Weekend Storm Potential for southern New England

By now I'm sure just about everyone has heard about the potential for a winter storm at some point this weekend with the time frame of impact somewhere between Saturday and Sunday.  At this stage, not only is trying to pin down the exact timing of the onset of any storm, but we are faced with many questions such as, will there be a storm and if so, how strong of a storm are we looking at?  When answering these questions we rely heavily on the many different computer forecast guidance models we have available and try to piece the puzzle together.  When there are differences among the computer forecast guidance models and not much consistency, it lowers the confidence of any forecast.  When looking at what potential this weekend may hold, we will look at the key pieces within the atmosphere and then see how different computer forecast guidance models are handling them.

Below we are looking at the 500mb level of the atmosphere (images on the left), or about 18,000ft AGL.  This level of the atmosphere is of major importance as the jet stream at this level, along with the upper-air pattern is the driver of our weather and we are also looking at 1000-500mb thicknesses along with sea-level pressure (images on the right)  The top image is from the European model (Euro) and the last two images are from the GFS model and the NAM model, two of the American models.  

With the images on the left, each model output graphic has two circled areas of interest.  These two features are what will eventually develop a storm off the East coast.  Each of these graphics are what each model is forecasting what the 500mb pattern will look like come 7:00 AM EST Saturday morning.  As you can see each model is handling the two features differently and that is one factor leading to a low confidence call right now, especially with regards to potential strength.

What each of these models do have in common is a rather strong northern branch of energy (the area near the US/Canadian border).  With each model in agreement on this, all 3 of the selected models will continue to lead to amplification of this feature (strengthening) and lead to a digging trough working into and through the central-tier of the US.  What we also notice is, the two American models, have another piece of very weak energy ahead of the more stronger energy.  The Euro has just the one piece of potent energy.

When looking into the southern stream energy (circled area across the southern-tier of the US), we can see various differences between each of these models.  The Euro and NAM models, however, keep the southern stream energies intact, with the NAM being stronger than the Euro with this energy.  The GFS, however, has the southern stream energy separate into two pieces.

The key for this weekend will be how each of these two streams interact with each other; when do they phase (combine), where do they phase, and how strong will each feature be?  As we go through the next few days, computer forecast guidance should begin to encounter a consensus of how the pattern will unfold and once this occurs, we will have a much better idea of exactly what to expect and when to expect it.

Going back to each model, we see what their projections are for 7:00 AM EST Sunday for the Euro/GFS models and for 1:00 AM EST Sunday from the NAM.  The NAM computer only runs out to 84 hours from it's initialized time.

From going by the latest data from this afternoon, what we see right now is the Euro/NAM computer forecast models are very close to the development of what would be a pretty strong storm system with the potential to produce plow-able snowfall across the entire region.  The GFS, however, wants to phase the two streams later rather than earlier, and that solution would reduce the risk for a more potent system.

Given how the GFS ensembles (a collection of different members associated with a particular model run) vary widely with it's solutions, this tells us that the GFS right now is really all over the place with this system and seeing this, certainly keeps the possibility in hand of a major coastal storm with the potential for plow-able snowfall throughout the region.

As new data continues to come in, over the course of the next 24-36 hours, the situation should become much more clear and details can start to be ironed out.