Frost Dates for 59917 — Eureka, Montana

Half of springs at Eureka Rs, MT see their last 32°F night by May 11; only one in ten is past it by April 28, and one in ten still has one after May 25. Planting tender crops after May 17 leaves a 30% risk of a frost on them, after May 21 20%, and after May 25 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 27 in half of years, as early as September 10 one year in ten, and holds off past October 9 one year in ten. A hard freeze of 28°F follows 10 days later, around October 7; in spring the one-in-ten date for 28°F is May 12, 13 days before the 32°F one. The freeze-free season lasts 138 days in a typical year, longer than at 72% of the 68 Montana stations; it reaches 156 days one year in ten and falls short of 116 one year in ten. On average 153 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 15 days longer, to about May 26, and return around September 13. Severe cold of 16°F or below comes every winter, usually between November 12 and March 9.

July is the warmest month at Eureka Rs, MT, averaging 87°F by day and 53°F at night; January is the coldest at 32°F and 19°F. A freezing night is all but certain in every month from October to April (a 90% chance or more). From June to August the chance of a 32°F night stays under 10% each month. In spring, May still brings one in 83% of years.

Warmth for growing adds up to 2,158 growing degree days a year (base 50°F), more than at 78% of the 68 Montana stations. 29% of them (627) have built up by the end of June, and July adds the most, 607. Precipitation totals 14.9 inches a year, most in June (2.5") and least in February (0.7"); May to September bring 52% of it.

The nearest other station, Libby Dam (Base), MT, 33 miles southwest of Eureka and 422 feet lower, has the same 50% last-frost date, a first frost 4 days later (October 1), a season 4 days longer.

The first measurable snow (0.1 inch or more) in Eureka usually arrives around November 10, and the last of the season around March 25. Those are the middle dates across the winters of 1991–2020 at Eureka Rs, MT, 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 47 days after the last snow.

ZIP 59917 is 5 miles from Eureka Rs, MT, which stands at 2,532 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. The 2023 USDA Plant Hardiness Zone Map lists ZIP 59917 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.