Frost Dates for 99349 — Mattawa, Washington

Half of springs at Priest Rapids Dam, WA see their last 32°F night by March 24; only one in ten is past it by March 4, and one in ten still has one after April 7. Planting tender crops after March 31 leaves a 30% risk of a frost on them, after April 3 20%, and after April 7 10%. The 14 days between the even-odds date and the one-in-ten date is narrower than at 88% of the 113 Washington stations behind this site's ZIP pages.

In fall the first 32°F night comes by November 4 in half of years, as early as October 23 one year in ten, and holds off past November 18 one year in ten. A hard freeze of 28°F follows 12 days later, around November 16; in spring the one-in-ten date for 28°F is March 25, 13 days before the 32°F one. The freeze-free season lasts 222 days in a typical year, longer than at 73% of the 113 Washington stations; it reaches 246 days one year in ten and falls short of 202 one year in ten. On average 75 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 16 days longer, to about April 9, and return around October 25. Nights of 24°F, cold enough to damage hardier plants, come in 100% of winters and 16°F in 81%.

July is the warmest month at Priest Rapids Dam, WA, averaging 92°F by day and 67°F at night; January 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 27% of years.

Warmth for growing adds up to 3,845 growing degree days a year (base 50°F), the most of the 113 Washington stations. 33% of them (1,267) have built up by the end of June, and July adds the most, 900. Precipitation totals 7.6 inches a year, most in December (1.4") and least in August (0.1"); May to September bring 21% of it.

The nearest other station, Smyrna, WA, 13 miles northeast of Mattawa, has a last frost 30 days later (April 23), a first frost 23 days earlier (October 12), a season 50 days shorter.

The first measurable snow (0.1 inch or more) in Mattawa usually arrives around January 3, and the last of the season around January 19. Those are the middle dates across the winters of 1991–2020 at Priest Rapids Dam, WA, 5.6 miles away: half of years saw snow earlier, half later. Snow fell in 77% of those winters. The last 32°F night of spring typically comes 64 days after the last snow.

ZIP 99349 is 6 miles from Priest Rapids Dam, WA, which stands at 428 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. The 2023 USDA Plant Hardiness Zone Map lists ZIP 99349 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.