Frost Dates for 61360 — Seneca, Illinois

Half of springs at Marseilles Lock, IL see their last 32°F night by April 27; only one in ten is past it by April 12, and one in ten still has one after May 13. Planting tender crops after May 3 leaves a 30% risk of a frost on them, after May 7 20%, and after May 13 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 29 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 May 1, 12 days before the 32°F one. The freeze-free season lasts 171 days in a typical year, shorter than at 84% of the 158 Illinois stations; it reaches 191 days one year in ten and falls short of 151 one year in ten. On average 139 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 10 days longer, to about May 7, and return around October 7. Severe cold of 16°F or below comes every winter, usually between November 27 and March 12.

July is the warmest month at Marseilles Lock, IL, averaging 84°F by day and 62°F at night; January is the coldest at 31°F and 14°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 37% of years.

Warmth for growing adds up to 3,084 growing degree days a year (base 50°F), fewer than at 80% of the 158 Illinois stations. 34% of them (1,063) have built up by the end of June, and July adds the most, 711. Precipitation totals 39.1 inches a year, most in June (4.9") and least in February (1.9"); May to September bring 54% of it.

The nearest other station, Morris 1 NW, IL, 10 miles east of Seneca, has a last frost 7 days earlier (April 20), a first frost 6 days later (October 23), a season 13 days longer.

The first measurable snow (0.1 inch or more) in Seneca usually arrives around December 2, and the last of the season around March 18. Those are the middle dates across the winters of 1991–2020 at Marseilles Lock, IL, 7.4 miles away: half of years saw snow earlier, half later. Snow fell in 96% of those winters. The last 32°F night of spring typically comes 40 days after the last snow.

ZIP 61360 is 7 miles from Marseilles Lock, IL, which stands at 490 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 61341 (Marseilles). The 2023 USDA Plant Hardiness Zone Map lists ZIP 61360 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.