Frost Dates for 51040 — Onawa, Iowa

Half of springs at Onawa 3NW, IA see their last 32°F night by May 1; only one in ten is past it by April 19, and one in ten still has one after May 13. Planting tender crops after May 6 leaves a 30% risk of a frost on them, after May 9 20%, and after May 13 10%. The 12 days between the even-odds date and the one-in-ten date is narrower than at 67% of the 132 Iowa stations behind this site's ZIP pages.

In fall the first 32°F night comes by October 3 in half of years, as early as September 21 one year in ten, and holds off past October 18 one year in ten. A hard freeze of 28°F follows 10 days later, around October 13; in spring the one-in-ten date for 28°F is May 6, 7 days before the 32°F one. The freeze-free season lasts 154 days in a typical year, shorter than at 84% of the 132 Iowa stations; it reaches 172 days one year in ten and falls short of 135 one year in ten. On average 151 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 8 days longer, to about May 9, and return around September 25. Severe cold of 16°F or below comes every winter, usually between November 8 and March 26.

July is the warmest month at Onawa 3NW, IA, averaging 86°F by day and 63°F at night; January is the coldest at 30°F and 9°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 54% of years.

Warmth for growing adds up to 3,119 growing degree days a year (base 50°F), more than at 64% of the 132 Iowa stations. 35% of them (1,099) have built up by the end of June, and July adds the most, 753. Precipitation totals 30.0 inches a year, most in May (4.6") and least in January (0.5"); May to September bring 64% of it.

The nearest other station, Tekamah, NE, 18 miles south of Onawa, has a last frost 3 days earlier (April 28), a first frost 7 days later (October 10), a season 11 days longer.

The first measurable snow (0.1 inch or more) in Onawa usually arrives around November 17, and the last of the season around March 29. Those are the middle dates across the winters of 1991–2020 at Onawa 3NW, IA, 3 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 33 days after the last snow.

ZIP 51040 is 3 miles from Onawa 3NW, IA, which stands at 1,060 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. The 2023 USDA Plant Hardiness Zone Map lists ZIP 51040 in zone 5a, the band where the coldest night of the year averaged -20 to -15°F (-28.9 to -26.1°C) over 1991–2020; a zone describes winter lows, not frost dates.