Why is the storm shifting south?

A few days ago, most models had the area getting hit with 10-15″ of snow, which generated a lot of hype. Even up until yesterday, some of the models were still showing that same solution. However, models were beginning to trend towards a change in the orientation of the Polar Vortex, which started to sound the alarms for a south trend. This is why we wrote this article yesterday, detailing the potential trends and why this storm might not produce that much snow.

Today, every model has shifted southward, and barely gives New York City a few inches. And the south trend might not be done.

On the left, we have Wednesday’s European Model forecast at 500mb valid for Monday morning, which showed a lot of snow. On the right, we have today’s European Model forecast at 500mb valid for Monday morning, which showed little in the way of snow. Note the differences with the handling of the Polar Vortex. On the left, the vortex was much more elongated and oriented from southwest to northeast. Additionally, energy is diving down and interacting with the southern stream storm. On the right, the vortex is compressed together, and has a due north to south orientation. This squashes the height field in the Northeast, and leaves little room for precipitation to advance northward. This also allows northern stream energy to dive down further east — out ahead of the storm — so the pieces of energy do not interact. Instead of the northern stream energy enhancing the storm like it did on the left, it inhibits the storm on the right.

The fact that the change in Polar Vortex orientation led to two separate changes to the weather pattern to inhibit the storm explains why the shifts to the south have been so dramatic.

We issued a snowfall map earlier, which can be viewed with this link. 

However, there is a chance that these amounts will have to be pushed even further south. Wave #1 looks to come into the region on Sunday afternoon, and give light snow from Central NJ and northward through around 3:00am on Monday morning. This could add up to 2-4 or 3-6″ in some spots from New York City’s latitude and south. South of Trenton, however, may see rain or ice with this initial batch of precipitation. Then wave #2 arrives for Monday morning, but this should miss the area entirely, except for areas south of Trenton. The question becomes how far north does wave #2 go — but it is even conceivable that the south push is so strong, that the northern half of the 8-12″ zone may have to be chopped in half or even more. If wave #1 is rain or ice for these locations, and wave #2 misses, then not much snow will fall at all for the northern sections of the 8-12″ zone. Areas further south and/or east of Philadelphia could still see significant snow from wave #2.

We will wait until tomorrow morning to make any changes to our map, since tonight’s data could still change. However, the trend has continued to be south, and based on the orientation of the Polar Vortex, we foresee that continuing.