* Winter weather advisory for northern Maryland, for one to two inches of snow through Saturday morning *

A fast-moving weather system swinging through the D.C. region early Saturday may put down roughly a coating to an inch of light snow.

Locations well north and northwest of Washington are most likely to see snow amounts up to an inch or a little more, while areas well south of the District may see only a few flurries, if that. For most of the immediate area, a light coating seems most likely.

Patchy light snow may break out between 1 and 4 a.m. Saturday and should taper off between 7 and 10 a.m. By midday and into the afternoon, a few flurries may linger, mainly northeast of Washington.

GFS model forecast of snow Saturday in Mid-Atlantic.

With temperatures in the 20s as the snow falls, it will be fluffy and light — easy to sweep away with a broom. Whatever falls should stick to everything, given the subfreezing ground temperatures. Slick spots on untreated roads are possible — so use caution driving Saturday morning.

This ranks as Level One “nuisance event” on our Winter Storm Impact Scale because of the potential for light accumulating snow and cold temperatures. Its impact will be lessened by its occurring early in the day on a weekend, when traffic is light.

In the southern part of the region, outside the zone where accumulation is forecast, this weather system is not rated.

Snow amount discussion

There is no question sufficient cold air is in place for snow, but the limiting factor for much accumulation will be moisture. The responsible disturbance originates from Canada and will not be able to tap into moisture from the Gulf of Mexico or Atlantic Ocean.

As the disturbance crosses the mountains, its moisture will be further depleted. Moreover, the center of the disturbance is passing right overhead, and, in such cases, the stripe of steadiest snow usually occurs to its north.

Essentially, our region will be served snow leftovers.

Every weather model along with the forecast from the National Weather Service agrees that a dusting to one inch is the most likely accumulation in the region with the highest amounts to the north and northwest.

American (GFS model forecast of snowfall Saturday. Note it assumes a 10:1 snow-to-liquid ratio. In this event, it could be higher or closer to 20:1 — meaning there is potential for twice as much snow as shown.)

The one factor that could lead to somewhat higher accumulations in some areas is the snow’s fluff factor. This will be a very dry snow, which will allow flakes to pile up faster than they normally do.

In a typical Washington snow event, a weather system that precipitates the equivalent of one-tenth of an inch of rain would produce one inch of snow. We call this a 10:1 snow-to-liquid ratio. But in this event, the equivalent of one-tenth of an inch of rain might equate to twice as much snow, signifying more like a 20:1 snow-to-liquid ratio. (This is the type of snow perfect for skiing and common in places like Utah and Colorado.)

Because models are predicting the equivalent of about 0.03 to 0.08 inches of rain for this event around Washington, that could reasonably convert to 0.6 to 1.6 inches of snow instead of 0.3 to 0.8 inches. As such, the snow boom scenario for this system is around 2 inches in Washington.

It’s also possible a lot of the snow fizzles out after it crosses the mountains, and we see little or no accumulation. That’s the bust scenario.

Beyond Saturday

The snow chances looking ahead into next week aren’t great. Yes, it will be very cold – which you need for snow. But moisture will again be a limiting factor.

The coastal storm the European model had been predicting for the middle of next week has vanished. All models now predict it to remain too far off the coast – although it still bears monitoring in case models trend it back westward.

Aside from that, we’ll need to watch fast-moving disturbances moving through the flow from the west that could produce some light snow or flurries, like Saturday’s system.

In general, snow has been hard to come by this winter and Washington is not alone. If we examine accumulated snow across the Lower 48 (see the graphic below) it has been in low supply outside the Rockies and the very northern tier of the nation.

While it’s been very cold, it’s also been very dry as most weather systems have come into the Lower 48 from Canada, deprived of much moisture. The southern branch of the jet stream along which storms can form and draw in a lot of moisture from the Gulf of Mexico has been very quiet, except for the one storm that brought snow to the South early in December. This is not all that unusual for a La Niña winter.