11:21 PM

As the model guidance for the 00Z NAM and 00Z GFS came in, my thoughts centered on a question that bugged me about the end of next week.  Just how cold will the boundary layer be?  Do the models introduce the impact of a one to two feet snowpack, which will keep the boundary layer cool if not downright cold.

The overall idea for the upper level pattern is that the high latitude blocking is expected to relax for a short time.  This is not an unreasonable development as even the most persistent upper level blocking tends to break down for a short time before a strong return.  Further, there is strong agreement that the negative EPO ridge will be slightly further to the west than in previous set ups, which will allow a trough to dig into the Southwestern United States and leads to a ridge building over the Southeast.  However, there area  few issues that trouble me with the overall 500 MB pattern.  For example, the MJO state does not support a negative EPO ridge to be as far west as guidance suggests.  The MJO wave went from a neutral status to stage 5 (La Nina supported) to rapidly a stage 7 position.  This development in the guidance MAY be the influence of the MJO briefly in stage 5, but the duration would suggest a colder pattern at a quick rate.

Friday Night

More importantly though, I don’t like how the model guidance is handling the 500 MB features.  For example, in the ooZ GFS run this evening suggests that the ridge will be building north while a strong Polar Vortex drops south.  Something has to give here, and I think this ridge is going to end up further east.  However, what is really interesting is the strong convergence and confluence that develops at the upper levels with this pattern and even more impressively if the ridge is further to the east.  If the ridge is further east, this means the upper low over the Plains may end up further east and the strongest PVA is focused towards the coast rather than over the Ohio Valley.

A strong ridge over the eastern Great Lakes makes this forecast much more complicated, especially with a 12+” snow pack to keep the boundary layer cool at the surface.  It should be noted that even the ECMWF 2M temperatures keeps the majority of the region below freezing into Friday morning. There is growing support for a CAD developing through Friday night, and the increasing thermal gradient at the coast line would support strong isentropic lifting over the region.  Given that Arctic air does not like to retreat quickly, and certainly not in a six hour period.

I believe as the guidances gets to a point of optimal resolution that the guidance will  illustrate the potential for secondary cyclogenessi along the Mid Atlantic coast.  I would look for a period of snow, sleet, and freezing rain on Thursday night into much of Friday before any change over to rain.  I believe the influence of the snow pack along the coast will have a stronger influence on this future storm than some believe.

More on this potentially dangerous situation tomorrow mornng!

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