Frost Dates for 59474 — Shelby, Montana

Half of springs at Shelby, MT see their last 32°F night by May 23; only one in ten is past it by May 11, and one in ten still has one after June 6. Planting tender crops after May 28 leaves a 30% risk of a frost on them, after June 1 20%, and after June 6 10%. The 14 days between the even-odds date and the one-in-ten date is narrower than at 60% of the 68 Montana stations behind this site's ZIP pages.

In fall the first 32°F night comes by September 16 in half of years, as early as September 3 one year in ten, and holds off past September 28 one year in ten. A hard freeze of 28°F follows 10 days later, around September 26; in spring the one-in-ten date for 28°F is May 22, 15 days before the 32°F one. The freeze-free season lasts 115 days in a typical year, shorter than at 71% of the 68 Montana stations; it reaches 132 days one year in ten and falls short of 97 one year in ten. On average 190 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 13 days longer, to about June 5, and return around September 5. Severe cold of 16°F or below comes every winter, usually between October 24 and April 7.

July is the warmest month at Shelby, MT, averaging 82°F by day and 52°F at night; January is the coldest at 31°F and 6°F. A freezing night is all but certain in every month from September 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 20% of years.

Warmth for growing adds up to 1,697 growing degree days a year (base 50°F), fewer than at 72% of the 68 Montana stations. 25% of them (429) have built up by the end of June, and July adds the most, 522. Precipitation totals 12.2 inches a year, most in June (2.9") and least in February (0.4"); May to September bring 63% of it.

The nearest other station, Conrad, MT, 23 miles south of Shelby and 225 feet higher, has a last frost 3 days earlier (May 20), a first frost 3 days later (September 19), a season 4 days longer.

The first measurable snow (0.1 inch or more) in Shelby usually arrives around November 11, and the last of the season around April 15. Those are the middle dates across the winters of 1991–2020 at Shelby, MT, 4.6 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 38 days after the last snow.

ZIP 59474 is 5 miles from Shelby, MT, which stands at 3,325 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. The 2023 USDA Plant Hardiness Zone Map lists ZIP 59474 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.