Frost Dates for 52159 — Monona, Iowa

Half of springs at Postville, IA see their last 32°F night by May 1; only one in ten is past it by April 18, and one in ten still has one after May 14. Planting tender crops after May 6 leaves a 30% risk of a frost on them, after May 9 20%, and after May 14 10%. The 13 days between the even-odds date and the one-in-ten date is narrower than at 42% 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 20 one year in ten, and holds off past October 17 one year in ten. A hard freeze of 28°F follows 9 days later, around October 12; in spring the one-in-ten date for 28°F is May 6, 8 days before the 32°F one. The freeze-free season lasts 153 days in a typical year, shorter than at 89% of the 132 Iowa stations; it reaches 172 days one year in ten and falls short of 134 one year in ten. On average 162 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 9 days longer, to about May 10, and return around September 24. Severe cold of 16°F or below comes every winter, usually between November 13 and March 26.

July is the warmest month at Postville, IA, averaging 83°F by day and 61°F at night; January is the coldest at 26°F and 7°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 52% of years.

Warmth for growing adds up to 2,659 growing degree days a year (base 50°F), fewer than at 89% of the 132 Iowa stations. 34% of them (906) have built up by the end of June, and July adds the most, 677. Precipitation totals 39.0 inches a year, most in June (5.9") and least in January (1.1"); May to September bring 61% of it.

The nearest other station, Prairie Du Chien, WI, 12 miles east of Monona and 507 feet lower, has a last frost 3 days earlier (April 28), a first frost 9 days later (October 12), a season 13 days longer.

The first measurable snow (0.1 inch or more) in Monona usually arrives around November 21, and the last of the season around March 28. Those are the middle dates across the winters of 1991–2020 at Postville, IA, 9.2 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 34 days after the last snow.

ZIP 52159 is 9 miles from Postville, IA, which stands at 1,165 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 1 other ZIP code shares the station; its figures are presented in full for 52162 (Postville). The 2023 USDA Plant Hardiness Zone Map lists ZIP 52159 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.