Frost Dates for 97883 — Union, Oregon

Half of springs at Union, OR see their last 32°F night by May 11; only one in ten is past it by April 26, and one in ten still has one after May 31. Planting tender crops after May 19 leaves a 30% risk of a frost on them, after May 24 20%, and after May 31 10%. The 20 days between the even-odds date and the one-in-ten date is narrower than at 64% of the 103 Oregon stations behind this site's ZIP pages.

In fall the first 32°F night comes by September 26 in half of years, as early as September 9 one year in ten, and holds off past October 10 one year in ten. A hard freeze of 28°F follows 11 days later, around October 7; in spring the one-in-ten date for 28°F is May 10, 21 days before the 32°F one. The freeze-free season lasts 136 days in a typical year, shorter than at 84% of the 103 Oregon stations; it reaches 158 days one year in ten and falls short of 112 one year in ten. On average 135 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 22 days longer, to about June 2, and return around September 10. Severe cold of 16°F or below comes every winter, usually between November 23 and February 20.

August is the warmest month at Union, OR, averaging 87°F by day and 50°F at night; December is the coldest at 39°F and 25°F. A freezing night is all but certain in every month from October to April (a 90% chance or more). From June to August the chance of a 32°F night stays under 10% each month. In spring, May still brings one in 79% of years.

Warmth for growing adds up to 2,086 growing degree days a year (base 50°F), fewer than at 59% of the 103 Oregon stations. 27% of them (554) have built up by the end of June, and August adds the most, 572. Precipitation totals 14.9 inches a year, most in May (2.5") and least in July (0.5"); May to September bring 40% of it.

The nearest other station, La Grande, OR, 14 miles northwest of Union, has a last frost 5 days earlier (May 6), a first frost 5 days later (October 1), a season 10 days longer.

The first measurable snow (0.1 inch or more) in Union usually arrives around November 25, and the last of the season around March 25. Those are the middle dates across the winters of 1991–2020 at Union, OR, 1.7 miles away: half of years saw snow earlier, half later. Every one of those winters saw snow. The last 32°F night of spring typically comes 47 days after the last snow.

ZIP 97883 is 2 miles from Union, OR, which stands at 2,777 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. The 2023 USDA Plant Hardiness Zone Map lists ZIP 97883 in zone 6b, the band where the coldest night of the year averaged -5 to 0°F (-20.6 to -17.8°C) over 1991–2020; a zone describes winter lows, not frost dates.