Frost Dates for 59823 — Bonner, Montana

Half of springs at Missoula 2 NE, MT see their last 32°F night by May 10; only one in ten is past it by April 27, and one in ten still has one after May 28. Planting tender crops after May 17 leaves a 30% risk of a frost on them, after May 22 20%, and after May 28 10%. The 18 days between the even-odds date and the one-in-ten date is wider than at 68% 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 16 one year in ten, and holds off past October 15 one year in ten. A hard freeze of 28°F follows 11 days later, around October 12; in spring the one-in-ten date for 28°F is May 11, 17 days before the 32°F one. The freeze-free season lasts 143 days in a typical year, longer than at 84% of the 68 Montana stations; it reaches 161 days one year in ten and falls short of 120 one year in ten. On average 163 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 22 days longer, to about June 1, and return around September 19. Severe cold of 16°F or below comes every winter, usually between November 14 and March 3.

July is the warmest month at Missoula 2 NE, MT, averaging 85°F by day and 52°F at night; December is the coldest at 33°F and 20°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 82% of years.

Warmth for growing adds up to 1,932 growing degree days a year (base 50°F), more than at 57% of the 68 Montana stations. 26% of them (502) have built up by the end of June, and July adds the most, 578. Precipitation totals 17.4 inches a year, most in June (2.6") and least in July (1.0"); May to September bring 46% of it.

The nearest other station, Missoula 6 NW WFO, MT, 21 miles west of Bonner and 223 feet lower, has a last frost 4 days earlier (May 6), a first frost 3 days later (October 4), a season 8 days longer.

The first measurable snow (0.1 inch or more) in Bonner usually arrives around November 5, and the last of the season around March 30. Those are the middle dates across the winters of 1991–2020 at Potomac, MT, 4 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 41 days after the last snow.

ZIP 59823 is 16 miles from Missoula 2 NE, MT, which stands at 3,420 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 4 other ZIP codes share the station; its figures are presented in full for 59801 (Missoula). The 2023 USDA Plant Hardiness Zone Map lists ZIP 59823 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.