Frost Dates for 80233 — Thornton, Colorado

Half of springs at Northglenn, CO see their last 32°F night by May 2; only one in ten is past it by April 18, and one in ten still has one after May 14. Planting tender crops after May 7 leaves a 30% risk of a frost on them, after May 9 20%, and after May 14 10%. The 12 days between the even-odds date and the one-in-ten date is narrower than at 81% of the 110 Colorado stations behind this site's ZIP pages.

In fall the first 32°F night comes by October 11 in half of years, as early as September 26 one year in ten, and holds off past October 24 one year in ten. A hard freeze of 28°F follows 10 days later, around October 21; in spring the one-in-ten date for 28°F is May 5, 9 days before the 32°F one. The freeze-free season lasts 160 days in a typical year, longer than at 83% of the 110 Colorado stations; it reaches 180 days one year in ten and falls short of 142 one year in ten. On average 149 nights a year drop to 32°F or below. Nights of 36°F, when frost can still settle on leaves in low, open spots, continue 10 days longer, to about May 12, and return around October 1. Severe cold of 16°F or below comes every winter, usually between November 17 and March 11.

July is the warmest month at Northglenn, CO, averaging 91°F by day and 59°F at night; December is the coldest at 48°F and 19°F. A freezing night is all but certain in every month from October to April (a 90% chance or more). From June to August the chance of a 32°F night stays under 10% each month. In spring, May still brings one in 56% of years.

Warmth for growing adds up to 3,168 growing degree days a year (base 50°F), more than at 82% of the 110 Colorado stations. 32% of them (1,025) have built up by the end of June, and July adds the most, 775. Precipitation totals 14.5 inches a year, most in May (2.4") and least in January (0.5"); May to September bring 56% of it.

The nearest other station, Brighton 3 SE, CO, 7 miles northeast of Thornton and 391 feet lower, has a last frost 1 day later (May 3), a first frost 4 days earlier (October 7), a season 5 days shorter.

The first measurable snow (0.1 inch or more) in Thornton usually arrives around October 29, and the last of the season around April 26. Those are the middle dates across the winters of 1991–2020 at Northglenn, CO, 3.1 miles away: half of years saw snow earlier, half later. Every one of those winters saw snow. The last 32°F night of spring typically comes 6 days after the last snow.

ZIP 80233 is 3 miles from Northglenn, CO, which stands at 5,407 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 11 other ZIP codes share the station; its figures are presented in full for 80229 (Thornton). The 2023 USDA Plant Hardiness Zone Map lists ZIP 80233 in zone 6a, the band where the coldest night of the year averaged -10 to -5°F (-23.3 to -20.6°C) over 1991–2020; a zone describes winter lows, not frost dates.