Frost Dates for 59859 — Plains, Montana

Half of springs at Superior, MT see their last 32°F night by May 17; only one in ten is past it by May 1, and one in ten still has one after June 8. Planting tender crops after May 25 leaves a 30% risk of a frost on them, after May 30 20%, and after June 8 10%. The 22 days between the even-odds date and the one-in-ten date is wider than at 93% of the 68 Montana stations behind this site's ZIP pages, so a fixed calendar date is a gamble here.

In fall the first 32°F night comes by September 23 in half of years, as early as September 6 one year in ten, and holds off past October 7 one year in ten. A hard freeze of 28°F follows 13 days later, around October 6; in spring the one-in-ten date for 28°F is May 19, 20 days before the 32°F one. The freeze-free season lasts 125 days in a typical year, shorter than at 53% of the 68 Montana stations; it reaches 150 days one year in ten and falls short of 96 one year in ten. On average 161 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 19 days longer, to about June 5, and return around September 9. Severe cold of 16°F or below comes every winter, usually between November 17 and March 6.

July is the warmest month at Superior, MT, averaging 89°F by day and 51°F at night; December is the coldest at 34°F and 22°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 89% of years and June still brings one in 15% of years.

Warmth for growing adds up to 2,129 growing degree days a year (base 50°F), more than at 72% of the 68 Montana stations. 28% of them (588) have built up by the end of June, and July adds the most, 606. Precipitation totals 16.5 inches a year, most in June (2.0") and least in July (0.9"); May to September bring 41% of it.

The nearest other station, Thompson Falls Ph, MT, 27 miles west of Plains and 330 feet lower, has a last frost 10 days earlier (May 7), a first frost 11 days later (October 4), a season 25 days longer.

The first measurable snow (0.1 inch or more) in Plains usually arrives around November 6, and the last of the season around April 2. Those are the middle dates across the winters of 1991–2020 at Saint Regis 1 NE, MT, 16.4 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 45 days after the last snow.

ZIP 59859 is 18 miles from Superior, MT, which stands at 2,710 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 1 other ZIP code shares the station. The 2023 USDA Plant Hardiness Zone Map lists ZIP 59859 in zone 6a, the band where the coldest night of the year averaged -10 to -5°F (-23.3 to -20.6°C) over 1991–2020; a zone describes winter lows, not frost dates.