Some media outlets have been waffling back and forth between the possibility of a significant or major storm and just the nuisance variety, based mostly on what the models at a particular hour are saying. I have not however. I still believe a major snowstorm across a large portion of the northern MidAtlantic and New England is likely Thursday into Friday, and below is a major reason why.
The first map below is the upper air pattern at hour 60 from the afternoon run of the NAM (12z) for this Thursday evening. It shows a positively tilted shallower trough, with it’s base/axis in the central part of north Georgia. The ridge out west is less amplified as well.
Now, compare that to the next image below from this evening NAM (18z). The trough is less positive and more neutral, with the base is slightly west and south in the western portion of central Georgia, meaning it is slightly deeper, and also a little sharper (lines in the upper midwest are steeper along the front of the trough).
So most would think – who cares? Not that big of a difference, right? And you’d be correct, it isn’t a big difference. But that’s the point. These little differences higher in the atmosphere at 500mb can have profound effects on what happens at the surface.
The third map below corresponds to the first 500 mb map we looked at, yet at the surface, for the 12z NAM. The low center is directly south of Prince Edward island, Canada, and the precip shield is sparse to non existent over the Mid-Atlantic.
Now look at the last map in the series below. This is the surface depiction off the 18z NAM. Those subtle changes at 500mb produce quite a difference at the surface, with the low about 150 miles west of its position versus the 12z. Additionally, the precip shield is much more robust, producing moderate to heavy snow over most of the area.
Now, these differences were only 6 hours apart between two model runs. And all of the runs did this this afternoon at 500mb. So you can see how the next 12-24 hours may make quite a difference as more accurate data comes in and the models adjust. I expect that the models will continue to adjust a quite a bit of wind blown, high ratio snow is on the way for most of us.