Frost Dates for 59521 — Box Elder, Montana

Half of springs at Big Sandy, MT see their last 32°F night by May 17; only one in ten is past it by May 4, and one in ten still has one after June 4. Planting tender crops after May 24 leaves a 30% risk of a frost on them, after May 29 20%, and after June 4 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 September 25 in half of years, as early as September 12 one year in ten, and holds off past October 7 one year in ten. A hard freeze of 28°F follows 9 days later, around October 4; in spring the one-in-ten date for 28°F is May 20, 15 days before the 32°F one. The freeze-free season lasts 128 days in a typical year, longer than at 51% of the 68 Montana stations; it reaches 147 days one year in ten and falls short of 107 one year in ten. On average 180 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 31, and return around September 15. Severe cold of 16°F or below comes every winter, usually between October 30 and March 31.

July is the warmest month at Big Sandy, MT, averaging 86°F by day and 55°F at night; January is the coldest at 31°F and 10°F. A freezing night is all but certain in every month from October to May (a 90% chance or more). From July to August the chance of a 32°F night stays under 10% each month. In spring, June still brings one in 16% of years.

Warmth for growing adds up to 2,205 growing degree days a year (base 50°F), more than at 79% of the 68 Montana stations. 28% of them (619) have built up by the end of June, and July adds the most, 628. Precipitation totals 13.4 inches a year, most in June (3.1") and least in February (0.4"); May to September bring 66% of it.

The nearest other station, Havre City Co Airport, MT, 18 miles northeast of Box Elder and 185 feet lower, has a last frost 2 days earlier (May 15), a first frost 2 days earlier (September 23), a season 1 day longer.

The first measurable snow (0.1 inch or more) in Box Elder usually arrives around November 8, and the last of the season around April 13. Those are the middle dates across the winters of 1991–2020 at Fort Assiniboine, MT, 14.2 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 34 days after the last snow.

ZIP 59521 is 14 miles from Big Sandy, MT, which stands at 2,770 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. The 2023 USDA Plant Hardiness Zone Map lists ZIP 59521 in zone 4a, the band where the coldest night of the year averaged -30 to -25°F (-34.4 to -31.7°C) over 1991–2020; a zone describes winter lows, not frost dates.