Frost Dates for 61755 — Mackinaw, Illinois

Half of springs at Congerville 2NW, IL see their last 32°F night by April 21; only one in ten is past it by April 7, and one in ten still has one after May 7. Planting tender crops after April 28 leaves a 30% risk of a frost on them, after May 2 20%, and after May 7 10%. The 16 days between the even-odds date and the one-in-ten date is narrower than at 22% of the 158 Illinois stations behind this site's ZIP pages.

In fall the first 32°F night comes by October 17 in half of years, as early as October 3 one year in ten, and holds off past October 30 one year in ten. A hard freeze of 28°F follows 10 days later, around October 27; in spring the one-in-ten date for 28°F is April 27, 10 days before the 32°F one. The freeze-free season lasts 178 days in a typical year, shorter than at 70% of the 158 Illinois stations; it reaches 195 days one year in ten and falls short of 155 one year in ten. On average 134 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 11 days longer, to about May 2, and return around October 6. Severe cold of 16°F or below comes every winter, usually between November 29 and March 8.

July is the warmest month at Congerville 2NW, IL, averaging 86°F by day and 65°F at night; January is the coldest at 33°F and 17°F. A freezing night is all but certain in every month from October to April (a 90% chance or more). From June to September the chance of a 32°F night stays under 10% each month. In spring, May still brings one in 22% of years.

Warmth for growing adds up to 3,532 growing degree days a year (base 50°F), more than at 53% of the 158 Illinois stations. 36% of them (1,262) have built up by the end of June, and July adds the most, 779. Precipitation totals 37.5 inches a year, most in May (4.8") and least in February (1.8"); May to September bring 53% of it.

The nearest other station, Bloomington 5W, IL, 14 miles east of Mackinaw and 212 feet higher, has the same 50% last-frost date, a first frost 3 days later (October 20), a season 3 days longer.

The first measurable snow (0.1 inch or more) in Mackinaw usually arrives around December 3, and the last of the season around March 15. Those are the middle dates across the winters of 1991–2020 at Mackinaw 3WSW, IL, 4.1 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 37 days after the last snow.

ZIP 61755 is 8 miles from Congerville 2NW, IL, which stands at 640 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 5 other ZIP codes share the station; its figures are presented in full for 61571 (Washington). The 2023 USDA Plant Hardiness Zone Map lists ZIP 61755 in zone 6a, the band where the coldest night of the year averaged -10 to -5°F (-23.3 to -20.6°C) over 1991–2020; a zone describes winter lows, not frost dates.