Frost Dates for 99320 — Benton City, Washington

Half of springs at Richland, WA see their last 32°F night by April 6; only one in ten is past it by March 22, and one in ten still has one after April 24. Planting tender crops after April 13 leaves a 30% risk of a frost on them, after April 17 20%, and after April 24 10%. The 18 days between the even-odds date and the one-in-ten date is narrower than at 56% of the 113 Washington stations behind this site's ZIP pages.

In fall the first 32°F night comes by October 24 in half of years, as early as October 10 one year in ten, and holds off past November 8 one year in ten. A hard freeze of 28°F follows 11 days later, around November 4; in spring the one-in-ten date for 28°F is April 5, 19 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 221 days one year in ten and falls short of 179 one year in ten. On average 91 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 April 25, and return around October 14. Nights of 24°F, cold enough to damage hardier plants, come in 100% of winters and 16°F in 89%.

July is the warmest month at Richland, WA, averaging 89°F by day and 60°F at night; December is the coldest at 40°F and 28°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 67% of years.

Warmth for growing adds up to 3,125 growing degree days a year (base 50°F), more than at 93% of the 113 Washington stations. 33% of them (1,018) have built up by the end of June, and July adds the most, 764. Precipitation totals 7.2 inches a year, most in December (1.1") and least in August (0.1"); May to September bring 25% of it.

The nearest other station, Prosser, WA, 14 miles west of Benton City and 457 feet higher, has a last frost 8 days later (April 14), a first frost 2 days earlier (October 22), a season 11 days shorter.

The first measurable snow (0.1 inch or more) in Benton City usually arrives around December 14, and the last of the season around January 21. Those are the middle dates across the winters of 1991–2020 at Sunnyside, WA, 25.7 miles away: half of years saw snow earlier, half later. Snow fell in 94% of those winters. The last 32°F night of spring typically comes 75 days after the last snow.

ZIP 99320 is 11 miles from Richland, WA, which stands at 373 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 99352 (Richland). The 2023 USDA Plant Hardiness Zone Map lists ZIP 99320 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.