Frost Dates for 59523 — Chinook, Montana

Half of springs at Chinook, MT see their last 32°F night by May 14; only one in ten is past it by May 1, and one in ten still has one after May 27. Planting tender crops after May 20 leaves a 30% risk of a frost on them, after May 23 20%, and after May 27 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 23 in half of years, as early as September 11 one year in ten, and holds off past October 3 one year in ten. A hard freeze of 28°F follows 8 days later, around October 1; in spring the one-in-ten date for 28°F is May 16, 11 days before the 32°F one. The freeze-free season lasts 130 days in a typical year, longer than at 57% of the 68 Montana stations; it reaches 148 days one year in ten and falls short of 112 one year in ten. On average 195 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 25, and return around September 12. Severe cold of 16°F or below comes every winter, usually between October 26 and April 2.

July is the warmest month at Chinook, MT, averaging 84°F by day and 54°F at night; January is the coldest at 27°F and 4°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 1,979 growing degree days a year (base 50°F), more than at 60% of the 68 Montana stations. 29% of them (575) have built up by the end of June, and July adds the most, 587. Precipitation totals 13.5 inches a year, most in June (2.9") and least in February (0.4"); May to September bring 67% of it.

The nearest other station, Fort Belknap 2 SW, MT, 18 miles east of Chinook, has a last frost 2 days later (May 16), a first frost 1 day earlier (September 22), a season 2 days shorter.

The first measurable snow (0.1 inch or more) in Chinook usually arrives around November 7, and the last of the season around April 9. Those are the middle dates across the winters of 1991–2020 at Chinook, MT, 7.5 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 35 days after the last snow.

ZIP 59523 is 8 miles from Chinook, MT, which stands at 2,420 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. The 2023 USDA Plant Hardiness Zone Map lists ZIP 59523 in zone 3b, the band where the coldest night of the year averaged -35 to -30°F (-37.2 to -34.4°C) over 1991–2020; a zone describes winter lows, not frost dates.