Frost Dates for 59829 — Darby, Montana

Half of springs at Darby, MT see their last 32°F night by May 24; only one in ten is past it by May 6, and one in ten still has one after June 14. Planting tender crops after June 2 leaves a 30% risk of a frost on them, after June 7 20%, and after June 14 10%. The 21 days between the even-odds date and the one-in-ten date is wider than at 90% 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 4 one year in ten, and holds off past October 8 one year in ten. A hard freeze of 28°F follows 11 days later, around October 4; in spring the one-in-ten date for 28°F is May 21, 24 days before the 32°F one. The freeze-free season lasts 118 days in a typical year, shorter than at 66% of the 68 Montana stations; it reaches 143 days one year in ten and falls short of 92 one year in ten. On average 167 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 23 days longer, to about June 16, and return around September 6. Severe cold of 16°F or below comes every winter, usually between November 9 and March 11.

July is the warmest month at Darby, MT, averaging 82°F by day and 50°F at night; December is the coldest at 32°F and 19°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 33% of years.

Warmth for growing adds up to 1,583 growing degree days a year (base 50°F), fewer than at 82% of the 68 Montana stations. 25% of them (397) have built up by the end of June, and July adds the most, 487. Precipitation totals 15.2 inches a year, most in June (2.1") and least in February (0.9"); May to September bring 45% of it.

The nearest other station, Hamilton, MT, 28 miles north of Darby and 295 feet lower, has a last frost 3 days earlier (May 21), a first frost 1 day earlier (September 22), a season 4 days longer.

The first measurable snow (0.1 inch or more) in Darby usually arrives around November 2, and the last of the season around April 2. Those are the middle dates across the winters of 1991–2020 at Sula 3 ENE, MT, 13.5 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 52 days after the last snow.

ZIP 59829 is 13 miles from Darby, MT, which stands at 3,880 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. The 2023 USDA Plant Hardiness Zone Map lists ZIP 59829 in zone 5b, the band where the coldest night of the year averaged -15 to -10°F (-26.1 to -23.3°C) over 1991–2020; a zone describes winter lows, not frost dates.