@PB GFI, as promised, here is my continuance of our friendly battle (haha) Since we have already gone over the synoptic and hemispheric arguments, I feel no need to revisit them now, unless you or others want to rehash all of that. I will note, however, that I feel that the modeling has largely been correcting in the direction that I have been expecting based on the points discussed in that previous post. From here, I will bring the focus a bit closer, and focus on some of the "broader details", which is where we left off yesterday. Again, my focus is going to largely be on the evolution of the H5 (anomalies) at times that I feel are crucial points, but instead of just using the EURO Ensemble plots, I will also pull in the ensembles of the GEFS and GEM as well to demonstrate the differences between them. All images will be annotated to help demonstrate my analysis (was thinking of doing a video initially, but they seem to get "lost in the sauce" so to speak lol ), but keep in mind there are some timing differences between the three ensembles, so the forecast periods used may not all match, but they match in the process of evolution.
Without further adieu, the first image will be of the EURO Ensemble at the point in which we begin to see our system of interest amplify in response to the downstream amplification/blocking (as I mentioned a few days ago):
The amplifying energies are outlined by the three lines near the Rockies, our building Eastern Seaboard ridge is denoted by the zig-zag line, and the circled region is the area in which blocking is beginning to develop (again, for reasons mentioned earlier). The last piece, and honestly, the most importance piece in my opinion, is the shortwave outlined by the box. This specific component is what will cause any difference of outcome, as I will show now. Up until this point, all three ensembles are in agreement. Here are all three ensembles valid at the point in which the western energies have bundled, and the system begins to amplify:
All three ensembles agree on a substantial enhancement of the (blocking) ridging across eastern North America, the low trapped underneath (blocked), and the general location of the system of interest over the central CONUS. I want to draw your attention to three key differences with the GEFS versus the other two, though, that are all connected. Notice how the GEFS keeps the northern stream energy elongated and fights it through the developing ridging over eastern North America, whereas the others keep that energy as a consolidated shortwave that harmlessly rockets by to the north. The GEFS' solution allows three key things to happen: 1. it starts to shear out the blocking ridge in eastern North America as it SLOWLY fights its way eastward, which both weakens the blocking ridge out in front of our system interest, and thereby the onshore/warmer low-level flow (initially in response to the higher pressure upstream of the blocked system over the western Atlantic as well the early development of the upper-level ridge across the northwestern Atlantic/eastern North America) 2. acts as a "lead" and helps to create a pathway for the system of interest which allows for a continued positive tilt concomitantly with 3. the substantial amplification of upstream ridging forced by the very slow progression of the elongated shortwave and continued rapid progression of systems into the West Coast. As the eastern North American ridging continues to shear out, it also shifts the axis of the Atlantic high eastward along with it, while also weakening its influence. Meanwhile, the trailing ridging continues amplify and reorient/develop increasing surface pressure in a way that enhances cold air advection into the northern tier of the eastern CONUS. Since we are all looking for a snowstorm, you should root for this to continue, and continue to trend in this direction. That said, I do not think this evolution is correct based on my earlier analysis, and wholeheartedly believe that that Canadian shortwave remains compact and races quickly through the higher latitudes, rather than elongating and fighting through the ridging. I just DO NOT think the larger pattern supports this - the ridging has STRONG support from a plethora of factors. So assuming, the GEFS corrects to look like the others, why else do I think that we don't see much, if any, wintry weather from this? Let's continue the evolution.
With the pattern amplifying across eastern North America, you have to look at the finer details, and the most important are the following: 1. Where is the ridging amplifying? 2. How is it oriented? Based on the above, the first is pretty obvious - across eastern North America, AHEAD of our system. When this is the case, systems lift out, they don't continue to dig. When a system begins to lift, it tilts negative, and changes its direction from mostly zonal to mostly meridional while also intensifying the lower-level features associated with it. Combine that with the amplifying ridging, both at the surface and aloft, it further enhances the southerly/southeasterly flow......warming flow, while directing the primary (and probably even a secondary low BEHIND the INITIAL BLOCKING). Eventually, the amplification process takes the mostly meridional orientation of the Eastern Seaboard ridge and through advective processes reorients it into a mostly zonal orientation across eastern Canada, which DOES halt its northward progression (cuts BENEATH this SECONDARY BLOCKING), at which point the system begins sliding east. However, by this point, it's too late as you are still tapping a warmer easterly flow off the Atlantic, and cannot establish enough cold air advection in time for the system to draw into it until it is passing us by (and much, if not all of the precipitation along with it). So you see, I agree with your premise that the low does eventually slip beneath the blocking, but not before it gains too much latitude behind it.
Keep in mind, that as the blocking ridge begins to tilt from meridional to zonal as it lifts northward into Canada, it also unblocks the Atlantic. Therefore, just as you start to establish your low-level flow in a favorable way to enhance cold advection, just like our most recent system, it immediately begins retreating to the northeast, and you're then forced to try to snow in a marginal airmass on a wicked southerly or southeasterly wind, which will more than likely just not work. The air is already contaminated, it's not fresh like our last event, so it's already too late. This is why I said earlier that even IF we see a system take a perfect track, it is still unlikely to snow EXCEPT in northern New England, where I think they can do fairly well with this. It also would not surprise me, as I already alluded to, to see multiple lows develop as we see repeated occlusions here, and may end up seeing a surface feature ride along the frontal boundary itself, but cannot begin wrapping itself up until its past us.
Anyway, this is why I still do not like this event at all for us, and I think such an outcome is being well-advertised on the guidance right now. As usual, though, this is always up for further discussion, and I am looking forward to it! At the very least, I hope that you all enjoyed this, and if anything requires further clarification, please don't be afraid to ask!