11:26 AM

This morning I discussed the potential outcomes of the storm for the period of Saturday through Sunday.  One of the solutions that I discussed was that the upper low over Quebec would be weaker and thus the disturbance dropping over central Ontario would have more of an influence while the negative NAO, still clearly in place, is weaker on Saturday night and Sunday.  The result would lead to a negatively tilted trough and a storm track that would bring an accumulating, if not significant, snowfall to much of the northern Mid Atlantic.

GFS- Saturday Night

The new 12Z GFS has taken a SIGNIFICANT shift towards the third possibility.  Note that instead of a large upper low over eastern Quebec, which would essentially force the coastal low to move east-northeast from the North Carolina coastal waters, a new upper low takes shape over northeastern Ontario.  This upper low is the combination of the retrograding upper low over Quebec earlier in the period and the upper disturbance dropping south from the Hudson Bay.  The development of this feature in THIS location completely changes the potential outcome of this storm along the east coast.  Now, with this feature in place note that the Sub Tropical disturbance and Polar disturbance (now phased as one solid disturbance over the southern Appalachians) can start to go into a negative tilt and thus the storm can move further north.

I do not believe the GFS has the complete potential of this storm, however this morning’s drastic changes in depiction of the 500 MB features of this storm strongly points me to a much snowier solution than the idea of this storm exiting stage right.

Naturally, the evolution of this storm will have to be monitored and the 12Z ECMWF is also going to give us some clues as well.  However I stress not to jump from model run to model run and look at the details.  That discussion will have to wait for a few more days.

12:24 PM

The 12Z GFS and PARA GFS are now in and I want to address a few features that I think are very important for the next 15 days.

Over the past few weeks, actually since October, I’ve been steady in advertising the threat that I saw coming for early December as the pattern changes.  The basis of these thoughts started with the warming in the stratosphere and how eventually these changes in combination with a negative QBO phase in the atmosphere would impact every aspect of the northern Hemisphere from the MJO waves to the developing El Nino, and finally the pattern for much of North America.  I won’t go into the details, but they can be found here.

This morning, the 12Z guidance has come in with a strong declaration that the cold pattern I’ve been foreseeing is on the way.  This new pattern will take shape in stages and not in one quick change, which is understandable given the current Pacific set up and the state of the MJO progressing into stage 6.

This post will not going into the details of the 12Z GFS/pare GFS guidance, but the fact that these models and the 00Z ECMWF all point to one major pattern shift.  A shift from a positive EPO pattern to a negative EPO pattern.  This shift strongly suggest a ridge is going to build over Alaska and western Canada, and as a result, cold air will be shunted south and east into the United States.  Even more impressive this morning is that all model guidance strongly suggest that a cross polar flow will set up and that means Arctic air.

Now, the initial push of the Polar and Arctic air masses will likely be directed along the Rockies, Plains, and western Great Lakes as the trough axis will be over the western Gulf of Mexico.  The storm around December 5th will bring rain to much of the northern Mid Atlantic, but will also represent another step towards the cold air over the Plains to bleed towards the East.  This will be a slow process.  However, this sets the stage for a more significant storm for the December 8th to 10th period, that’s I’ve been concerned about.  The northern Mid Atlantic will be on the edge of a new Arctic air mass and the thermal gradient will be very strong by this time period.  With a new trough in place and a strong ridge over Alaska and western Canada, plenty of strong disturbances will be diving south from the Polar Branch.  Meanwhile, the MJO data this morning is hinting the Sub Tropical jet stream will become more active by this time period as well.  Fresh cold air, active jet streams, tight thermal gradients: all ingredient for a storm.  The question is not if, but when and I’d mark the period surround December 10th as the when.

You can beat I’ll be chewing on these thoughts as I’m feasting on my turkey!

1:00 PM

Update 3:00 PM: The ECMWF is now in and the model guidance suggests a more organized upper level disturbance at 500 MB with a strong primary low off the New Jersey coast on Friday morning.  The ECMWF outcome would lead to strong sustained winds of at least 35 mph with higher gusts and would produce true Nor’ Easter conditions for much of the Philadelphia and New York City metropolitan areas.  The new GFS Ensemble guidance does lean towards the ECMWF solution, but still has a wide range of solutions in terms of timing of when the peak of this storm will be experienced.  All in all though, the theme for Thursday afternoon through Friday afternoon is wet and windy.

Update 1:35 PM: The 12Z Canadian guidance also continues the multi-low pressure structure, however the rainfall on the Canadian does not reach the forecast area until Thursday evening and the precipitation is focused along the New Jersey coast, New York City metropolitan area, and eastern Long Island.  The pressure gradient associated with the first low on Thursday night would be strong enough to support sustained winds at 25 mph or greater.

The new 12Z GFS model guidance just came out and the trend continues to point towards a multi-low pressure structure with a more progressive movement to the overall storm.

500 MB Pattern Thursday Morning

The difference in the model guidance the past few runs has been to depict the 500 MB disturbance as a weaker and more unorganized system than in previous runs.  As a result, instead of one dominant low pressure system, there are several waves of low pressure moving through the New Jersey coastal waters.

The similarities in the model guidance however will still lead to a heavy rain event for the forecast area, even though the winds would be weaker if this type of storm development is realized.

Even with the weaker 500 MB disturbance, the Sub Tropical jet stream is going to feed these low pressure system with plenty of tropical moisture and the thermal gradient discussed this morning just as strong on the 12Z GFS.  The major change for the forecast is a faster moving storm, which produces heavy rainfall from late Thursday morning through Friday morning.  A third low pressure system develops over the coastal waters of North Carolina on Friday night, however this low track slightly too far east to produce widespread heavy rainfall over the forecast area and will be more of an issue for the coastal waters than then coastal plain.  Again, the difference in the guidance over the past few days is that instead of one dominant low pressure system that moves slowly, the 500 MB disturbances break down to three impulses developing three moderate to weak low pressure systems, which produce heavy rainfall but not the strong northeast winds that would have been possible.

12:42 PM

Surface Map/850 T on Thursday Night

The 12Z GFS is coming in as I type and the stormy potential is becoming more impressive for the Mid Atlantic.

The latest guidance continues the theme of a disturbance over the eastern Rockies from the Sub Tropical Jet Stream interacting with a disturbance from the Polar Jet stream.  By  Thursday morning, this disturbance intensifies over the Mississippi Valley and starts to carve out a deep and impressive trough (especially for this time of year) over the Mississippi Valley.  The lifting from this disturbance will produce waves of low pressure along the stationary front over the Southeast and southern Mid Atlantic.  Meanwhile, convergence and confluence over the northern Plains and Great Lakes will continue to support high pressure stretching from the northern Plains to New England.  This high pressure system will support cool/cold air over the northern Mid Atlantic and New England.  Again, if this was winter, there will be plenty of white snow flakes and ice for the forecast area, at least at the start.

On Thursday night through Friday and into Saturday morning, the GFS depicts a coastal low taking shape and tracking over the coastal waters of New Jersey.  In fact, the latest GFS forecast would go as far as suggesting a “bench mark” track for this storm.  The one aspect I picked up on here was that the GFS is scaling back on the intensity to a more reasonable solution.  The strength of the low pressure system is more in line with the ensemble guidance from last night and is basically an average of the 00Z ECMWF/GFS guidance.

So Friday looks to be very wet and windy with the potential for coastal flooding and urban flooding from Thursday night through Saturday morning.  Temperatures will be rather cool and I like where I am in the forecast.  The combination of the cold air mass over New England and the marine influence from the Atlantic will likely keep temperatures from rising much on Friday with many locations in the upper 40′s (interior) to upper 50′s (eastern Long Island) through the day.

I’ll post new forecast thoughts later this evening once I get all the 12Z and 18Z guidance.

11:28 AM

So far, the 12Z NAM and part of the 12Z GFS has come out and there are some points I like to make.  

There are some comments that the GFS/NAM are warmer.  Given the account for some error, the rate of warming at 850 MB is about the same.  The warm air at 850 MB reaches Philadelphia and south/central New Jersey by around 6AM tomorrow then by 1 PM the freezing line at 850 MB reaches southern Connecticut.  Here’s the catch though, and it is an important one.

Take a look at 700 MB.  The majority of the lifting and moisture associated with this storm occurs between 00Z tonight and 12Z tomorrow.  12 hours of intense precipitation that produces 0.5″ to 1.50″ of liquid precipitation or 0.04″ to 0.12″ of precipitation per hour based on guidance.  Most of this precipitation in that time frame is going to be frozen, not liquid.  

So, yes the warm air advection is coming and will blast through the coastal locations of the forecast area, but by the time it does most of the precipitation ends as the forecast area is dry slotted.  

Point two from the model guidance so far is that the air mass below 850 MB is still significantly colder and you can bet the heavy snowfall ahead of the warming will have an impact, although small.  

I still want to wait for the 12Z ARW/NMM data and the 12Z MM5 data to get a good look at the atmosphere around 925 MB.  The thickness between 1000 and 850 MB on the GFS would suggest that locations from Philadelphia to Atlantic City on northward may be dealing with a very icy mess tomorrow morning before ending as a brief period of rain.

The plot thickens.