Frost Dates for 06378 — Stonington, Connecticut

Half of springs at Westerly State Airport, RI see their last 32°F night by April 11; only one in ten is past it by March 31, and one in ten still has one after April 26. Planting tender crops after April 17 leaves a 30% risk of a frost on them, after April 20 20%, and after April 26 10%. The 15 days between the even-odds date and the one-in-ten date is wider than at 50% of the 28 Connecticut stations behind this site's ZIP pages.

In fall the first 32°F night comes by October 31 in half of years, as early as October 16 one year in ten, and holds off past November 12 one year in ten. A hard freeze of 28°F follows 11 days later, around November 11; in spring the one-in-ten date for 28°F is April 10, 16 days before the 32°F one. The freeze-free season lasts 201 days in a typical year, longer than at 89% of the 28 Connecticut stations; it reaches 221 days one year in ten and falls short of 181 one year in ten. On average 109 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 16 days longer, to about April 27, and return around October 17. Severe cold of 16°F or below comes every winter, usually between December 21 and February 28.

July is the warmest month at Westerly State Airport, RI, averaging 82°F by day and 66°F at night; January is the coldest at 40°F and 25°F. A freezing night is all but certain in every month from November to March (a 90% chance or more). From May to September the chance of a 32°F night stays under 10% each month. In spring, April still brings one in 86% of years.

Warmth for growing adds up to 3,008 growing degree days a year (base 50°F), more than at 64% of the 28 Connecticut stations. 28% of them (854) have built up by the end of June, and July adds the most, 736. Precipitation totals 42.9 inches a year, most in October (4.3") and least in February (2.8"); May to September bring 40% of it.

The nearest other station, Groton, CT, 6 miles west of Stonington, has a last frost 3 days earlier (April 8), a first frost 2 days later (November 2), a season 6 days longer.

The first measurable snow (0.1 inch or more) in Stonington usually arrives around December 13, and the last of the season around March 14. Those are the middle dates across the winters of 1991–2020 at Groton, CT, 6.5 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 28 days after the last snow.

ZIP 06378 is 6 miles from Westerly State Airport, RI, which stands at 81 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 6 other ZIP codes share the station; its figures are presented in full for 02891 (Westerly). The 2023 USDA Plant Hardiness Zone Map lists ZIP 06378 in zone 7a, the band where the coldest night of the year averaged 0 to 5°F (-17.8 to -15.0°C) over 1991–2020; a zone describes winter lows, not frost dates.