Storm Briefing for Saturday, December 14th

Another storm is brewing for the Mid-Atlantic and Northeast states, and this time, it could pack quite a punch compared to our last couple storm systems. Although the pattern is not optimal for one that would condone a major snow-maker in the Northeast since the NAO / AO continue to remain positive and the PNA is negative, we are still on track as of tonight for parts of the northeast to receive a significant snowfall. Another component I am also concerned about is the possibility of ice accretion for areas along the immediate coast of the Northeast. 

Here are a couple images of the current water vapor loop. The bottom picture is simply a zoomed in version of the SW Conus from the top image. Both pieces of energy that will work in conjunction to bring a storm system together for this Saturday are on land and beginning to advance east. There are a couple things that should be noted:

1. The STJ, or sub-tropical jet stream, is much more active than it was this time last year. This is helping the southern vorts, or upper level low's, to strengthen before they even reach the eastern U.S. and there is a feed of energy from the eastern Pacific flowing through the jet and into the vort. 

2. The Gulf of Mexico's warm sea surface temperatures were not hindered much this past summer due to the  inactive hurricane season. It is open for business and providing additional energy to the storm that develop in the southeastern U.S. There has already been a few examples of moisture-laden storms this fall that have brought high rainfall amounts to parts of the southeast. 

3. Our northern branch energy is likely NOT to phase with the southern branch energy with this upcoming storm system. Much of this is due to the lack of "real" blocking, or -NAO, and the zonal flow in the western U.S. (-PNA). I say "real" blocking because, although the NAO is positive, there is a PV (Polar Vortex) over Greenland helping to provide confluence over eastern Canada which is one of the reasons why cold air will remain in place (for some locations) when this storm arrives. 

The 2 biggest factors that will ultimately determine who sees rain, snow, ice or a mixture of all three is the positioning of the upper level High Pressure system(s) and the strength of the southern vort. There will be partial phasing with the northern branch which will help develop a broad LLC (low-level center) and bring ample precipitation amounts to PA, MD, NJ, NY and eventually New England, but how strong it gets is significant. A stronger system brings up warm air from the south and could potentially initialize a dangerous ice situation (specifically for NJ), while a weaker system keeps cold air intact and keeps most of the precipitation in the form of snow. 

Once the next set of tonight's model runs come out, I plan to release a 1st call snowfall map so everyone can see what my initial thought's are. Given the active STJ and presence of GOM energy filtering into the storm system once it develops, I am favoring a more warmer solution (plus ice) at this time for the Mid-Atlantic and Northeast up to NYC. But we'll see what happens...stay tuned!

Francesco Paparatto