10:52 AM

I’m going to cover all the thoughts for the next seven (7) days in this one post.  The reason is because the short and medium range periods are really interconnected so there’s no point in splitting them up.  So get that coffee cup and prepare to read a lot.  I’m going to add more description in this post in exactly what I think will play out here.  The good news is that despite the jumping around of the guidance, a basic idea is unfolding the question is now the details.

Today, the cold air is on the move through the forecast area.  Most locations have already seen the highs for today and temperatures will only fall from this point on through the 40′s, 30′s, and 20′s.  Before moving forward, we need to understand the type of cold air that is building into the forecast area.  MOS Guidance has been trying to play catch up with the type of cold air moving in.  This isn’t just cold air, it is dry, cold air.  The type of cold air that does not leave quickly.  More importantly, the cold air source over Southeast Canada is very impressive with temperatures in the 20 below range and dew points in the upper 20′s below range.  

Another observation we should look at is the interaction of the moisture along the cold front and behind the cold front.  Note that the cold front is well to the south, cutting through North Carolina.  Yet, there is basically a “virga” storm over central/southern New Jersey, the Philadelphia metro, and points south.  The strong initial dry air advection below 850 MB is preventing precipitation, but enhancing the fall of temperatures.  The key point here though is the developments seen on the satellite images, strengthening mid level forcing over the southern half of the forecast area.  This forcing is being produced by a tightening thermal gradient at 850 MB that will build down to the surface as the cold air moves into the forecast area.  Is this development by itself meaningful?  No, not by itself.  But when you combine these observations with the pattern going forward, I’m getting some insights on how this pattern will unfold.  So let’s go forward and look at this coming week!

Sunday Evening/Monday:  Moisture is pulling over central Rockies and beginning to move east once again.  For much of tonight through Sunday afternoon should remain dry as the area of moisture over the Southeast exits and some dry air works in ahead of the next area of moisture.  The movement of moisture and the cold air in place is what is going to drive this pattern, and with disturbances exiting from the opening upper low over the Southwest, models are having a difficult time handling this pattern.  Knowing this, that is exactly why I am not jumping around from model run to model run.  Sorry, little bit of a rant.  

Anyway, the moisture will move moving towards the southern half of the forecast area by Sunday evening.  The strong thermal gradient in place over the forecast area at the surface and up through the mid layers of the atmosphere (850, 700 MB) will produce vertical and horizontal lifting of the atmosphere.  In a more basic term, the atmosphere is favorable for rising air in this time period.  The environment is set, the question is if a match will be struck.  

Introducing match:  Various model guidance is starting to pick up on a mid level disturbance at 700 and 500 MB that drives through the Mid Atlantic on Sunday night and Monday morning.  Note, model guidance did not have any indication of this possibility 48 hours ago, but there it is now.  Why?  Because that is the type of pattern we are dealing with.  This isn’t about one major storm, but a series of disturbances that will bring periods of light snow.  An inch there.  Two inches here.  Before you know it, you have a snow pack developing and a more organized low on the way to complicate matters, but I’m getting ahead of myself.

The disturbance will race through Maryland and Delaware on Sunday night and Monday morning.  I expect a band of light snow to develop to the north and east of this racing disturbance, which would be set up over central New Jersey through the Philadelphia metro and on south.  Note the map here:

Blue:  A period of light snow on early Monday morning.  A dusting is expected overall, but some locations may reach an isolated 2 inches in a heavier band.  The snow is exiting the forecast area by late Monday morning or early afternoon.

Light Blue:  A few flurries, but nothing significant to impact travel.

Grey:  Cold, a few clouds.  A disappointed sigh.  

I wasn’t expecting much in this time period at all, but the point here is that the models could not and have not handled this potential well at all.  Note a few days ago, most model guidance had RH at 700 MB below 50%, now we have indications of snow.  That’s why this is meteorology and not modelology.  

Monday Night Through Tuesday Morning:  The weak disturbance exits and clearing skies return.  The thermal gradient at the vertical and horizontal remains, but the dry air advection behind this exiting disturbance will allow some clearing to take place.  Temperatures will be cold, in fact I am staying below MOS guidance here in this period.  I wouldn’t rule out a flurry from a passing mid level disturbance at 700 or 800 MB, but otherwise a dry period ahead of a more significant system.

Tuesday Night Through Wednesday Morning:  I’m going with an average of model guidance for this period with some tips from the HPC, which has an excellent discussion this morning on this period.  A significant amount of moisture is seen exiting from the west as the upper low opens and begins to move east as a progressive trough.  An initial impulse or disturbance will move along the stationary front over the Mississippi Valley and Mid Atlantic on Tuesday night.  I think model guidance is starting to pick up on the threat, but there is a lot of “noise” out there to make an accurate forecast.  Taking a blend of guidance reduces that “noise” and taking a clue from previous similar patterns gives me an idea what to expect.  The majority of the significant snow that falls on Tuesday night and Wednesday morning will be focused from central New Jersey through the Philadelphia metro and into Baltimore.  This is based on what I think the low level and mid level thermal gradient will set up and the amount of moisture that will be present in the atmosphere.  This disturbance will have influence from the Pacific but also interactions from the Gulf of Mexico.  Be careful with the forecasted QPF amounts here.  There is a growing potential for someone to get under a heavy band of snow due to the strengthening WAA.  This disturbance has the potential to produce over 4 inches of snow from central New Jersey on southwards through the Philadelphia metro on Tuesday night.  As for locations further north, I think there is a significant cut off in the precipitation area, and there is a potential for New York City to miss out here.  In these types of storms there is a have and have nots that is pretty well defined.  Monmouth County (for example only) could get 5 inches of snow while JFK gets just a few flurries.  Yes, I think the gradient will be that tight.  As far as precipitation type, with high pressure right to the north of this low pressure track seen again on the average of model guidance and using GEFS data, I expect all snow for this period throughout the forecast area.  Again, to stress, this storm looks to be for locations SOUTH of New York City.

Wednesday Night Through Saturday:  This period is like the end of a great classical ensemble.  The trumpets are blowing, the drums are thundering, and everything is coming to a climax.  To this point, the disturbances that have come through the forecast area are pieces broken off from the main upper low/disturbance over the Southwest coast.  The driver of the southern branch is coming east here and will have a big story to tell.  

First, I want to warn about the forecast of precipitation types here.  I would advise hold off on any “rain” ideas just yet until we see the type of snowpack on the ground on Wednesday morning and the type of cold air advection that develops behind the departing low.  Also, beware of the weak yet still present convergence zone over eastern Quebec on Wednesday night, which is supporting the CAD seen on the 06Z GFS and ECMWF.  The cold air is not going to give up and leave.  Yes, the warm air will eventually win out along the immediate coastal plain on Wednesday night and Thursday morning, but not before there is a significant period of mixed precipitation.  The low level boundary layer is going to be difficult to determine and what’s even more troubling is the set up of the storm track.  This storm track with a pronounced CAD in the mid range remember is from central Virginia through Delaware and southeast of the New Jersey coast.  This storm track argues for a north to northeasterly wind, not east or southeast.  We’ll have to watch how this plays out as far as wind direction and the boundary layer, but don’t rule out frozen precipitation type for a good deal of Wednesday night through Thursday.

Overall set up here is still not one strong low, but two developed, organized yet weak low pressure systems.  The model guidance on the ECMWF suggest at most a 1004 MB low.  Not exactly historic.  That’s a huge indication on the type of pattern we are dealing with.  The position of the main PVA is going to be key, but I am willing to bet the storm track is going to remain the same, but a slightly colder solution will play out.  I expect two weak low pressure system to move south and east of the forecast area, with cold air remaining over the forecast area, especially at the low levels.  I also think there is potential for more precipitation than what guidance is suggesting at the beginning of this next phase on Wednesday night.  The initial strong push of warm air ahead of the develop low screams of a strong isentropic lifting over the Philadelphia metro and up through New York City on Wednesday night into Thursday morning.  

So that’s where I pretty much stand for this up coming week.  To recap:

1.  Most of the action will be south of New York City  Sunday through much of Wednesday.

2. There is a growing potential for a moderate snowfall over the Philadelphia metro on Tuesday night through Wednesday morning.

3. A more organized, but still overall weak series of low pressure systems will impact the forecast area Wednesday night through early Saturday morning.  A variety of precipitation is expected with the potential for significant snow/ice for the majority of the forecast area.  The cold air will lose some ground, but will not give in to the warm air advection as easily as model guidance suggest.  

4.  Models will continue to waffle with a great deal of uncertainty.  Count on it!  

5. The best way to forecast this pattern is take each disturbance individually.  NOW-CASTING and reevaluation of the forecast will be needed.  Someone is going to get a good deal of snowfall out of this pattern, the question is who and how much.