Frost Dates for 84320 — Lewiston, Utah

Half of springs at Trenton, UT see their last 32°F night by May 17; only one in ten is past it by May 3, and one in ten still has one after June 6. Planting tender crops after May 25 leaves a 30% risk of a frost on them, after May 31 20%, and after June 6 10%. The 20 days between the even-odds date and the one-in-ten date is wider than at 61% of the 66 Utah stations behind this site's ZIP pages.

In fall the first 32°F night comes by September 18 in half of years, as early as September 1 one year in ten, and holds off past October 4 one year in ten. A hard freeze of 28°F follows 13 days later, around October 1; in spring the one-in-ten date for 28°F is May 17, 20 days before the 32°F one. The freeze-free season lasts 121 days in a typical year, shorter than at 73% of the 66 Utah stations; it reaches 142 days one year in ten and falls short of 98 one year in ten. On average 180 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 5, and return around September 6. Severe cold of 16°F or below comes every winter, usually between November 10 and March 11.

July is the warmest month at Trenton, UT, averaging 89°F by day and 51°F at night; January is the coldest at 32°F and 12°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 18% of years.

Warmth for growing adds up to 2,082 growing degree days a year (base 50°F), fewer than at 79% of the 66 Utah stations. 28% of them (573) have built up by the end of June, and July adds the most, 623. Precipitation totals 16.9 inches a year, most in May (2.2") and least in July (0.6"); May to September bring 36% of it.

The nearest other station, Richmond, UT, 5 miles southeast of Lewiston and 225 feet higher, has a last frost 6 days earlier (May 11), a first frost 17 days later (October 5), a season 22 days longer.

The first measurable snow (0.1 inch or more) in Lewiston usually arrives around November 7, and the last of the season around April 5. Those are the middle dates across the winters of 1991–2020 at Trenton, UT, 4.4 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 42 days after the last snow.

ZIP 84320 is 4 miles from Trenton, UT, which stands at 4,455 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. The 2023 USDA Plant Hardiness Zone Map lists ZIP 84320 in zone 6a, the band where the coldest night of the year averaged -10 to -5°F (-23.3 to -20.6°C) over 1991–2020; a zone describes winter lows, not frost dates.