Frost Dates for 59725 — Dillon, Montana

Half of springs at Dillon U Of Montana Western, MT see their last 32°F night by June 5; only one in ten is past it by May 18, and one in ten still has one after June 23. Planting tender crops after June 13 leaves a 30% risk of a frost on them, after June 18 20%, and after June 23 10%. The 18 days between the even-odds date and the one-in-ten date is wider than at 68% of the 68 Montana stations behind this site's ZIP pages.

In fall the first 32°F night comes by September 13 in half of years, as early as August 29 one year in ten, and holds off past September 27 one year in ten. A hard freeze of 28°F follows 12 days later, around September 25; in spring the one-in-ten date for 28°F is June 3, 20 days before the 32°F one. The freeze-free season lasts 98 days in a typical year, shorter than at 90% of the 68 Montana stations; it reaches 121 days one year in ten and falls short of 74 one year in ten. On average 189 nights a year drop to 32°F or below. Severe cold of 16°F or below comes every winter, usually between October 27 and April 5.

July is the warmest month at Dillon U Of Montana Western, MT, averaging 83°F by day and 48°F at night; December is the coldest at 32°F and 13°F. A freezing night is all but certain in every month from September to May (a 90% chance or more). July is the only month with under a 10% chance of a 32°F night. In spring, June still brings one in 61% of years.

Warmth for growing adds up to 1,461 growing degree days a year (base 50°F), fewer than at 88% of the 68 Montana stations. 24% of them (347) have built up by the end of June, and July adds the most, 473. Precipitation totals 11.4 inches a year, most in May (1.9") and least in January (0.3"); May to September bring 57% of it.

The first measurable snow (0.1 inch or more) in Dillon usually arrives around October 10, and the last of the season around May 13. Those are the middle dates across the winters of 1991–2020 at Grant 5 SE, MT, 16.8 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 23 days after the last snow.

ZIP 59725 is 8 miles from Dillon U Of Montana Western, MT, which stands at 5,129 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. The 2023 USDA Plant Hardiness Zone Map lists ZIP 59725 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.