Frost Dates for 81321 — Cortez, Colorado

Half of springs at Cortez, CO see their last 32°F night by May 12; only one in ten is past it by April 30, and one in ten still has one after May 31. Planting tender crops after May 19 leaves a 30% risk of a frost on them, after May 23 20%, and after May 31 10%. The 19 days between the even-odds date and the one-in-ten date is wider than at 95% of the 110 Colorado stations behind this site's ZIP pages.

In fall the first 32°F night comes by October 7 in half of years, as early as September 26 one year in ten, and holds off past October 21 one year in ten. A hard freeze of 28°F follows 10 days later, around October 17; in spring the one-in-ten date for 28°F is May 14, 17 days before the 32°F one. The freeze-free season lasts 147 days in a typical year, longer than at 55% of the 110 Colorado stations; it reaches 165 days one year in ten and falls short of 127 one year in ten. On average 168 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 14 days longer, to about May 26, and return around September 30. Severe cold of 16°F or below comes every winter, usually between November 18 and March 7.

July is the warmest month at Cortez, CO, averaging 91°F by day and 57°F at night; January is the coldest at 43°F and 17°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 87% of years.

Warmth for growing adds up to 2,805 growing degree days a year (base 50°F), more than at 68% of the 110 Colorado stations. 30% of them (848) have built up by the end of June, and July adds the most, 739. Precipitation totals 12.0 inches a year, most in September (1.5") and least in June (0.4"); May to September bring 45% of it.

The nearest other station, Yellow Jacket 4NE, CO, 15 miles north of Cortez and 923 feet higher, has a last frost 3 days later (May 15), a first frost 1 day later (October 8).

The first measurable snow (0.1 inch or more) in Cortez usually arrives around November 20, and the last of the season around April 14. Those are the middle dates across the winters of 1991–2020 at Cortez, CO, 0.5 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 28 days after the last snow.

ZIP 81321 is under a mile from Cortez, CO, which stands at 6,167 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. The 2023 USDA Plant Hardiness Zone Map lists ZIP 81321 in zone 7a, the band where the coldest night of the year averaged 0 to 5°F (-17.8 to -15.0°C) over 1991–2020; a zone describes winter lows, not frost dates.