Frost Dates for 59457 — Lewistown, Montana

Half of springs at Lewistown Municipal Airport, 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 8. Planting tender crops after May 29 leaves a 30% risk of a frost on them, after June 2 20%, and after June 8 10%. The 16 days between the even-odds date and the one-in-ten date is narrower than at 40% 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 8 one year in ten, and holds off past October 3 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 24, 15 days before the 32°F one. The freeze-free season lasts 119 days in a typical year, shorter than at 63% of the 68 Montana stations; it reaches 138 days one year in ten and falls short of 98 one year in ten. On average 184 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 19 days longer, to about June 11, and return around September 8. Severe cold of 16°F or below comes every winter, usually between October 28 and April 6.

July is the warmest month at Lewistown Municipal Airport, MT, averaging 82°F by day and 51°F at night; January is the coldest at 34°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 24% of years.

Warmth for growing adds up to 1,632 growing degree days a year (base 50°F), fewer than at 78% of the 68 Montana stations. 23% of them (377) have built up by the end of June, and July adds the most, 501. Precipitation totals 17.4 inches a year, most in June (3.4") and least in February (0.5"); May to September bring 64% of it.

The first measurable snow (0.1 inch or more) in Lewistown usually arrives around September 30, and the last of the season around May 12. Those are the middle dates across the winters of 1991–2020 at Lewistown 11 SSE, MT, 10.8 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 11 days after the last snow.

ZIP 59457 is 1 mile from Lewistown Municipal Airport, MT, which stands at 4,145 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. The 2023 USDA Plant Hardiness Zone Map lists ZIP 59457 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.