Frost Dates for 97233 — Portland, Oregon

Half of springs at Portland WFO, OR see their last 32°F night by March 13; only one in ten is past it by February 20, and one in ten still has one after April 5. Planting tender crops after March 24 leaves a 30% risk of a frost on them, after March 29 20%, and after April 5 10%. The 23 days between the even-odds date and the one-in-ten date is wider than at 56% of the 103 Oregon 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 22 in half of years, as early as November 3 one year in ten, and holds off past December 14 one year in ten. A hard freeze of 28°F follows 18 days later, around December 10; in spring the one-in-ten date for 28°F is March 7, 29 days before the 32°F one. The freeze-free season lasts 251 days in a typical year, longer than at 87% of the 103 Oregon stations; it reaches 286 days one year in ten and falls short of 224 one year in ten. On average 29 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 28 days longer, to about April 10, and return around November 5. Nights of 24°F, cold enough to damage hardier plants, come in 63% of winters and 16°F in 4%.

August is the warmest month at Portland WFO, OR, averaging 82°F by day and 57°F at night; December is the coldest at 46°F and 36°F. A freezing night is all but certain in every month from December to January (a 90% chance or more). From May 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 72% of years and April still brings one in 14% of years.

Warmth for growing adds up to 2,677 growing degree days a year (base 50°F), more than at 74% of the 103 Oregon stations. 31% of them (836) have built up by the end of June, and August adds the most, 608. Precipitation totals 43.9 inches a year, most in December (7.0") and least in August (0.6"); May to September bring 18% of it.

The nearest other station, Portland Troutdale Airport, OR, 5 miles northeast of Portland, has a last frost 3 days later (March 16), a first frost 2 days earlier (November 20), a season 1 day shorter.

Measurable snow is rare here: only 5 of 27 winters from 1991 to 2020 recorded 0.1 inch or more at Troutdale, OR, 6.1 miles away, so there is no typical first-snow date to plan around.

ZIP 97233 is 4 miles from Portland WFO, OR, which stands at 21 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 7 other ZIP codes share the station; its figures are presented in full for 97230 (Portland). The 2023 USDA Plant Hardiness Zone Map lists ZIP 97233 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.