Frost Dates for 80501 — Longmont, Colorado

Half of springs at Longmont 2 ESE, CO see their last 32°F night by May 7; only one in ten is past it by April 24, and one in ten still has one after May 22. Planting tender crops after May 13 leaves a 30% risk of a frost on them, after May 16 20%, and after May 22 10%. The 15 days between the even-odds date and the one-in-ten date is wider than at 56% of the 110 Colorado stations behind this site's ZIP pages.

In fall the first 32°F night comes by October 5 in half of years, as early as September 23 one year in ten, and holds off past October 16 one year in ten. A hard freeze of 28°F follows 7 days later, around October 12; in spring the one-in-ten date for 28°F is May 12, 10 days before the 32°F one. The freeze-free season lasts 151 days in a typical year, longer than at 62% of the 110 Colorado stations; it reaches 169 days one year in ten and falls short of 131 one year in ten. On average 178 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 11 days longer, to about May 18, and return around September 28. Severe cold of 16°F or below comes every winter, usually between November 7 and March 20.

July is the warmest month at Longmont 2 ESE, CO, averaging 91°F by day and 55°F at night; January is the coldest at 45°F and 14°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 71% of years.

Warmth for growing adds up to 2,765 growing degree days a year (base 50°F), more than at 65% of the 110 Colorado stations. 32% of them (872) have built up by the end of June, and July adds the most, 716. Precipitation totals 15.5 inches a year, most in April (2.0") and least in December (0.5"); May to September bring 57% of it.

The nearest other station, Boulder, CO, 15 miles southwest of Longmont and 534 feet higher, has a last frost 1 day earlier (May 6), the same first-frost date.

The first measurable snow (0.1 inch or more) in Longmont usually arrives around October 26, and the last of the season around May 1. Those are the middle dates across the winters of 1991–2020 at Longmont 1.7 NNE, CO, 1.7 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 80501 is 1 mile from Longmont 2 ESE, CO, which stands at 4,950 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 8 other ZIP codes share the station; its figures are presented in full for 80504 (Longmont). The 2023 USDA Plant Hardiness Zone Map lists ZIP 80501 in zone 5b, the band where the coldest night of the year averaged -15 to -10°F (-26.1 to -23.3°C) over 1991–2020; a zone describes winter lows, not frost dates.