Frost Dates for 99362 — Walla Walla, Washington

Half of springs at Walla Walla Regional Airport, WA see their last 32°F night by March 27; only one in ten is past it by March 3, and one in ten still has one after April 22. Planting tender crops after April 5 leaves a 30% risk of a frost on them, after April 13 20%, and after April 22 10%. The 26 days between the even-odds date and the one-in-ten date is wider than at 89% of the 113 Washington stations behind this site's ZIP pages, so a fixed calendar date is a gamble here.

In fall the first 32°F night comes by October 31 in half of years, as early as October 15 one year in ten, and holds off past November 17 one year in ten. A hard freeze of 28°F follows 14 days later, around November 14; in spring the one-in-ten date for 28°F is March 27, 26 days before the 32°F one. The freeze-free season lasts 219 days in a typical year, longer than at 72% of the 113 Washington stations; it reaches 246 days one year in ten and falls short of 189 one year in ten. On average 70 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 25 days longer, to about April 21, and return around October 19. Nights of 24°F, cold enough to damage hardier plants, come in 99% of winters and 16°F in 81%.

July is the warmest month at Walla Walla Regional Airport, WA, averaging 90°F by day and 62°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 February (a 90% chance or more). From May to September the chance of a 32°F night stays under 10% each month. In spring, March still brings one in 90% of years and April still brings one in 38% of years.

Warmth for growing adds up to 3,267 growing degree days a year (base 50°F), more than at 98% of the 113 Washington stations. 31% of them (999) have built up by the end of June, and July adds the most, 813. Precipitation totals 18.8 inches a year, most in November (2.3") and least in August (0.4"); May to September bring 26% of it.

The nearest other station, Milton Freewater, OR, 16 miles west of Walla Walla and 196 feet lower, has a last frost 2 days earlier (March 25), a first frost 2 days earlier (October 29), a season 1 day shorter.

The first measurable snow (0.1 inch or more) in Walla Walla usually arrives around December 11, and the last of the season around February 13. Those are the middle dates across the winters of 1991–2020 at Mill Creek Dam, WA, 11.1 miles away: half of years saw snow earlier, half later. Snow fell in 90% of those winters. The last 32°F night of spring typically comes 42 days after the last snow.

ZIP 99362 is 12 miles from Walla Walla Regional Airport, WA, which stands at 1,166 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. The 2023 USDA Plant Hardiness Zone Map lists ZIP 99362 in zone 7a, the band where the coldest night of the year averaged 0 to 5°F (-17.8 to -15.0°C) over 1991–2020; a zone describes winter lows, not frost dates.