Frost Dates for 61104 — Rockford, Illinois

Half of springs at Rockford Gtr Rockford Airport, IL see their last 32°F night by April 24; only one in ten is past it by April 10, and one in ten still has one after May 8. Planting tender crops after April 30 leaves a 30% risk of a frost on them, after May 2 20%, and after May 8 10%. The 14 days between the even-odds date and the one-in-ten date is narrower than at 82% 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 2 one year in ten, and holds off past October 30 one year in ten. A hard freeze of 28°F follows 11 days later, around October 28; in spring the one-in-ten date for 28°F is April 27, 11 days before the 32°F one. The freeze-free season lasts 174 days in a typical year, shorter than at 76% of the 158 Illinois stations; it reaches 192 days one year in ten and falls short of 154 one year in ten. On average 136 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 4, and return around October 6. Severe cold of 16°F or below comes every winter, usually between November 26 and March 13.

July is the warmest month at Rockford Gtr Rockford Airport, IL, averaging 84°F by day and 64°F at night; January is the coldest at 29°F and 15°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 26% of years.

Warmth for growing adds up to 3,083 growing degree days a year (base 50°F), fewer than at 80% of the 158 Illinois stations. 34% of them (1,059) have built up by the end of June, and July adds the most, 736. Precipitation totals 37.2 inches a year, most in June (5.2") and least in January (1.6"); May to September bring 56% of it.

The nearest other station, Beloit, WI, 17 miles north of Rockford, has a last frost 7 days later (May 1), a first frost 5 days earlier (October 12), a season 12 days shorter.

The first measurable snow (0.1 inch or more) in Rockford usually arrives around November 21, and the last of the season around March 31. Those are the middle dates across the winters of 1991–2020 at Rockford Gtr Rockford Airport, IL, 4.4 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 24 days after the last snow.

ZIP 61104 is 4 miles from Rockford Gtr Rockford Airport, IL, which stands at 730 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 13 other ZIP codes share the station; its figures are presented in full for 61108 (Rockford). The 2023 USDA Plant Hardiness Zone Map lists ZIP 61104 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.