Frost Dates for 20171 — Herndon, Virginia

Half of springs at Washington Dc Dulles Airport, VA see their last 32°F night by April 15; only one in ten is past it by March 31, and one in ten still has one after April 30. Planting tender crops after April 21 leaves a 30% risk of a frost on them, after April 25 20%, and after April 30 10%. The 15 days between the even-odds date and the one-in-ten date is narrower than at 70% of the 128 Virginia stations behind this site's ZIP pages.

In fall the first 32°F night comes by October 24 in half of years, as early as October 10 one year in ten, and holds off past November 5 one year in ten. A hard freeze of 28°F follows 11 days later, around November 4; in spring the one-in-ten date for 28°F is April 16, 14 days before the 32°F one. The freeze-free season lasts 194 days in a typical year, shorter than at 54% of the 128 Virginia stations; it reaches 214 days one year in ten and falls short of 170 one year in ten. On average 102 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 29, and return around October 14. Severe cold of 16°F or below comes every winter, usually between December 22 and February 24.

July is the warmest month at Washington Dc Dulles Airport, VA, averaging 88°F by day and 67°F at night; January is the coldest at 43°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 87% of years.

Warmth for growing adds up to 3,886 growing degree days a year (base 50°F), more than at 52% of the 128 Virginia stations. 36% of them (1,381) have built up by the end of June, and July adds the most, 843. Precipitation totals 43.2 inches a year, most in May (4.7") and least in February (2.6"); May to September bring 48% of it.

The nearest other station, WFO Sterling, VA, 6 miles northwest of Herndon, has a last frost 2 days earlier (April 13), a first frost 2 days later (October 26), a season 1 day longer.

The first measurable snow (0.1 inch or more) in Herndon usually arrives around December 14, and the last of the season around March 14. Those are the middle dates across the winters of 1991–2020 at Washington Dc Dulles Airport, VA, 3.4 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 32 days after the last snow.

ZIP 20171 is 3 miles from Washington Dc Dulles Airport, VA, which stands at 290 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 5 other ZIP codes share the station. The 2023 USDA Plant Hardiness Zone Map lists ZIP 20171 in zone 7b, the band where the coldest night of the year averaged 5 to 10°F (-15.0 to -12.2°C) over 1991–2020; a zone describes winter lows, not frost dates.