Frost advisory and freeze warning for DC’s western suburbs Saturday night

The National Weather Service has issued frost and freeze alerts for western parts of the DC area late Saturday night into early Sunday morning, with the coldest temperatures of the season so far expected.

A freeze warning covers western Loudoun County in Virginia, Frederick County in Maryland, and areas farther west. A frost advisory covers northwestern Montgomery and Howard counties in Maryland, plus eastern Loudoun, northwestern Prince William, and Fauquier counties in Virginia. Additional areas to the north and south are also under advisory.

Both alerts run from 1 a.m. to 9 a.m. Sunday.

Clear skies, fresh cold air, and slackening winds will promote overnight cooling. Much of the DC area should fall deep into the 30s. Milder spots like downtown DC may only dip to around 40, while locations right along the water could be a touch warmer.

Although many of the temperatures on the National Weather Service forecast map above are just above freezing in the frost and freeze alert zones, readings are likely to be locally colder. Plus, frost can form with air temperatures in the mid-30s, as officially measured at 2 meters above the ground, when temperatures at ground level fall to freezing.

The high-resolution NAM model, below, illustrates the potential for locally colder readings.

DC is still about a month away from its average first freeze, but western parts of the area are at or approaching the time when freezes typically begin in our current climate.

The average first freeze at Dulles Airport is Oct. 22, compared with Oct. 25 in Damascus, and Nov. 22 in Annapolis. Since frost can form with air temperatures in the mid-30s, average first frost dates are generally about a week to 10 days earlier.