Frost Dates for 99350 — Prosser, Washington

Half of springs at Prosser, WA see their last 32°F night by April 14; only one in ten is past it by March 24, and one in ten still has one after May 7. Planting tender crops after April 26 leaves a 30% risk of a frost on them, after May 1 20%, and after May 7 10%. The 23 days between the even-odds date and the one-in-ten date is wider than at 81% 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 22 in half of years, as early as October 5 one year in ten, and holds off past November 7 one year in ten. A hard freeze of 28°F follows 11 days later, around November 2; in spring the one-in-ten date for 28°F is April 22, 15 days before the 32°F one. The freeze-free season lasts 189 days in a typical year, longer than at 50% of the 113 Washington stations; it reaches 217 days one year in ten and falls short of 161 one year in ten. On average 109 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 18 days longer, to about May 2, and return around October 9. Severe cold of 16°F or below comes in 97% of winters, usually between December 8 and February 3.

July is the warmest month at Prosser, WA, averaging 90°F by day and 60°F at night; December is the coldest at 40°F and 26°F. A freezing night is all but certain in every month from November to March (a 90% chance or more). From June to September the chance of a 32°F night stays under 10% each month. In spring, April still brings one in 72% of years and May still brings one in 20% of years.

Warmth for growing adds up to 3,098 growing degree days a year (base 50°F), more than at 91% of the 113 Washington stations. 32% of them (1,000) have built up by the end of June, and July adds the most, 773. Precipitation totals 8.8 inches a year, most in December (1.3") and least in July (0.2"); May to September bring 26% of it.

The nearest other station, Sunnyside, WA, 15 miles northwest of Prosser, has a last frost 17 days later (May 1), a first frost 16 days earlier (October 6), a season 32 days shorter.

The first measurable snow (0.1 inch or more) in Prosser 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, 15 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 83 days after the last snow.

ZIP 99350 is under a mile from Prosser, WA, which stands at 830 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. The 2023 USDA Plant Hardiness Zone Map lists ZIP 99350 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.