Frost Dates for 59821 — Arlee, Montana

Half of springs at Saint Ignatius, MT see their last 32°F night by May 12; only one in ten is past it by April 28, and one in ten still has one after June 1. Planting tender crops after May 19 leaves a 30% risk of a frost on them, after May 24 20%, and after June 1 10%. The 20 days between the even-odds date and the one-in-ten date is wider than at 84% of the 68 Montana stations behind this site's ZIP pages.

In fall the first 32°F night comes by October 2 in half of years, as early as September 19 one year in ten, and holds off past October 14 one year in ten. A hard freeze of 28°F follows 10 days later, around October 12; in spring the one-in-ten date for 28°F is May 17, 15 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 162 days one year in ten and falls short of 119 one year in ten. On average 152 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 20 days longer, to about June 1, and return around September 22. Severe cold of 16°F or below comes every winter, usually between November 15 and March 5.

July is the warmest month at Saint Ignatius, MT, averaging 86°F by day and 52°F at night; December is the coldest at 37°F and 23°F. A freezing night is all but certain in every month from October to April (a 90% chance or more). From July to August the chance of a 32°F night stays under 10% each month. In spring, May still brings one in 80% of years and June still brings one in 10% of years.

Warmth for growing adds up to 2,070 growing degree days a year (base 50°F), more than at 65% of the 68 Montana stations. 27% of them (556) have built up by the end of June, and July adds the most, 592. Precipitation totals 16.8 inches a year, most in June (2.6") and least in March (0.9"); May to September bring 51% of it.

The nearest other station, Missoula 6 NW WFO, MT, 17 miles south of Arlee and 282 feet higher, has a last frost 6 days earlier (May 6), a first frost 2 days later (October 4), a season 9 days longer.

The first measurable snow (0.1 inch or more) in Arlee usually arrives around November 13, and the last of the season around April 2. Those are the middle dates across the winters of 1991–2020 at Saint Ignatius, MT, 10.3 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 40 days after the last snow.

ZIP 59821 is 10 miles from Saint Ignatius, MT, which stands at 2,915 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 1 other ZIP code shares the station; its figures are presented in full for 59865 (St Ignatius). The 2023 USDA Plant Hardiness Zone Map lists ZIP 59821 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.