I don't know why I'm so surprised that each snowfall seems to fall apart 2-3 days before getting here. I'm the one who predicted a below average snowfall season with only 15" total accumulation in Harrisonburg. I guess it's the inner snow lover in me that secretly hopes my prediction is wrong - even though the meteorologist side is saying "good forecast". Such internal battles.
So here's what still leaves me a bit on edge. The current models - all of them from the 12z (morning) runs are NOT predicting the current precipitation structure very well. There's still a lot of precipitation on the NW side of the storm, bringing snow to Dallas and NE Texas. Icing started late Sunday morning in LA, MS, and AL - very ahead of schedule because of a long finger of precipitation that has extended 100 miles east of any model prediction.
This being said, I don't think it's going to make a major difference in our forecast. While the northern stream system (going through the IA, IL, MO, IN, OH) looks to stay strong for a while, it loses steam moving into the Appalachians. The southern stream system (bringing all the ice/snow to TX, LA, MS, AL) will be most powerful in the SE and lose strength moving up the coast. What models are agreeing on is a very weak southern stream storm and late phasing of the northern & southern stream - which leaves us out in the cold - with little snow.
Sometimes meteorologists look at "ensembles". Basically what we see as a GFS or CMC on the internet is a single run of the model, the base run. After that main run is completed, there are minor tweaks to different variables from that base (let's say, a 20 mile shift in track, or a 1 degree difference in surface temperature) and we get 10 or 15 ensembles (depending on the model) to look at. We look at these to see other plausible solutions if those minor tweaks were to actually play out in the atmosphere. Short range ensemble forecasts (SREF's), and even some GFS ensembles are actually bringing in some higher precipitation amounts (because of earlier phasing), but I'm still not sold - yet.
Of course, storm track is so important. If the southern stream low is farther west and can connect with the northern stream energy sooner- we have a shot of still seeing 2-4". Otherwise, we're looking at some meager amounts in the Valley as we stay in the "black hole" of snow for another storm.
Three things we have to keep an eye on over the next 24 hours.
1) The western extent of the precip in the southern stream storm
2) The track of the southern stream storm
3) The speed of the northern stream storm
Right now, I'm going to have to lessen the projected amounts based on the past 24 hours of model runs to about 1-2". Areas in the northern Valley, Frederick/Clarke counties, may pick up an additional inch because phasing may occur fast enough that we see some heavier snows develop closer to the northern VA/WV & MD panhandles Tuesday evening.
Of course we will re-evaluate tomorrow and I'll update the blog at least twice. Feel free to ask questions/comment!