Frost Dates for 19401 — Norristown, Pennsylvania

Half of springs at Norristown, PA see their last 32°F night by April 8; only one in ten is past it by March 28, and one in ten still has one after April 23. Planting tender crops after April 15 leaves a 30% risk of a frost on them, after April 18 20%, and after April 23 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 31 in half of years, as early as October 14 one year in ten, and holds off past November 12 one year in ten. A hard freeze of 28°F follows 10 days later, around November 10; in spring the one-in-ten date for 28°F is April 12, 11 days before the 32°F one. The freeze-free season lasts 203 days in a typical year, longer than at 91% of the 176 Pennsylvania stations; it reaches 222 days one year in ten and falls short of 183 one year in ten. On average 107 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 14 days longer, to about April 22, and return around October 20. Severe cold of 16°F or below comes every winter, usually between December 27 and February 26.

July is the warmest month at Norristown, PA, averaging 89°F by day and 67°F at night; January is the coldest at 42°F and 23°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 81% of years.

Warmth for growing adds up to 3,815 growing degree days a year (base 50°F), more than at 96% of the 176 Pennsylvania stations. 34% of them (1,305) have built up by the end of June, and July adds the most, 859. Precipitation totals 50.7 inches a year, most in August (5.0") and least in February (3.0"); May to September bring 46% of it.

The nearest other station, Phoenixville 1 E, PA, 8 miles west of Norristown, has a last frost 10 days later (April 18), a first frost 4 days earlier (October 27), a season 11 days shorter.

The first measurable snow (0.1 inch or more) in Norristown usually arrives around December 11, and the last of the season around March 4. Those are the middle dates across the winters of 1991–2020 at Norristown, PA, 1 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 35 days after the last snow.

ZIP 19401 is 1 mile from Norristown, PA, which stands at 62 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 17 other ZIP codes share the station; its figures are presented in full for 19403 (Norristown). The 2023 USDA Plant Hardiness Zone Map lists ZIP 19401 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.