year, 32°F
  • Hardiness zone5bcalculated*

The year at a glance

Frost season at Brighton, COSpring: a frost after April 22 in 9 years out of 10, after May 3 in half of the years, after May 16 in 1 year out of 10. Fall: the first frost comes before September 26 in 1 year out of 10, before October 7 in half of the years, before October 19 in 9 years out of 10. Typical frost-free season: 155 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 3First frost ~Oct 7Frost season at Brighton, COSpring: a frost after April 22 in 9 years out of 10, after May 3 in half of the years, after May 16 in 1 year out of 10. Fall: the first frost comes before September 26 in 1 year out of 10, before October 7 in half of the years, before October 19 in 9 years out of 10. Typical frost-free season: 155 days.JFMAMJJASONDLast frost ~May 3First frost ~Oct 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 25 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 16September 26
50% (typical)May 3October 7
90% (likely)April 22October 19
Frost-free season (32°F, 50% chance)
155 days
Hard frost (28°F), last in spring, 50% chance after
April 23
Hard frost (28°F), first in fall, 50% chance before
October 16

The last 15 years (2006–2020 normals)

Typical last frost May 5 (2 days later than the 30-year normal), typical first fall frost October 11 (4 days later). Frost-free season 157 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 Brighton: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 16, 20% May 12, 30% May 8, 40% May 6, 50% May 3, 60% May 1, 70% April 30, 80% April 27, 90% April 22. Observed last spring frost in recent years: 2021 May 12, 2022 May 21, 2023 April 24, 2024 April 21, 2025 April 30, 2026 May 7.Spring: last frostChance of a frost after this date10%50%90%May 1May 16May 3Apr 222021: May 122022: May 212023: April 242024: April 212025: April 302026: May 7Fall at Brighton: chance of a frost before this dateFall, chance of a frost before the date: 10% September 26, 20% September 30, 30% October 2, 40% October 5, 50% October 7, 60% October 10, 70% October 13, 80% October 16, 90% October 19. Observed first fall frost in recent years: 2021 October 13, 2022 October 17, 2023 October 14, 2024 October 25, 2025 October 19.Fall: first frostChance of a frost before this date10%50%90%Oct 1Nov 1Sep 26Oct 7Oct 192021: October 132022: October 172023: October 142024: October 252025: October 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 12October 13
2022May 21October 17
2023April 24October 14
2024April 21October 25
2025April 30October 19
2026May 7season not over yet

What these dates mean

Setting out tender crops on the early spring date, April 22, carries a 90% chance of a light frost, making it a gamble for growers in Brighton. The cautious alternative is the later date, May 16, where only a 10% chance of frost remains.

The 50% spring date, May 3, marks the point where the odds of a frost later than that day are even, while the 50% fall date, October 7, indicates an even chance of a frost earlier than that day. For a careful schedule, the conservative fall date is September 26, which has only a 10% chance that a frost has already arrived. The median hard-frost dates are April 23 in spring and October 16 in fall; a hard frost at 28 °F can injure plants that survive a light frost at 32 °F, so it represents a more severe threat.

How to read the probabilities

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

  • Spring: "10% after May 16" 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 26" 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 Brighton, CONumbered nearest stations: 1 — Northglenn, CO — 10 mi; 2 — near Aurora, CO — 13 mi; 3 — Wheat Ridge, CO — 19 mi; 4 — Boulder, CO — 23 mi; 5 — near Coal Creek, CO — 29 mi.25 mi · 40 km1. Northglenn, CO — 10 mi12. near Aurora, CO — 13 mi23. Wheat Ridge, CO — 19 mi34. Boulder, CO — 23 mi45. near Coal Creek, CO — 29 mi5
  1. Northglenn, CO 10 mi (16 km)
  2. near Aurora, CO 13 mi (20 km)
  3. Wheat Ridge, CO 19 mi (30 km)
  4. Boulder, CO 23 mi (37 km)
  5. near Coal Creek, CO 29 mi (47 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: 39.9436, -104.8361.