Frost Dates for 59323 — Colstrip, Montana

Half of springs at Colstrip, MT see their last 32°F night by May 15; only one in ten is past it by May 2, and one in ten still has one after May 28. Planting tender crops after May 20 leaves a 30% risk of a frost on them, after May 24 20%, and after May 28 10%. The 13 days between the even-odds date and the one-in-ten date is narrower than at 74% of the 68 Montana stations behind this site's ZIP pages.

In fall the first 32°F night comes by September 30 in half of years, as early as September 15 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 10; in spring the one-in-ten date for 28°F is May 16, 12 days before the 32°F one. The freeze-free season lasts 138 days in a typical year, longer than at 72% of the 68 Montana stations; it reaches 157 days one year in ten and falls short of 118 one year in ten. On average 162 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 26, and return around September 20. Severe cold of 16°F or below comes every winter, usually between November 4 and March 30.

July is the warmest month at Colstrip, MT, averaging 85°F by day and 57°F at night; January is the coldest at 36°F and 14°F. A freezing night is all but certain in every month from October to May (a 90% chance or more). From June to August the chance of a 32°F night stays under 10% each month.

Warmth for growing adds up to 2,267 growing degree days a year (base 50°F), more than at 85% of the 68 Montana stations. 27% of them (612) have built up by the end of June, and July adds the most, 654. Precipitation totals 16.4 inches a year, most in May (3.1") and least in December (0.6"); May to September bring 57% of it.

The nearest other station, Busby, MT, 30 miles southwest of Colstrip and 212 feet higher, has a last frost 8 days later (May 23), a first frost 15 days earlier (September 15), a season 25 days shorter.

The first measurable snow (0.1 inch or more) in Colstrip usually arrives around November 6, and the last of the season around April 16. Those are the middle dates across the winters of 1991–2020 at Colstrip, MT, 1.8 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 29 days after the last snow.

ZIP 59323 is 2 miles from Colstrip, MT, which stands at 3,218 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 59327 (Forsyth). The 2023 USDA Plant Hardiness Zone Map lists ZIP 59323 in zone 5a, the band where the coldest night of the year averaged -20 to -15°F (-28.9 to -26.1°C) over 1991–2020; a zone describes winter lows, not frost dates.