I am not usually one for technical discussions, as I like to leave that for Mike DeFino. And this is kind of a piggyback to his blog... but there is something that stands out to me on the model guidance that is skewing the outcome downstream...
Take a look at 500mb on the 12z GFS, for example (since the HPC didn't throw this run out)... I circled two areas, one is a shortwave over Michigan, the 2nd near our low pressure system is energy that is sitting at the base of the trough:
I don't care what anybody says, the pattern change never really happened, even though many have kept arguing all Winter long that it was coming... yes myself included for a time before I gave up on that idea. With that being said, why would anybody argue against an entrenched pattern of this Winter right now with everything that has[n't] happened thus far?
The reason I point out the Michigan energy is this:
The Northern stream energy dives ahead of the system as modeled and the storm stays to the South. However... the separate pieces of energy diving down through Michigan with the Northern stream seems to be hanging back on several models. The trend all winter in this La Nina pattern has been to propagate shortwaves associated with the Northern stream a lot faster than modeled, time after time... that piece of energy that is at the base of the polar vortex trough is actually pulling the shortwave coming through Michigan further south... and as a result, it is significantly slowing this feature. Since it has been the same trend all Winter to propagate these shortwaves with the Northern stream faster, I can't agree with this solution or go against the trend that has been there this entire time. This is a big red flag to me.
Where I am going with this:
There is no reason in a progressive La Nina pattern that this trend wouldn't continue. It has happened all year long where the Northern stream energy pieces have been faster than modeled, so gotta go with the trend of the pattern here. By the s/w kicking East faster, it allows for the confluence in New England to break down, and currently under-modeled Southern Vortmax will allow for increased heights along the Eastern seaboard. Increased heights = NW propagation of the storm and will not miss S&E. What's crazy is the models are a mere 200 miles in a spread apart from each other with this feature. But in this case, as little as 200 miles makes a big difference to what you get downstream as the end result.
I am also currently against the idea the 50/50 low bombs well NE of Maine... my theory of the confluence disruption goes right out the window if the storm bombs out that has affected us today. Strong 50/50 = no storm for even Philly.
Anybody putting a first call map out with totals right now is an absolute clown and asking for people to rip them apart. So obviously, I will do no such thing. Similar to Mdaniel91 and what he did in his blog, I am issuing a "probabilities" map, which is precent chance of seeing SOME accumulation... whether that's 1/2" of snow or a foot doesn't matter... this is the % chances at seeing at least SOME accumulation Saturday night and Sunday as I see it right now.
My thoughts on the models are this... right now they are all over the place, and although the Northern stream is collectively modeled as more progressive, it hasn't been sampled yet until that energy comes onshore. Once it does, and observations are taken, we will see how the Northern stream and Southern Stream interact, if at all. I think the keys are now shifting focus to
1) how strong the 50/50 low gets... too strong and we're screwed
2) the Northern stream energy over Michigan... does it propagate East faster and help the storm come NW with the increased Southern stream vortmax and increased heights along the Eastern seaboard?
My vote right now is yes it does, and let the NW trend commence... may not be until 12z tomorrow or later, but we will find out soon enough...