A look at the 500mb height anomaly map valid 7am Friday will tell the story. A near record-breaking negative NAO (deep purple shade over Greenland) will cause the upper level jet stream to buckle. This causes low pressure systems to essentially cut-off at the surface and even retrograde back into the east coast instead of the typical east-northeast exit. High latitude blocking on the Atlantic and Pacific (-EPO) develops a near perfect pathway for our Nor'easter to track this way.
One reason why your local Meteorologists and others are not calling for a blizzard is because - if you look at the 500mb anomaly map again - the trough over the eastern U.S. is not deep. In other words, we're not working with an arctic air mass. Do not get me wrong...there will be snow - a lot for some - but it will not be your DC to Boston 12"+ Godzilla snowstorm. The Godzilla snow amounts will be confined to area N&W of NYC. But I am not completely ruling NYC out of a 12" storm simply because models are still trending with the exact placement of the secondary low pressure system that will develop. But I would place probability of those types of amounts at less than 10%. Notice in the 500mb vorticity map above we're seeing the 500mb low close off off the coast of NJ. At the surface, this will produce a rapidly deepening low pressure system drawing cold air into its core (or to the coast). So we're likely looking at rain changing to snow for many across NJ and NYC Metro late Thursday into FRI morning.
As cyclogenesis occurs 850mb temps will crash. These maps (EURO top and GFS bottom) illustrate that. At 7am, 850mb are at or above freezing for most but they are rapidly crashing as the morning goes on. This gives you a rough idea of when precipitation will change from rain to snow.
There is more I can go into specifics on. The evolution of the 250mb jet, how the mid level vorts (700mb, 850mb) stack, where the best frontogenesis will be, etc. Once there is clarity from the models I will expand on these things in the forum. For now, I think NYC Metro can expect a rain to snow event with at least 3 inches falling. The immediate coast, such as Jersey Shore, will primary see a high wind and coastal flooding event with snow as your "cherry on top." N&W of NYC I think could see Godzilla-type snowfall amounts if it all plays out perfectly. The primary will lend way to the secondary (energy transfer to the coast) and a huge CCB is likely to develop. Under this CCB snowfall rates could approach 2-3"/hour. Wind gusts could be an excess of 50mph at times along the coast and Long Island. I do not see much of a wind threat for those in the interior. But the north winds turning east would bring about coastal flooding so please be aware of that.
A 1st call snow map will be released tomorrow on the forum.
Happy Tracking!
Francesco Paparatto
