Forecast radar at 1 a.m. Monday from high-resolution NAM model. (PivotalWeather.com/PivotalWeather.com)

* Winter weather advisory along and west of Interstate 95 from 7 p.m. Sunday to 10 a.m. Monday | SchoolCast /FedCast *

4:10 p.m. update: All of the information below still applies but the latest data suggests the onset of precipitation is likely to be on the later side of the 6 to 10 p.m. window, beginning first in our west and northwest areas between 8 and 10 p.m., and over the rest of the region from 9 to 11 p.m., from northwest to southeast. We can’t rule out some very light and spotty precipitation before this time, but it’s unlikely to be of consequence.

Original post from 12:15 p.m.

A weak weather disturbance will scoot through the region tonight and, with cold air in place, a wintry mix of precipitation is expected. We expect the biggest impact in our northern and northwestern suburbs, where precipitation will be steadiest and temperatures will be coldest.

The onset time of precipitation is uncertain. The precipitation is running into dry air and may, at first, evaporate. Some snow and mixed precipitation could creep into our far northwestern areas as early as after sunset but may take until after 9 or 10 p.m. to reach the Beltway. Getting around this evening may be okay, but check radar and our updates before venturing out.


Precipitation is most likely during the overnight hours to around sunrise, when travel should be most difficult. It will be a mix: more snow in Northern Maryland (Zone 1), and a combination of freezing rain, sleet and snow in the immediate area and to the west and southwest (Zones 1 and 2). Areas south and southeast of downtown (Zone 3) may just see rain and sleet, with little accumulating frozen precipitation.

Snow accumulation is most likely along and north of Interstate 70 in Maryland, where an inch or two is possible. While just a coating or so of snow may accumulate in the immediate metro area, an icy mix of sleet and freezing rain could cause slick spots, especially west and northwest of downtown.


(CWG/CWG)

Weekend temperatures have remained cold enough that snow and freezing rain should be able to cause areas of iciness, especially on untreated sidewalks, parking lots and driveways. On the roads, neighborhood streets, ramps, bridges and overpasses could also turn slick. Pedestrians and motorists alike should use caution.

While a light glaze of ice could accumulate in Zones 1 and 2, not enough is expected to build up on trees and power lines to trigger outages.


Predicted ice accumulation from National Weather Service through Monday morning. (National Weather Service)

Precipitation should tend to decrease toward morning and change to plain rain from southeast to northwest as temperatures gradually rise above freezing. But particularly in our colder areas (Zones 1 and 2), slick spots and school delays are quite possible before conditions thaw somewhat.

There is the chance of additional mixed precipitation Monday night into Tuesday morning. However, the latest information suggests iciness may be more of an issue north of Interstate 70 and along and west of Interstate 81, rather than being much of a problem in the immediate area where temperatures could remain above freezing. We will analyze this potential more in depth on Monday. The focus of this article is the wintry mix tonight into Monday morning.

Timeline


Forecast radar from high-resolution NAM model Sunday night into Monday morning. (PivotalWeather.com/PivotalWeather.com)

6 p.m. to 10 p.m. Sunday: Light snow may slowly develop north and northwest of the Beltway. A little light, patchy, mixed precipitation may develop elsewhere, but it’s equally likely to stay dry. Temperatures range from 30 degrees in our colder areas to 33 to 35 degrees elsewhere.

10 p.m. Sunday to 4 a.m. Monday: Expect light snow in our northern areas, possibly mixing with and changing to sleet and freezing rain. Elsewhere, a wintry mix of snow, sleet and freezing rain (except rain and sleet in from downtown Washington to the south and southeast). Temperatures range from 28 to 31 in our colder areas to 30 to 33 degrees elsewhere.

4 a.m. to 10 a.m. Monday: Mixed precipitation changes to mostly rain and drizzle from southeast to northwest and decreases in intensity. Temperatures range from 30 to 34 in our colder areas, to 32 to 36 degrees elsewhere.

Storm impact

This event rates as a Category 1 “nuisance event” on our winter storm impact scale for Zones 1 and 2 (it is not rated for Zone 3). Air and pavement temperatures are cold enough for roads and sidewalks to become slick, which is likely to lead to areas of hazardous travel and delays overnight Sunday into early Monday.


(Washington Post/Washington Post)

But precipitation amounts should be light, the duration of the event should be relatively short, and the worst of it should occur before the morning commute on Monday. All of these factors will mitigate the overall impact of the event.