Frost Dates for 61350 — Ottawa, Illinois

Half of springs at Ottawa 5SW, IL see their last 32°F night by April 20; only one in ten is past it by April 6, and one in ten still has one after May 5. Planting tender crops after April 26 leaves a 30% risk of a frost on them, after April 30 20%, and after May 5 10%. The 15 days between the even-odds date and the one-in-ten date is narrower than at 54% of the 158 Illinois stations behind this site's ZIP pages.

In fall the first 32°F night comes by October 22 in half of years, as early as October 7 one year in ten, and holds off past November 4 one year in ten. A hard freeze of 28°F follows 10 days later, around November 1; in spring the one-in-ten date for 28°F is April 24, 11 days before the 32°F one. The freeze-free season lasts 183 days in a typical year, shorter than at 53% of the 158 Illinois stations; it reaches 203 days one year in ten and falls short of 162 one year in ten. On average 129 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 1, and return around October 11. Severe cold of 16°F or below comes every winter, usually between November 30 and March 10.

July is the warmest month at Ottawa 5SW, IL, averaging 83°F by day and 64°F at night; January is the coldest at 31°F and 15°F. A freezing night is all but certain in every month from November 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 18% of years.

Warmth for growing adds up to 3,202 growing degree days a year (base 50°F), fewer than at 72% of the 158 Illinois stations. 35% of them (1,121) have built up by the end of June, and July adds the most, 733. Precipitation totals 36.1 inches a year, most in May (4.5") and least in February (1.5"); May to September bring 54% of it.

The nearest other station, Marseilles Lock, IL, 5 miles east of Ottawa, has a last frost 7 days later (April 27), a first frost 5 days earlier (October 17), a season 12 days shorter.

The first measurable snow (0.1 inch or more) in Ottawa usually arrives around December 5, and the last of the season around March 15. Those are the middle dates across the winters of 1991–2020 at Ottawa 5SW, IL, 3.7 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 36 days after the last snow.

ZIP 61350 is 4 miles from Ottawa 5SW, IL, which stands at 525 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. The 2023 USDA Plant Hardiness Zone Map lists ZIP 61350 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.