Sorry to say, but at this time Friday still remains a mystery. I know you want me to tell you how much snow to expect, but I just can't yet do that for you. I will say that most of us in Maryland will experience snow, but how much is still the question.
WEDNESDAY EVENING / THURSDAY MORNING
A weak clipper system moves through the area late Wednesday bringing with it the chance for snow. This is nothing major and should only be a dusting to an inch in isolated locales. This is true for central and S MD.
Moving from S MD up to central MD, the precip should begin later in the morning to early evening, respectively. I won't yet say how much accumulation, but I am confident in saying the precip will be snow with a chance of sleet in S MD mixed with the snow. The atmosphere will be too cold for rain or freezing rain. Although, the local NWS is still forecasting freezing rain and rain for Friday. It appears they believe that as the Low approaches it will bring in warmer southern air over top of the cold air. That set up will produce sleet, but I just don't see that much warm air making it into the atmosphere. All other local NWS in the region are forecasting all snow. I agree.
But, I do have a concern with this system. As I mentioned yesterday, cold is not the issue, and the position of the Low is obvious in regards to its impact on our accumulation, but I think the greater concern for us will be dry air. In the past couple storms it was the dry air that stopped our snow accumulation. I think that is a concern for Friday as well. The cold air today is quite dry, and the system moving in Friday is not an overly strong one, therefore, a lack of moist air. I'm not saying it won't snow Friday, but we might not see large accumulations.
WHY THE DELAY?
So, given all of our technology, model data, and experience, why don't we yet know what will happen 3 days from now? First,the storm which will impact us on Friday is currently over the Pacific Ocean! It isn't even in the US yet! The other issue is with the models. Very briefly, model data works by inputting weather observations into a computer programmed to analyse the data based on a pre-programmed equation and historical outcomes in similar situations. This is one of the reasons why the models do better as the storm nears since the model is being fed more current and regional data. In this case, the data being entered is from sources over the Pacific. Once the storm comes on land we will have more observation points, and the models will have more historical data to compute. Within a day or so of a storm I start to compare acutal radar images along with satellite imagery to where the models said the events would happen. This tells me how well the models are doing with the data. In this sorm's case I need to see some consistency in the models, but that won't happen until more data points can be entered (please note that the above explanation is greatly condensed).
I will have another update later Wednesday, and will hopefully have more details by then. Follow me daily on Twitter (@wxmanmd) for weather info and tidbits.