9:40 PM

This evening, my focus is not going to be on any particular model, but on the overall pattern that will set up for Thursday through Saturday morning.

The 500 MB pattern is not one that is atypical for a major winter storm for the northern Mid Atlantic.  Unlike normal winter storm 500 MB set up, there is no negative NAO in place.  Instead a neutral to positive NAO is in control.  However, the Polar jet stream will be in a state of change as the Arctic Oscillation goes negative once again.  As a result, a strong upper low will be dropping south over the Canadian Maritimes and a strong trough will move through the Northeast on Wednesday night.  This trough will feature a cold front that will introduce a Polar air mass into the northern Mid Atlantic.  Meanwhile, a ridge will be building over the Canadian Plains, which supports a strong surface high pressure over Quebec.  This 500 MB Polar jet stream pattern is not stable and thus is progressive in nature, but does provide the mechanism to introduce cold air into the Philadelphia and New York City metropolitan areas.

Meanwhile, a strong Sub Tropical disturbance will be racing east along a stationary front located from northern Oklahoma through the Tennessee Valley and off the Mid Atlantic coast.  This disturbance will be capable of drawing warm, moist air from the Gulf of Mexico northward towards the Mid Atlantic.  The Sub Tropical jet stream is also rather progressive right now with a deep trough along the West coast sending disturbance after disturbance through the Rockies and into the Plains.  However, this disturbance does have the potential to produce heavy precipitation for much of the northern Mid Atlantic given the open moisture source from the Gulf of Mexico.

This precipitation event is going to be drive by isentropic lifting which is basically warm air moving over a cold air mass that is already in place.  However, the cold air mass in this case is limited in mid level and low level support as the surface high retreats over Quebec.  There are some signs of Cold Air Damning (CAD) on various model guidance, but it is clear that the development of the low pressure system will be a key in determining the boundary layer.

Basically the problem with this forecast is that if the surface low is too strong when approaching the coastal plain, the result will be a mix of snow, sleet, and rain over the interior and mostly rain along the coast.  This is due to the surface low able to alter with wind direction along the coast to a more easterly component, thus driving in a marine air mass.  However, if the low is too weak than suppression of the entire precipitation shield is possible as the Polar jet stream drives a jet streak from the northwest into the Mid Atlantic.

The point here is timing is the key to the storm for Friday.  In order to get a snow event out of this scenario what must happen is the low pressure system remains weak enough not to force an easterly wind along the coast, but strong enough to enhance the thermal gradient in place.  This fine line in development on Thursday night into Friday morning is very tricky to forecast at this time and will have a major impact on what to expect going forward.  As the surface low jumps towards the Delaware coast, there is pretty strong agreement that the low intensifies and the cold air is pulled south and east, thus changing all precipitation over to snow, possibly allowing for some accumulation even along the coast.  The fast nature of the disturbance involved clearly points to a quick exit with drying conditions likely by Friday night.

As of this forecast, I would lean towards a rain event along the coast as the boundary layer set up for a cold boundary layer will have to rely on a near perfect solution.  Is an all snow or all frozen precipitation event possible on Friday?  Yes, the threat is certainly there, but when conditions have to come together just right for such a solution, it is wise to lean on the warm side.

While I am breaking down the current radar and satellite images, I think it is a good time to discuss the latest guidance with the set up for next week.

First and foremost, it is NOT wise to try to figure out the rain/snow line positions of any storm 4 or 5 days out.  You can do a general idea, like location A is in the warm sector and B is in the cold sector.  However, the idea of saying who gets snow, sleet, or rain is basically pushing the reasonable use of the data available.  

The most important aspect of the upcoming pattern is that we are progressing towards a cold and wet pattern rather than a cold and dry pattern.  What this mean is that the track of the high pressure system will cut across the Great Lakes, which basically increases the potential for CAD over the coastal plain.  The pattern developing is basically a split jet stream pattern.  The southern branch cuts across the Pacific Northwest around the developing trough off the West coast and runs across the Plains and towards the southern Mid Atlantic.  Meanwhile, the northern branch is significantly amplified by the strong -EPO signal, which builds a ridge over western Alaska and continues to support a northwestern flow from central Canada to the Plains and then off to the East.  What we have setting up is a pattern where the Plains to the northern Mid Atlantic is the battle ground for some active disturbances over the next few weeks.  While this is NOT the best pattern for those south of the forecast area, I think this pattern will make many snow/winter lovers happy as we progress through the rest of December.  

As for the storm for the end of the week, I have said and continue to say that the risk is certainly there.  Before I start doing my own little snow dance, I would wait to see how the 00Z guidance progresses.  The reason why is that the disturbances that will drive this developing pattern will begin to enter an area of better data resolution and as a result, the influence of pattern data for the GFS and ECMWF will be important to observe tonight.