Frost Dates for 59072 — Roundup, Montana

Half of springs at Roundup, MT see their last 32°F night by May 20; only one in ten is past it by May 6, and one in ten still has one after June 2. Planting tender crops after May 26 leaves a 30% risk of a frost on them, after May 29 20%, and after June 2 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 21 in half of years, as early as September 5 one year in ten, and holds off past October 4 one year in ten. A hard freeze of 28°F follows 10 days later, around October 1; in spring the one-in-ten date for 28°F is May 21, 12 days before the 32°F one. The freeze-free season lasts 124 days in a typical year, shorter than at 54% of the 68 Montana stations; it reaches 142 days one year in ten and falls short of 101 one year in ten. On average 163 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 2, and return around September 10. Severe cold of 16°F or below comes every winter, usually between November 1 and March 31.

July is the warmest month at Roundup, MT, averaging 89°F by day and 54°F at night; January is the coldest at 37°F and 13°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 14% of years.

Warmth for growing adds up to 2,260 growing degree days a year (base 50°F), more than at 82% of the 68 Montana stations. 27% of them (620) have built up by the end of June, and July adds the most, 657. Precipitation totals 14.9 inches a year, most in June (2.9") and least in January (0.4"); May to September bring 64% of it.

The first measurable snow (0.1 inch or more) in Roundup usually arrives around November 19, and the last of the season around March 24. Those are the middle dates across the winters of 1991–2020 at Roundup, MT, 7 miles away: half of years saw snow earlier, half later. Snow fell in 96% of those winters. The last 32°F night of spring typically comes 57 days after the last snow.

ZIP 59072 is 7 miles from Roundup, MT, which stands at 3,200 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. The 2023 USDA Plant Hardiness Zone Map lists ZIP 59072 in zone 4b, the band where the coldest night of the year averaged -25 to -20°F (-31.7 to -28.9°C) over 1991–2020; a zone describes winter lows, not frost dates.