Frost Dates for 21084 — Jarrettsville, Maryland

Half of springs at Conowingo Dam, MD see their last 32°F night by April 3; only one in ten is past it by March 22, and one in ten still has one after April 18. Planting tender crops after April 9 leaves a 30% risk of a frost on them, after April 14 20%, and after April 18 10%. The 15 days between the even-odds date and the one-in-ten date is narrower than at 52% of the 50 Maryland stations behind this site's ZIP pages.

In fall the first 32°F night comes by November 6 in half of years, as early as October 26 one year in ten, and holds off past November 19 one year in ten. A hard freeze of 28°F follows 11 days later, around November 17; in spring the one-in-ten date for 28°F is April 6, 12 days before the 32°F one. The freeze-free season lasts 217 days in a typical year, longer than at 64% of the 50 Maryland stations; it reaches 236 days one year in ten and falls short of 197 one year in ten. On average 106 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 15 days longer, to about April 18, and return around October 28. Severe cold of 16°F or below comes in 99% of winters, usually between January 2 and February 20.

July is the warmest month at Conowingo Dam, MD, averaging 88°F by day and 69°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 64% of years.

Warmth for growing adds up to 3,882 growing degree days a year (base 50°F), fewer than at 52% of the 50 Maryland stations. 34% of them (1,328) have built up by the end of June, and July adds the most, 868. Precipitation totals 48.2 inches a year, most in September (5.1") and least in February (2.6"); May to September bring 47% of it.

The nearest other station, Aberdeen Phillips Field, MD, 18 miles southeast of Jarrettsville, has a last frost 7 days later (April 10), a first frost 7 days earlier (October 30), a season 16 days shorter.

The first measurable snow (0.1 inch or more) in Jarrettsville usually arrives around December 16, and the last of the season around March 16. Those are the middle dates across the winters of 1991–2020 at New Park, PA, 8.9 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 18 days after the last snow.

ZIP 21084 is 15 miles from Conowingo Dam, MD, which stands at 40 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 9 other ZIP codes share the station; its figures are presented in full for 21050 (Forest Hill). The 2023 USDA Plant Hardiness Zone Map lists ZIP 21084 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.