The following is provided by DC Severe Winter Weather:

Sunday Evening Update (Potential Snowstorm update):

After the dusting of snow across the area this afternoon, its beginning to finally feel like winter. Late this week, there is growing potential for a significant, possibly major snowstorm. While it is important to realize that this is 5 days out and there is a wide margin for error, this threat is not one to be taken lightly. Friday morning, a storm system developing in the deep south will approach the area and collide with a dome of cold high pressure. Given the fading -NAO block, low pressure over the Canadian maritimes, and strong southern jet stream, the setup for this storm is very similar to major snowstorms in the past. There are two principle scenarios on the table: Either the two branches of the jet stream phase and create a major storm or they do not phase and the storm slides to our south. Right now, models, ensembles, and the pattern favor the first scenario, that of a major storm, but things can change quickly.

SCENARIO A: Major Storm: In this scenario, the northern and southern jet stream systems combine to our west and produce a surface low that tracks up the coast from the deep south. This scenario could lead to major, double-digit accumulations of snow as well as strong winds, coastal flooding, and icing. Even in this scenario, there is a chance strong winds blow off the Ocean and turn the snow to rain for a while along I-95 and east. The snow would likely begin Friday morning (1/22) and last until Saturday afternoon and evening (1/23). We give this scenario a 70% probability. The 12z Euro (depicted below) as well as the 18z GFS showed this scenario.

SCENARIO B: Minor Storm: In this scenario, the two branches of the jet stream stay separate. This would prevent the low pressure from deepining off shore and likely spare the area from any major impacts. That said, we could get a few inches from the southern jet stream system Saturday. We give this scenario a 30% probability. Some of the GEFS ensemble members (shown below) depicted this scenario.

School cancellations, other cancellations, and delays are very likely Friday (1/22). A cancellation or delay for teachers on professional day, next Monday (1/25), is possible as well. To give you an idea of the maximum potential here, the 12z Euro on the left produced 20-24″ of snow for the area between Friday morning (1/15) and Saturday evening (1/16). It’s shaping up to be an interesting and high-stakes week. We’ll keep you up to date as our thinking changes.