In the midst of a record 767 days since the DC area last received 2 inches or more of snowfall from one event, a system is currently organizing over the Intermountain West that will give the DC metro area it’s best chance since Snowmageddon for significant snow accumulations late Tuesday through Wednesday. Before taking drastic measures to prepare for “Snowquester,” it is worth noting there is still a significant amount of uncertainty with this system. The uncertainty surrounding this system isn’t so much “if” the DC area and surrounding counties will be impacted with significant amounts of precipitation, but more so what form of precipitation we will receive. While models are becoming more consistent with the track of system, minor disparities in track and intensity could mean the difference between DC metro receiving a mostly rain/wintry mix of precipitation that accumulates 1-2 inches at most and a major snow event that could accumulate 8-10 inches of snow. I’m expecting models to continue shifting between now through when the system is impacting our area given the complexity of this system, so at this point I think it would be most useful to summarize the current scenarios projected by the GFS and ECMWF (the two more reliable models) and provide my preliminary forecast.
1) GFS scenario — The system takes a more northerly track and intensifies significantly off the Southern Virginia/North Carolina coast after phasing with a weaker low from the South. Precipitation begins as rain in the DC metro area and surrounding counties Tuesday night, but as the system strengthens off the coast precipitation rates will increase, helping a faster and sustainable transition into heavy and wet snowfall that makes it to the ground before melting. The combination of air temperatures and intense snowfall rates will dictate surface temperatures — which will be above freezing at the onset of precipitation — and be a key factor in how much accumulation occurs. Assuming surface temperatures decrease enough to support accumulating snowfall by early/mid Wednesday, snowfall accumulations in the DC Metro area could range between 6 – 10 inches.
2) ECMWF scenario — The system takes a more southerly track and experiences less intensification than expected in the GFS scenario as it moves off the Atlantic Coast. Precipitation begins as rain in the DC metro area Tuesday night and the lack of intensification allows atmospheric and ground temperatures to remain above freezing through most of Wednesday, resulting in the majority of precipitation to fall in the form of rain. As the system exits Wednesday night and colder air is fed into the area from the backside of the system’s northerly winds, a transition to snow has the potential to accumulate 1-2 inches in the city. **It’s worth noting the ECMWF significantly outperformed the GFS in modeling Sandy and the recent blizzard in the Northeast.**
3) My forecast — At this time I believe the latest output of the GFS and ECMWF to be the extreme maximum and minimum impact scenarios of what could happen, respectively, and the most likely scenario is an average of the two (although most models have begun favoring more snowfall). I expect precipitation in the DC metro area to begin Tuesday evening in the form of rain, intensifying through the day Wednesday that allows for a transition to moderate wet snowfall by late morning/early afternoon Wednesday that accumulates 1-4 inches (4 inches more likely than 1 inch) by the time precipitation ends late Wednesday. The further West, SW, and NW you are from the city (and especially at higher elevations) your chances to see closer to 6 inches of snowfall increase significantly, and places like Charlottesville and Roanoke could see closer to 10-12 inches. Areas East of I-95 will likely see limited — if any — precipitation in the form of snow from this event.
Again, there is high confidence that the DC metro area will see significant amounts of precipitation from this system, but the remaining uncertainty revolves around what type of precipitation we will receive. Models have been trending more favorably for accumulating snowfall over the last day, but don’t be surprised to see forecasts waver between now and even while the storm is impacting our area since small shifts in the track of the system will have significant impacts on where the snow/rain line meanders.
I plan on posting an updated forecast Monday evening or Tuesday morning.