Frost Dates for 61074 — Savanna, Illinois

Half of springs at Mt Carroll, IL see their last 32°F night by May 2; only one in ten is past it by April 18, and one in ten still has one after May 18. Planting tender crops after May 9 leaves a 30% risk of a frost on them, after May 13 20%, and after May 18 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 7 in half of years, as early as September 24 one year in ten, and holds off past October 20 one year in ten. A hard freeze of 28°F follows 9 days later, around October 16; in spring the one-in-ten date for 28°F is May 9, 9 days before the 32°F one. The freeze-free season lasts 154 days in a typical year, the shortest of the 158 Illinois stations; it reaches 171 days one year in ten and falls short of 136 one year in ten. On average 155 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 12 days longer, to about May 14, and return around September 27. Severe cold of 16°F or below comes every winter, usually between November 17 and March 20.

July is the warmest month at Mt Carroll, IL, averaging 85°F by day and 60°F at night; January is the coldest at 31°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 56% of years.

Warmth for growing adds up to 2,917 growing degree days a year (base 50°F), fewer than at 88% of the 158 Illinois stations. 35% of them (1,015) have built up by the end of June, and July adds the most, 702. Precipitation totals 40.3 inches a year, most in June (5.5") and least in January (1.7"); May to September bring 56% of it.

The nearest other station, Fulton L&D #13, IL, 14 miles south of Savanna, has a last frost 19 days earlier (April 13), a first frost 19 days later (October 26), a season 41 days longer.

The first measurable snow (0.1 inch or more) in Savanna usually arrives around December 1, and the last of the season around March 20. Those are the middle dates across the winters of 1991–2020 at Mt Carroll, IL, 7.4 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 43 days after the last snow.

ZIP 61074 is 7 miles from Mt Carroll, IL, which stands at 640 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 2 other ZIP codes share the station. The 2023 USDA Plant Hardiness Zone Map lists ZIP 61074 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.