U.S. Frost Dates by Weather Station

NOAA station USC00055984 · Colorado

Frost Dates for Northglenn, CO

NOAA weather station Northglenn (USC00055984), Colorado · elevation 5407 ft (1648 m) · hardiness zone 6a*

  • Lower-risk planting dateafter May 14only 1 spring in 10 still has a frost later
  • Typical last frostMay 250% chance of a frost after it
  • Typical first fall frostOctober 1150% chance of a frost before it
  • Frost-free season160 daystypical year, 32°F
  • Hardiness zone6acalculated*

The year at a glance

Frost season at Northglenn, COSpring: a frost after April 18 in 9 years out of 10, after May 2 in half of the years, after May 14 in 1 year out of 10. Fall: the first frost comes before September 26 in 1 year out of 10, before October 11 in half of the years, before October 24 in 9 years out of 10. Typical frost-free season: 160 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 2First frost ~Oct 11Frost season at Northglenn, COSpring: a frost after April 18 in 9 years out of 10, after May 2 in half of the years, after May 14 in 1 year out of 10. Fall: the first frost comes before September 26 in 1 year out of 10, before October 11 in half of the years, before October 24 in 9 years out of 10. Typical frost-free season: 160 days.JFMAMJJASONDLast frost ~May 2First frost ~Oct 11
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 26 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 14September 26
50% (typical)May 2October 11
90% (likely)April 18October 24
Frost-free season (32°F, 50% chance)
160 days
Hard frost (28°F), last in spring, 50% chance after
April 20
Hard frost (28°F), first in fall, 50% chance before
October 21

The last 15 years (2006–2020 normals)

Typical last frost May 1 (1 day earlier than the 30-year normal), typical first fall frost October 13 (2 days later). Frost-free season 164 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 Northglenn: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 14, 20% May 9, 30% May 7, 40% May 4, 50% May 2, 60% April 30, 70% April 27, 80% April 23, 90% April 18. Observed last spring frost in recent years: 2022 May 22, 2023 April 24, 2024 April 22, 2025 April 20, 2026 May 7.Spring: last frostChance of a frost after this date10%50%90%May 1May 14May 2Apr 182022: May 222023: April 242024: April 222025: April 202026: May 7Fall at Northglenn: chance of a frost before this dateFall, chance of a frost before the date: 10% September 26, 20% October 1, 30% October 4, 40% October 7, 50% October 11, 60% October 13, 70% October 16, 80% October 19, 90% October 24. Observed first fall frost in recent years: 2022 October 17, 2023 October 13, 2024 October 25, 2025 October 19.Fall: first frostChance of a frost before this date10%50%90%Oct 1Nov 1Sep 26Oct 11Oct 242022: October 172023: October 132024: 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
2021incomplete recordincomplete record
2022May 22October 17
2023April 24October 13
2024April 22October 25
2025April 20October 19
2026May 7season not over yet

What these dates mean

September 26 is the cautious fall date for finishing the harvest of tender crops in Northglenn, because there is only a 10 % chance that a light frost has arrived earlier. In spring the cautious date for setting out tender crops is May 14, when the chance of a light frost later than that date is also 10 %; planting on April 18 carries a 90 % chance of a frost later and is a gamble.

The 50 % dates are May 2 and October 11; these give an even chance of a light frost after (spring) or before (fall) the date. The 50 % hard-frost dates are April 20 and October 21, and a hard frost of 28 °F can damage plants that survive a light frost of 32 °F, making it a more critical concern.

How to read the probabilities

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

  • Spring: "10% after May 14" 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 Northglenn, CONumbered nearest stations: 1 — Brighton, CO — 10 mi; 2 — Wheat Ridge, CO — 10 mi; 3 — near Aurora, CO — 12 mi; 4 — Boulder, CO — 15 mi; 5 — near Coal Creek, CO — 20 mi.25 mi · 40 km1. Brighton, CO — 10 mi12. Wheat Ridge, CO — 10 mi23. near Aurora, CO — 12 mi34. Boulder, CO — 15 mi45. near Coal Creek, CO — 20 mi5
  1. Brighton, CO 10 mi (16 km)
  2. Wheat Ridge, CO 10 mi (16 km)
  3. near Aurora, CO 12 mi (19 km)
  4. Boulder, CO 15 mi (24 km)
  5. near Coal Creek, CO 20 mi (32 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: 39.8989, -105.0133.