GFS forecast for 1 p.m. on Saturday. (

* Winter weather advisory for D.C. metro, winter storm warning for western Loudoun and Frederick counties *

Update at 3:40 p.m.: A winter weather advisory has been issued for the immediate D.C. metro area. The National Weather Service is expecting 1 to 3 inches of snow along with a wintry mix across the metro. A winter storm warning has been issued for western Loudoun and Frederick counties, and points west, for snow accumulation of 5 to 8 inches. (NWS snow accumulation forecast)

Winter storm warning in pink, winter weather advisory in purple. (National Weather Service)

Original post:

A potent, moist low pressure system is developing over Texas on Friday, forecast to track northeast and bring a large swath of snow and wintry mix along with it. Forecast models are in agreement that the storm will bring accumulating snow to the D.C. area Saturday before changing to a wintry mix and then finally to rain late Saturday night.

Timing: Precipitation will start as snow, beginning mid-morning in the far northwest suburbs and working its way southeast across the area. The Beltway should expect precipitation to begin somewhere between the late morning and the early afternoon. While snow is expected through at least the mid-afternoon, sleet – and even brief periods of freezing rain – could begin to mix in somewhere between the mid- to late afternoon hours in the southeast, working its way to the northwest. Precipitation eventually turns to all rain overnight for the entire area and will last into Sunday morning.

[A 120-year-old record has been broken in D.C.]

Impacts: Given how frigid it has been this week, roads will surely be very cold and prone to ice. Unless they are very well treated, snow will begin to accumulate on roads almost immediately on Saturday morning, and travel will be hazardous. Even though an influx of warmer air will cause the snow to change to sleet and eventually rain, the roads won’t warm up that quickly, which could allow for some very slick spots during the transition in the late afternoon and early evening. Once it has changed over to all rain after 7 to 10 p.m., road conditions should improve in the immediate metro area but may remain hazardous in the far northwest suburbs overnight.

Snow accumulation forecast: The heaviest snow will accumulate in the northwest suburbs — includes northern Loudoun and Frederick counties (as well as the northern Baltimore metro area) — from 4 to 8 inches. Across the immediate D.C. metro area, 2 to 5 inches of snow is expected before it transitions to sleet and then rain – though there’s a 30 percent change of a “bust” scenario with fewer than 2 inches. Areas to the southeast may see up to 2 inches of snow.

The storm at hand will build northeast out of Texas on Friday. The storm will bring snow, wintry mix and rain to parts of the eastern U.S. from the lower Mississippi Valley to the Northeast. Winter storm watches and warnings are already in effect along the storm’s path, and we suspect that, at the very least, a winter weather advisory – if not a winter storm warning – will be posted for the D.C. metro area. By Saturday morning, the precipitation will reach the Mid-Atlantic.

[Friday evening skies will feature a cosmic trio]

Forecast models are bullish on bringing in precipitation quickly enough to produce a period of moderate to heavy snow during the day on Saturday. The heaviest accumulations are likely to occur to the north and west of the city with the lowest accumulations occurring to the the city’s south and east.

In these type of cases, trying to forecast how much snow will fall is always tricky – exactly how much often depends on how quickly the heavy band of precipitation spreads across your location before the mid-level warming leads to a changeover to sleet or even rain. Relatively small shifts in how rapidly the heavy precipitation moves into the area or changes in how quickly the warm layer aloft rises to above freezing can have a big impact on snowfall totals.

[Bitter cold = dead batteries (and stranded drivers)]

The 90 mph southerly wind being forecast at 3,000 feet by 4 p.m. Saturday argues strongly that warming will take place aloft. This will eventually change the snow into sleet – and possibly freezing rain – and then to rain from south to north.

The big question mark with this storm is how long the cold air will linger at ground-level. Arctic air can be stubborn, especially in those colder locations well west and north of the city. Still, the deep southerly flow should eventually win out and temperatures should rise to above freezing across the entire area by Saturday night, except for the far west and northwest areas.

More cold and snow:

How much snow is too much for your roof?

D.C. forecast through the weekend

Polar vortex turns Mid-Atlantic into Mid-Antarctic

Will D.C. be mocked again for its weekend snow?

Cold causes headaches for commuters, schools