7:14 AM

I want to be cautious here in discussing the potential for a major and possibly history winter storm for the forecast area.  The 500 MB pattern will strongly support the potential for a major winter storm Tuesday night through Thursday morning as several key pieces are showing up on all the model guidance.

1. A moderately strong negative NAO will be in place to slow down the disturbance that enters the eastern United States.  However, unlike this weekend, the 50/50 low will be slightly further north and will not suppress the strong disturbance over the eastern United States.  The negative NAO and 50/50 low will also force any developing low pressure system over the Ohio Valley to redevelop off the Mid Atlantic coast.

2.  A fresh supply of cold air will be in place and will be reinforced over the forecast area as strong high pressure will be to the north of the low pressure track.  Upper level dynamics would suggest a very strong area of high pressure will be over central Ontario through western Quebec in this time period.  As such, boundary layer issues will be less of a concern and the thermal gradient at mid levels will likely be focused along and off the coast.  

3.  A ridge will build over the Rockies, which will support an amplification of the trough over the East.  This means the trough over the east will be allowed to intensify and eventually would support a negatively tilted trough axis in this time period.

So there is growing support for a winter storm in this time period.  I am staying away from details at this time, because there is still a lot I don’t know about the structure of this storm.  For example, we don’t know the exact rate of intensification, when/where the 500 MB low will form with this storm, and just how cold the boundary layer and mid level layers will be to determine snow ratios.  However, if one was to take the ECMWF as verbatim, then over a foot of snow would fall over the forecast area.

1:15 PM

The past few hours sure have been interesting in model guidance and when looking at the observations.

Precipitation is breaking out over the Delmarva, Maryland, and beginning to develop all over Virginia.  Pressure falls are still developing over the Southeast, but the primary low is still in the Great Lakes.  As a result, warm air advection is still dominating the forecast area.  Temperatures are above freezing over much of central/southern New Jersey and southeastern Pennsylvania.  However, notice that dew points are well below freezing.  Any precipitation that does eventually fall, will force temperatures to fall and evaporational cooling may be rather strong this afternoon and evening.

Now, let’s discuss models.  If you take them at face value, they’ve already busted.  Why?  Well, in terms of the supposed dry slot over the Mid Atlantic, there should be little if any moisture nor precipitation over southern New Jersey, Maryland, nor southern Pennsylvania.  This region at was suppose to have strong dry air advection producing RH values falling below 90% and 70%.  You don’t have virga when RH values are below 70% in Virginia.  Clearly, there is a lot of moisture to work with over the East coast and this is seen very well on the water vapor images. 

So what are the models say here?  The models are producing this low on Monday night due to all the PVA rotating through this trough.  The best area of weakness starting this afternoon is going to be just off the coastal plain, thus the precipitation and low development.  It’s time to stop taking model verbatum and look at the overall pattern that is developing here. 

1. We have plenty of moisture in the atmosphere.

2.  We have a developing thermal gradient from the surface to 850 MB with much of the forecast area ending up on the cold side of that thermal gradient.  The thermal gradient will not intensify until the cold front moves off the coast to “refresh” the cold air in place. 

3. The pattern is dicating that a strong low will not develop along the coast, but rather a series of weak waves.

What we will need to watch for is how each disturbance develops and forms off the coast this evening.  Each wave of precipitation will have the ability to produce light to moderate precipitation, likely snow, over the forecast area.  However, given the fast nature of the 500 MB pattern, a concentrated well developed low pressure system seems unlikely.  This pattern is very difficult to forecast for and will have to be NOW-CAST through the next 24 to 48 hours. 

I will continue to update through the afternoon and keep an eye on those pressure falls along the Southeast coast.  There’s a lot of moisture down there, but no well developed area of low pressure, which means that while the radar shows heavy precipitation move ENE, that doesn’t mean the show is over as there is plenty of mid and low level moisture around to kick up another area of moderate precipitation.

8:59 PM

If you put your ear to the ground, you could hear the fall of a million snow lovers faint to the ground after seeing the 12Z GFS, which basically if taken verbatum would produce a significant snowfall for most of the forecast area, save southern New Jersey (even that location would still get measureable snow if the 12Z guidance is correct).  However, before we get ready for an all snow event, we better reason a bit on what the GFS and other guidance is suggesting.  Let’s deal with the 12Z GFS right off the bat.

12Z GFS:  The basic idea of the GFS is this.  The strong shortwave, which is a closed 500 MB low over the central Great Lakes by Friday morning runs into one very impressive 50/50 low at 500 MB.  Sensing that there is no way that the disturbance can go much further east, the strong PVA dives towards the Mid Atlantic coast.  The short wave trough tilts to the negative towards the coast and a coastal low is spawned.  The cold air on the model is kept over the coastal plain as the developing coastal low bombs outs and pulls the deeper cold air towards the center of the low pressure system.  Heights crash.  Intense lifting is seen along the coastal plain, and everyone gets into the snow parade over the forecast area along the coast.  The position of the high pressure is mute in this scenario as the idea is the cold air behind the clipper reenforces and strengthens the marginal cold air in place left by the previous clipper on New Years Eve.  So can this solution happen? 

Verbatum, I think that the idea of no mixing of sleet and/or rain along the coast is a bit of wishful thinking especially with a high pressure system off the coast.  The strongest high over eastern North America is over Ontario, but too far away to have a direct impact on the forecast area at this point.  The key here is that the coastal low boms and tracks in such a way that cold air has no where else to go but towards the coastal plain, and any warm air advection is forced above 700 MB to not produce significant change in precipitation type.  Although even in this situation, you have to expect some sleet.  So by the naked eye, yes the solution looks like a significant snow storm for the forecast area.  However, reasoning of the set up should lead up to the idea of some mixing back and forth through the evolution of the storm.

As for precipitation amount, the 12Z GFS produces intense Omega at 850 MB as a new 850 MB low quickly develops and significant low level forcing develops as a result of the bombing low pressure system.  The guidance suggest a deepening rate of around 2 MB per hour between 18Z Friday and 06Z Saturday.

The 500 MB set up is possible.  Yes.  There is also support of this solution with the Ensemble guidance and the UKMET.  The NAM hints at the possiblity as well.

12Z ECMWF:

Then we have the ECMWF, which likely caused snow lovers to faint again.  The ECMWF and the 18Z GFS for that matter completely loses the storm.  Why?  Well, because of the other possible solution.  The ECMWF and GFS up to 96 hours or Friday morning is rather similar in the upper level set up.  A strong 50/50 is in place.  A strong disturbance is driving towards the Great Lakes.  Then BAM.  The ECMWF loses the storm.  Where did it go?  The answer is that the ECMWF decides to try to merge the disturbance with the 50/50 low.  Basically shearing the storm appart and not allowing any sort of redevelopment.  Why would the low need to redevelop if the low simply falls apart?  Well, I have some problems with this idea.  First, I could understand if the disturbance was weak and unorganized, but this disturbance is anything but.  Although the blocking is strong, I doubt that the entire upper level system will just fall apart and those lead to a non-issue of a storm for not only the forecast area but the entire Northeast.  The idea just sounds suspect to me.  So for now I am throwing the solution out.  However, if the ECMWF is trying to pick up on the idea that this disturbance is much weaker than what has been forecasted, then the idea of this storm being sheared apart is plausable. 

00Z Guidance:

The 00Z guidance is going to be very interesting to watch tonight.  Why?  Because the models will be getting a better read on the disturbance, which is currently over the coastal waters of British Columbia, really is.  If the disturbance is as strong as what has been advertised, then I think we end up seeing guidance similar to the 12Z GFS.  If this disturbance is a 98 lb weakling, then the 50/50 low will crush this disturbance like a tin can, thus leading to a non-event for the forecast area. 

From this point on, we will get a much better feel on not only the evolution of this storm, but the track and some boundary layer characteristics. 

So the next few days are going to be hectic for all of us, but hey we have the summer to rest!