Frost Dates for 19373 — Thornton, Pennsylvania

Half of springs at W Chester 2 NW, PA see their last 32°F night by April 26; only one in ten is past it by April 10, and one in ten still has one after May 11. Planting tender crops after May 2 leaves a 30% risk of a frost on them, after May 6 20%, and after May 11 10%. The 15 days between the even-odds date and the one-in-ten date is narrower than at 72% of the 176 Pennsylvania stations behind this site's ZIP pages.

In fall the first 32°F night comes by October 18 in half of years, as early as October 4 one year in ten, and holds off past November 1 one year in ten. A hard freeze of 28°F follows 12 days later, around October 30; in spring the one-in-ten date for 28°F is April 26, 15 days before the 32°F one. The freeze-free season lasts 174 days in a typical year, shorter than at 52% of the 176 Pennsylvania stations; it reaches 194 days one year in ten and falls short of 152 one year in ten. On average 131 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 7, and return around October 7. Severe cold of 16°F or below comes every winter, usually between December 6 and March 7.

July is the warmest month at W Chester 2 NW, PA, averaging 86°F by day and 64°F at night; January is the coldest at 40°F and 20°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 35% of years.

Warmth for growing adds up to 3,270 growing degree days a year (base 50°F), more than at 77% of the 176 Pennsylvania stations. 34% of them (1,098) have built up by the end of June, and July adds the most, 773. Precipitation totals 51.0 inches a year, most in September (5.7") and least in February (3.1"); May to September bring 45% of it.

The nearest other station, Marcus Hook, PA, 7 miles southeast of Thornton and 365 feet lower, has a last frost 27 days earlier (March 30), a first frost 29 days later (November 16), a season 56 days longer.

The first measurable snow (0.1 inch or more) in Thornton usually arrives around January 8, and the last of the season around March 9. Those are the middle dates across the winters of 1991–2020 at Wilmington Porter Rsch, DE, 8.7 miles away: half of years saw snow earlier, half later. Snow fell in 92% of those winters. The last 32°F night of spring typically comes 48 days after the last snow.

ZIP 19373 is 7 miles from W Chester 2 NW, PA, which stands at 375 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 4 other ZIP codes share the station; its figures are presented in full for 19382 (West Chester). The 2023 USDA Plant Hardiness Zone Map lists ZIP 19373 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.