7:54 AM

This morning a strengthening upper low over New England will be the driving force behind a very cold and breezy day over much of the northern Mid Atlantic.  The exiting upper low will enhance the development of a ridge over much of the Mid Atlantic at 500 MB and set up an environment where wintery precipitation returns to much of the northern Mid Atlantic.

As a new Polar Vortex drops south from the North Pole, the developing ridge (currently over the Great Lakes) will slide into position with the ridge axis developing from the New Jersey coast to the southern tip of the Hudson Bay.  The strengthening convergence and confluence over northern Ontario and Quebec will enhance an area of surface high pressure over Ontario, leading to the high pressure system strengthening to a 1036 MB high or stronger depending on the model guidance.  Climatology with this set up suggest that a 1040 MB high, which has been shown at times on the model guidance is a favorable solution in this pattern.

The strengthening high pressure system will extend south as a ridge over the Northern Mid Atlantic, producing a strong Cold Air Damning or CAD signature down into the western Carolinas.  The developing CAD will be felt today as temperatures struggle to brake out of the 30′s.  The key note here is to take special care to examine the boundary layer by this evening for the potential storm for Friday evening through Sunday morning.

Over the past several days, I have been researching the potential for a more significant ice event than the guidance has been showing.  Let me be clear, my focus here is the interior locations at this time given the position of the high over central/northern Quebec sliding to northern Maine.  Although I think some precipitation will start as a mix of snow, sleet, and rain over the Delaware River Valley, I think the majority of the I-95 corridor will go over to rain relatively quickly given the data in front of me, however I want to stress the thoughts I presented last night once again.

There has been hints in the model guidance of three developments that have caught my interest.  The first is the impact of the boundary layer on the development of the coastal front.  The coastal front is usually a mesoscale derived feature as the low level thermal gradient is based on the interaction of the warmer Atlantic, which is very warm right now, and the cold air mass over the land.  The complicating factor in this set up is the snow pack that is present over the coastal plan, specifically over central/southern New Jersey and much of Long Island.  Naturally, some of this snow has melted, however at least 6 inches is present over most locations along with a new Arctic air mass to ensure any additional melting will be slow to occur.  Now, this factor alone does not drive the forecast as this mesoscale issue can be easily overwhelmed by the stronger synoptic signal that has been present the past few days.  Namely the strong primary low over the Mid West and the strong southwesterly flow at 850 MB and the eventual easterly flow off the Atlantic at the surface.

However, a new and interesting factor is now coming into play.  The model guidance strongly agrees that the primary low over the Mid West becomes vertically stacked, which means the primary low well to the west is in a state of weakening.  Meanwhile, model guidance continues to hint that a coastal low takes shape over the Southeast and tracks towards the Virginia coast.  From there, the various guidance either pulls the coastal low towards the Upper Low, tracking towards the St. Lawrence Valley or simply weakens into a elongated trough.  However, there are some synoptic clues that would suggest a stronger coastal solution.  Besides the vertically stack nature of the dying primary low, there is also a strong jet streak developing over theSoutheast coast.  All guidance has this feature, but in slightly different positions.  The axis of this jet streak is going to be very important.  If the jet streak sets up where the best divergence aloft is moved towards the coast under the conditions of a stacked old primary low, then a coastal wave will develop.  This wave would eventually become the primary low as the best dynamics transition towards the coast.  This solution would cause a lot of problems for interior eastern Pennsylvania through the central Hudson Valley and Mohawk Valley of New York.  Such a development would enhance the CAD and force winds to remain from the north and northeast rather than the east over these locations.  Given the structure of the atmosphere in these circumstances, a significant ice threat would be an issue and the change over to rain would be prevent or at least delayed for some time.

To be honest, I’m not comfortable going beyond this point in terms of details.  Over the next 24 hours, a lot of details with this set up will become clear.  I’ll have a clear view of the type of cold air mass in place along with the dry air associated with this Arctic air mass.  The model guidance will have a better handle on the jet stream structure as I don’t like to use the NAM out to 84 hours as a reason to go into details on this event.  However, it should be noted that the NAM lately has been on a roll this month with picking up the boundary layer considerations of winter storms along with at least hinting at a heavy precipitation event than medium range models have suggested.  Why the NAM has done so well overall is unknown to me, but it is something keep in mind.  What I will say is that in terms of exiting Arctic air masses, it is usually smart to lean on the side of a slow exit at the surface rather than a fast exit.  This is a situation where those in higher elevations would see a fast change over as temperatures even at 950 MB rise rapidly under the strong assault of warm air advection while those in the valleys have a prolonged freezing rain event.

I expect this storm to exit slowly as the 500 MB pattern though not supportive for a typical major winter storm is still a pattern supporting a slow exit.  I expect the precipitation to start on Friday afternoon and pick up in intensity through Saturday morning and evening.  The precipitation and the exit of the low pressure systems will be expected on Sunday morning followed by a return to below normal temperature conditions or basically highs in the 30′s.

Dry and cold conditions can be expected through Wednesday as Arctic high pressure remains in control.  However, by the end of the week a new threat will be approaching from the Gulf of Mexico.  All guidance illustrates a disturbance from the Sub Tropical jet stream interacting with the Polar jet stream from next Thursday through Friday, which happens to be New Years Eve and New Years Day.  The teleconnections of a building ridge within the Polar jet stream along the West coast and close to the central Rockies along with a developing negative NAO pattern with a new 50/50 low suggest another significant accumulating snowfall is possible for the northern Mid Atlantic.  Naturally the GFS is the most aggressive with this threat, however there is support with the ensemble guidance.  The ECMWF and Canadian also illustrates this threat for this time period.  What I like about this potential is that much like the last major winter storm, cold air will not be an issue to worry about.  A fresh air mass will be in place to support temperatures in the 20′s and 30′s for this storm.  Plenty of moisture from the Sub Tropical jet stream will also be present along with disturbances that I’m betting are not being handled very well right now.  In short, this year’s New Year’s Eve shows may turn into weather telecasts than anything else.  Of course, for the latest information and coverage, you know where to turn!