Frost Dates for 99403 — Clarkston, Washington

Half of springs at Lewiston Nez Perce Co Airport, ID see their last 32°F night by April 6; only one in ten is past it by March 20, and one in ten still has one after April 26. Planting tender crops after April 14 leaves a 30% risk of a frost on them, after April 19 20%, and after April 26 10%. The 20 days between the even-odds date and the one-in-ten date is wider than at 52% of the 113 Washington stations behind this site's ZIP pages.

In fall the first 32°F night comes by October 26 in half of years, as early as October 10 one year in ten, and holds off past November 12 one year in ten. A hard freeze of 28°F follows 13 days later, around November 8; in spring the one-in-ten date for 28°F is April 4, 22 days before the 32°F one. The freeze-free season lasts 200 days in a typical year, longer than at 58% of the 113 Washington stations; it reaches 225 days one year in ten and falls short of 176 one year in ten. On average 80 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 23 days longer, to about April 29, and return around October 13. Nights of 24°F, cold enough to damage hardier plants, come in 100% of winters and 16°F in 86%.

July is the warmest month at Lewiston Nez Perce Co Airport, ID, averaging 91°F by day and 61°F at night; December is the coldest at 41°F and 30°F. A freezing night is all but certain in every month from November to March (a 90% chance or more). From May to September the chance of a 32°F night stays under 10% each month. In spring, April still brings one in 65% of years.

Warmth for growing adds up to 3,143 growing degree days a year (base 50°F), more than at 94% of the 113 Washington stations. 30% of them (935) have built up by the end of June, and July adds the most, 801. Precipitation totals 12.8 inches a year, most in May (1.7") and least in July (0.5"); May to September bring 36% of it.

The nearest other station, Pullman Moscow Regional Airport, WA, 24 miles north of Clarkston and 1,115 feet higher, has a last frost 23 days later (April 29), a first frost 24 days earlier (October 2), a season 41 days shorter.

The first measurable snow (0.1 inch or more) in Clarkston usually arrives around December 3, and the last of the season around February 28. Those are the middle dates across the winters of 1991–2020 at Lewiston Nez Perce Co Airport, ID, 3.7 miles away: half of years saw snow earlier, half later. Snow fell in 95% of those winters. The last 32°F night of spring typically comes 37 days after the last snow.

ZIP 99403 is 4 miles from Lewiston Nez Perce Co Airport, ID, which stands at 1,436 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 3 other ZIP codes share the station; its figures are presented in full for 83501 (Lewiston). The 2023 USDA Plant Hardiness Zone Map lists ZIP 99403 in zone 7b, the band where the coldest night of the year averaged 5 to 10°F (-15.0 to -12.2°C) over 1991–2020; a zone describes winter lows, not frost dates.