Starting yesterday at 12 Z basically all the guidance started to trend south with the system around the 19-20th. The 18 Z GFS continued with the south trend and the 00z guidance also continued the south trend with one exception to the guidance being the 00z NAM.
However…as we look at all the models at hour 72 you will notice that they are all in fairly good agreement with the position of this system. The difference with the NAM and to a lesser extent the GFS is the north extent of the QPF shield which gets into southern PA..
Lets look at the guidance…
Now in the above images we have all the guidance from the 00z GFS/GGEM/UKMET/ECM/NOGAPS. You can see that they generally agree on the low position at 72 hrs. with probably the ECM being the most south of the solutions.
Now this image above is the 00z ECM ensemble means and it basically supports the operational run . It can be argued that the system on the means is just a touch NE of the op at the same time ..However..for this system to effect PA and Northeast ..you do not want the means to show a touch NE but more N then anything.
We are going to attempt to show WHY this is not going to make it any further to the north then what we are seeing on the guidance and we are going to use the ECM @ 48 hrs. to show this…
First thing we notice is the 50/50 low and this is part of the problem in that the 50/50 low is to strong which causes a NW flow behind the Northern stream system (that all the guidance trended faster with) which causes the NW flow or confluence to NOT allow the heights to rise on the east coast. Which leaves us with..
Black lines indicate the flow which is ENE…
Another problem is lack of a + PNA ridge that goes across the west into Canada as the white line represents. If this would have occurred we would be looking at a sizeable storm coming up the coast. However ..we do not have this happening…
Third the lack of blocking. Even if we had a full phase between the northern stream and the southern stream..with a positive NAO in place there really would be nothing to stop this from cutting west.
So instead of having the heights rise along the east coast…we are left with lower heights due to the NW flow or confluence from the 50/50 low and this system has no choice but to follow the steering flow ENE and off the coast around Hatteras and bring the Mid Atlantic region a decent snowfall…
As of 6 Z the NAM ..while still bringing some light QPF north of the PA/MD border has really backed off and gone more south and east as well coming into harmony with the rest of the 00z models.
One thing is certain as you can see by the radar image above that this system is not going to have a lack of moisture. And I would not be surprised if we see a very expansive moisture shield with this system..
So with over whelming guidance support to suggest this system has little fanfare to the north of the PA/MD border …taking a blend of the guidance with less emphasis on the ECM but more emphasis instead on the ECM means this would be the most likely area that has the potential to see a heavy wet snowfall…
At this time we think that around 4-8 inches of snow could fall across the area that is in blue..
We will continue to monitor this system for any changes but with the NAM as of 6 Z now coming in more suppressed we pretty much feel that the models are coming into agreement on the ultimate track and resulting location of the storm…
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