The disturbance riding the jet stream that will be the key player in how the weekend pattern evolves is still over the Pacific Ocean. This upper level feature won’t even arrive above the Pacific Northwest landmass until Thursday evening. Confidence and Consensus will begin to improve when the feature reaches land as there will be better information inputs into the forecast models (the equation will have more accurate data for a more accurate solution). Think of the term “garbage in garbage out” and it becomes clear why we have such low confidence during the Winter several days out ahead of a potential storm. The jet stream is after all a river of air that is very hard to predict and behaves very similarly to a river of water. What the model is trying to do is predict where a twig “the mid-level disturbance currently over the Pacific Ocean” dropped in the “river of water” (jet stream) will end up downstream (this weekend). The calculus that is involved in this equation is highly dependent upon good input data. We just don’t have enough coverage of buoys over the ocean to have excellent input data until the storm system reaches a landmass.
Image below shows the disturbance (circled in red) I am referring to at 1pm Thursday (still just off of the Pacific Northwest coast):
By Friday morning this disturbance is forecast to have reached land (image below Friday at 7am):
Finally by Sunday the disturbance is interacting with the upper level trough carving out across the northeast:
Exactly where this disturbance tracks and its timing and position in relation to the upper level trough diving over eastern Canada will ultimately influence who gets snow (and right now the models are in disagreement if the snow falls over the DC Region or to our south across southern Virginia and northern North Carolina.
This morning’s GFS ensembles still show snow impacting our region over the weekend with slight variations in track still evident (images below courtesy WxBell). How far south the arctic front tracks on Friday into the day on Saturday will also play a huge roll in where the snow ends up on Sunday.
GFS Ensembles for Snowfall Accumulation:
The European and Canadian currently are the furthest south in track:
- Latest GEM (Canadian forecast model) currently shows snow remaining well south of us across central North Carolina, but also shows a disturbance over the northern plains and a “juicy” storm developing along the Gulf coast that could phase providing the 2nd opportunity for an east coast storm around the 14th/15th of March.
European Ensembles (image below) are also not in agreement on where the weekend system tracks:
- Some of the 50 ensemble members show snow accumulating in the DC Area, while others keep accumulation suppressed to the south of us over North Carolina.
DC Storms 
It seems to be characteristic for the models to show the snowstorm too far south at some point prior to the event. For example, prior to our monster storm of last winter, there were model runs that had the heaviest snow south of DC. But when all was said and done, the heaviest was in the usual band from Damascus to the Baltimore suburbs.
I know there has been a trend to keep pushing the track further south. Let’s home that stops soon! The upper system goes right about over us, so that is one good sign,
At least it looks plenty cold enough, both surface and aloft, to get accumulating snow and not the typical March slop storm. If it starts at night as forecast, that should help as well.
The drama!!! I can’t handle it! BUT, I do love this post, especially the overview at the top with where the “twig” will track. Very informative.