Frost Dates for 59923 — Libby, Montana

Half of springs at Libby Dam (Base), MT see their last 32°F night by May 11; only one in ten is past it by April 30, and one in ten still has one after May 27. Planting tender crops after May 17 leaves a 30% risk of a frost on them, after May 21 20%, and after May 27 10%. The 16 days between the even-odds date and the one-in-ten date is narrower than at 40% of the 68 Montana stations behind this site's ZIP pages.

In fall the first 32°F night comes by October 1 in half of years, as early as September 14 one year in ten, and holds off past October 18 one year in ten. A hard freeze of 28°F follows 15 days later, around October 16; in spring the one-in-ten date for 28°F is May 11, 16 days before the 32°F one. The freeze-free season lasts 142 days in a typical year, longer than at 78% of the 68 Montana stations; it reaches 160 days one year in ten and falls short of 119 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 19 days longer, to about May 30, and return around September 20. Severe cold of 16°F or below comes every winter, usually between November 23 and March 6.

July is the warmest month at Libby Dam (Base), MT, averaging 85°F by day and 50°F at night; January is the coldest at 35°F and 22°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 1,864 growing degree days a year (base 50°F), more than at 51% of the 68 Montana stations. 26% of them (488) have built up by the end of June, and July adds the most, 552. Precipitation totals 17.1 inches a year, most in June (2.4") and least in August (0.7"); May to September bring 40% of it.

The nearest other station, Troy, MT, 29 miles northwest of Libby and 181 feet lower, has a last frost 3 days later (May 14), a first frost 6 days earlier (September 25), a season 10 days shorter.

The first measurable snow (0.1 inch or more) in Libby usually arrives around December 2, and the last of the season around March 2. Those are the middle dates across the winters of 1991–2020 at Libby Dam (Base), MT, 7 miles away: half of years saw snow earlier, half later. Snow fell in 96% of those winters. The last 32°F night of spring typically comes 70 days after the last snow.

ZIP 59923 is 7 miles from Libby Dam (Base), MT, which stands at 2,110 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. The 2023 USDA Plant Hardiness Zone Map lists ZIP 59923 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.