Frost Dates for 53097 — Mequon, Wisconsin

Half of springs at Germantown, WI see their last 32°F night by May 6; only one in ten is past it by April 22, and one in ten still has one after May 25. Planting tender crops after May 14 leaves a 30% risk of a frost on them, after May 19 20%, and after May 25 10%. The 19 days between the even-odds date and the one-in-ten date is the widest of the 182 Wisconsin stations behind this site's ZIP pages.

In fall the first 32°F night comes by October 8 in half of years, as early as September 26 one year in ten, and holds off past October 24 one year in ten. A hard freeze of 28°F follows 12 days later, around October 20; in spring the one-in-ten date for 28°F is May 12, 13 days before the 32°F one. The freeze-free season lasts 154 days in a typical year, shorter than at 53% of the 182 Wisconsin stations; it reaches 175 days one year in ten and falls short of 131 one year in ten. On average 156 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 13 days longer, to about May 19, and return around September 29. Severe cold of 16°F or below comes every winter, usually between November 22 and March 22.

July is the warmest month at Germantown, WI, averaging 81°F by day and 60°F at night; January is the coldest at 27°F and 11°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 64% of years.

Warmth for growing adds up to 2,381 growing degree days a year (base 50°F), fewer than at 57% of the 182 Wisconsin stations. 31% of them (733) have built up by the end of June, and July adds the most, 625. Precipitation totals 34.3 inches a year, most in June (4.4") and least in February (1.4"); May to September bring 56% of it.

The nearest other station, Grafton Wwtp, WI, 6 miles northeast of Mequon, has a last frost 13 days earlier (April 23), a first frost 16 days later (October 24), a season 28 days longer.

The first measurable snow (0.1 inch or more) in Mequon usually arrives around November 17, and the last of the season around April 9. Those are the middle dates across the winters of 1991–2020 at Germantown, WI, 5.7 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 27 days after the last snow.

ZIP 53097 is 6 miles from Germantown, WI, which stands at 850 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 2 other ZIP codes share the station; its figures are presented in full for 53051 (Menomonee Falls). The 2023 USDA Plant Hardiness Zone Map lists ZIP 53097 in zone 5b, the band where the coldest night of the year averaged -15 to -10°F (-26.1 to -23.3°C) over 1991–2020; a zone describes winter lows, not frost dates.