The posts in this forum are NOT official forecast and should not be used as such. They are just the opinion of the poster and may or may not be backed by sound meteorological data. They are NOT endorsed by any professional institution or 
It’s all a matter of timing and phasing. Here’s this afternoon’s 18z ETA.
84 hour ETA 300mb
84 hour ETA 500mb
The ETA this particular run is indicating a stronger northern branch s/w (which is also closed off at both 300 and 500mb as you can tell from the figures above), but on the other hand is much weaker with the southern branch energy, which because the vort energy is less concentrated, the system is an open wave.
It would also appear as if the ETA is trying to hold some energy back, notice the other s/w sliding down the western side of the positively tilted trough axis. The key here though is that the vort energy is strung out over a wide area from northern Mexico through the southeastern part of the country.
But, Notice the stronger northern branch s/w which is closed off at 500mb and much stronger than both the GFS 12z and 18z runs. So, it drops in and phases with the strung out southern branch energy across the southeast.
What is very interesting to me; however is that the GFS 18z run shows a more concentrated vort and therefore stronger southern branch system across the southern plains (552dm closed low) which is stronger than the ETA’s less concentrated solution, so therefore, it’s probably not a question of the southern branch energy being strong enough to feed back heat as it lifts and turns up, but more a question of phasing.
This is 18z GFS 500mb valid at 84 hours. Take a look at how different it is from the ETA at the same time, in regard to the strength of both the northern and southern stream systems.
18z GFS 500mb valid at 90 hours
The GFS indicates a delayed and weaker phasing solution than what the ETA or its 12z run does. The northern stream s/w is significantly weaker than the ETA or what it has on the 12z run (which would be roughly the middle ground on the intensity of the northern stream system).
The main thing here is that the fait of this system lies in what happens with the northern stream system. If a situation similar to the 18z GFS solution unfolds, it will be a storm cancel. If the 18z ETA idea or the 12z GFS/GGEM is correct, we’re looking at a major winter storm for many, especially south of New York City.
I’ll discuss the ECMWF later once it comes out (good or bad, lol). But a note about it now, the ECMWF runs have been having a problem holding back the energy too much, so we should really be careful of them, at least until we get closer to verification.
The ECMWF’s performance has not been that great this winter either, (especially in comparison to last) recall with the December nor’easter, at the last minute it shifted the track to the southeast, against virtually every other model solution.
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