Frost Dates for 99338 — Kennewick, Washington

Half of springs at Kennewick, WA see their last 32°F night by March 28; only one in ten is past it by March 9, and one in ten still has one after April 10. Planting tender crops after April 2 leaves a 30% risk of a frost on them, after April 5 20%, and after April 10 10%. The 13 days between the even-odds date and the one-in-ten date is the narrowest of the 113 Washington stations behind this site's ZIP pages.

In fall the first 32°F night comes by October 30 in half of years, as early as October 18 one year in ten, and holds off past November 15 one year in ten. A hard freeze of 28°F follows 12 days later, around November 11; in spring the one-in-ten date for 28°F is March 29, 12 days before the 32°F one. The freeze-free season lasts 216 days in a typical year, longer than at 70% of the 113 Washington stations; it reaches 239 days one year in ten and falls short of 196 one year in ten. On average 76 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 17 days longer, to about April 14, and return around October 21. Nights of 24°F, cold enough to damage hardier plants, come in 100% of winters and 16°F in 84%.

July is the warmest month at Kennewick, WA, averaging 91°F by day and 62°F at night; January is the coldest at 42°F and 29°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 37% of years.

Warmth for growing adds up to 3,502 growing degree days a year (base 50°F), more than at 99% of the 113 Washington stations. 33% of them (1,165) have built up by the end of June, and July adds the most, 830. Precipitation totals 8.0 inches a year, most in January (1.1") and least in July (0.2"); May to September bring 26% of it.

The nearest other station, Pasco Tri Cities Airport, WA, 8 miles northeast of Kennewick, has a last frost 30 days later (April 27), a first frost 19 days earlier (October 11), a season 48 days shorter.

Measurable snow is rare here: only 9 of 21 winters from 1991 to 2020 recorded 0.1 inch or more at Ice Harbor Dam, WA, 17.6 miles away, so there is no typical first-snow date to plan around.

ZIP 99338 is 7 miles from Kennewick, WA, which stands at 390 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 2 other ZIP codes share the station; its figures are presented in full for 99336 (Kennewick). The 2023 USDA Plant Hardiness Zone Map lists ZIP 99338 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.