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7:52 AM

As with the theme for much of this winter so far, model guidance continues to appear a bit lost in the handling of short wave disturbances over North America.  For the time period of Thursday night through Saturday night, there is a spread of solutions within the operational and ensemble guidance which generally point to a poor handling of the disturbances circled to the left, at least as of the 00Z guidance.  There is a theme in one camp, which the 00Z NAM and 00Z ECMWF support and the 06Z GFS and ensembles trended towards, which was basically a complete miss of the northern Mid Atlantic as the low pressure system forms too far out.  The 00Z UKMET and the Canadian along with various individual ensemble guidance disagree to varying degrees.

So what is happening here?

The problem with the model guidance is simply a case of timing.  There is no question at this point that the Sub Tropical and Polar disturbances will phase into a very strong upper level low that will lead to a very strong and long lasting negative NAO (more on that in a minute).  The question is simply WHEN!

As noted in the water vapor image above, the two disturbances are now in a much more favorable area of data collection.  The 12Z guidance this morning will likely finally trend towards a solution that will be the final forecast as there will now be an accurate depiction in the initialization of the model guidance.  In the above Water Vapor image I illustrate two potential out comes or camps in this scenario.  The first in magenta is a case where the phase of these two disturbances happens faster than currently forecasted.  The surface low is closer to the coast and the storm track is found from the North Carolina coastal waters to eastern Massachusetts.  The second solution is supported by the ECMWF/NAM camp from 00Z which leads to a developing surface low over the eastern edge of the Gulf Stream and thus leads to a complete miss of the northern Mid Atlantic.  The bend in the storm tracks is due to the developing upper level low that pulls the storm back west.

The storm track in magenta is a worse case scenario that I want to present because I frankly am uncomfortable with this current atmospheric set up.  Much like forecasting a perfect storm track, the same is true for a complete miss.  If timing off just a little bit, those scattered clouds can easily transition to six plus inches of snow along the coast in a short period of time.  In this case, the low pressure system rapidly deepens off the New Jersey coast leading to strong winds, heavy precipitation, and rapidly falling temperatures.  Precipitation type issues will have to be considered in this situation initially but all precipitation would go over to snow as the thermal gradient tights off the coast.

My current forecast is the storm track in red given the strong model support and trend of the ECMWF, NAM, and recent shifts in the GFS guidance at 06Z.  However, I do want to stress that confidence for this period is very low.

Beyond this storm, regardless of the outcome, the upper level pattern will be locked into one of the most favorable set ups for potential winter storms in some time.  Initially, the strength of the negative NAO will cause a suppression of the Sub Tropical jet stream through the middle of the week while cold Arctic air dominates.  Don’t be surprised if some locations pick up a quick 1 to 3 or 2 to 4 inches by passing disturbances around the upper low to the north in this arctic air mass.

Beyond Wednesday, there is very strong agreement of a building PNA ridge, a moderate negative NAO, and a contention between those two ridges.  This strongly suggest a high latitude blocking set up that not only will be cold but also very slow to break down.  Further, the ensemble guidance strongly suggest a trough axis over the eastern Mississippi Valley in this time period.  There is some disagreement on the placement of some upper level low features over southeastern Canada, but the overall idea is a pattern that is cold, slow, and supporting a high potential for coastal low pressure development.  In fact, there is growing support of a coastal storm in this time period from Thursday on through next weekend.  Again, much like the current storm, timing of these disturbances will be everything.  However, the point is that this pattern is a very favorable pattern, and I don’t expect a break down in this pattern through at least January.

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