Frost Dates for 98115 — Seattle, Washington

Half of springs at Seattle Sand Pt Wsfo, WA see their last 32°F night by March 8; only one in ten is past it by February 13, and one in ten still has one after March 29. Planting tender crops after March 16 leaves a 30% risk of a frost on them, after March 21 20%, and after March 29 10%. The 21 days between the even-odds date and the one-in-ten date is wider than at 65% 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 November 23 in half of years, as early as November 4 one year in ten, and holds off past December 16 one year in ten. A hard freeze of 28°F follows 18 days later, around December 11; in spring the one-in-ten date for 28°F is March 1, 28 days before the 32°F one. The freeze-free season lasts 259 days in a typical year, longer than at 97% of the 113 Washington stations; it reaches 289 days one year in ten and falls short of 231 one year in ten. On average 25 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 27 days longer, to about April 4, and return around November 5. Nights of 24°F, cold enough to damage hardier plants, come in 59% of winters and 16°F in 0%.

August is the warmest month at Seattle Sand Pt Wsfo, WA, averaging 77°F by day and 57°F at night; December is the coldest at 47°F and 37°F. A freezing night is all but certain in every month from December to January (a 90% chance or more). From April to October the chance of a 32°F night stays under 10% each month. In spring, February still brings one in 87% of years and March still brings one in 64% of years.

Warmth for growing adds up to 2,207 growing degree days a year (base 50°F), more than at 60% of the 113 Washington stations. 29% of them (644) have built up by the end of June, and August adds the most, 529. Precipitation totals 38.0 inches a year, most in November (5.9") and least in July (0.8"); May to September bring 19% of it.

The nearest other station, Seattle Boeing Field, WA, 11 miles south of Seattle, has a last frost 5 days later (March 13), a first frost 6 days earlier (November 17), a season 12 days shorter.

The first measurable snow (0.1 inch or more) in Seattle usually arrives around December 26, and the last of the season around February 8. Those are the middle dates across the winters of 1991–2020 at Seattle Sand Pt Wsfo, WA, 2.1 miles away: half of years saw snow earlier, half later. Snow fell in 62% of those winters. The last 32°F night of spring typically comes 28 days after the last snow.

ZIP 98115 is 2 miles from Seattle Sand Pt Wsfo, WA, which stands at 60 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 27 other ZIP codes share the station; its figures are presented in full for 98052 (Redmond). The 2023 USDA Plant Hardiness Zone Map lists ZIP 98115 in zone 9a, the band where the coldest night of the year averaged 20 to 25°F (-6.7 to -3.9°C) over 1991–2020; a zone describes winter lows, not frost dates.