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8:30 AM

This morning the pieces for a potential major winter storm starting on New Years Eve and continuing on through Saturday is starting to finally enter a better area of data collection.  To the left is a water vapor image with the features that will interact to form the potential two part storm.

One piece that is locked and loaded is the negative NAO pattern.  The upper level low and surface low that will allow the Polar and Sub Tropical jet streams to slow down and interact is rapidly developing over the Canadian Maritimes this morning and producing very windy conditions over the Philadelphia and New York City metropolitan areas, which is expected to last through the day.

The negative NAO set up is not perfect by any means.  The most significant problem with this set up is the upper low is forced too far to the south and east, which leads to convergence and confluence off the Mid Atlantic and New England coast.  As a result, surface high pressure will move over and to the east of the northern Mid Atlantic by tomorrow evening, which will allow for a moderation of the air mass from the mid levels down to the boundary layer.  Basically, the atmosphere will be in an above freezing state (still in the 30′s but above freezing none the less) ahead of the first tropical disturbance due to the slight displacement of the upper low over the Canadian Maritimes which leads to a significant displacement of the surface high pressure system.  If the upper low was going to remain around 50N/50W then this would be a totally different ball game with a sustained Arctic air mass and a storm track more favorable for the enter East coast.  Alas, this development which is supported by all model guidance and by current developments on the water vapor satellite animations leads to a situation where timing and interaction will be a key factor.

Back to the water vapor image, lets focus on Sub Tropical Disturbance one.  The model guidance is still having a difficult time with separating the first subtropical disturbance and the second tropical disturbance.  Consider the first disturbance like a blocking running back (football term!).  In other words, this disturbance is paving the way for the second.  Ahead of the first disturbance is a weak Sub Tropical ridge that will pop over the Southeast and Mid Atlantic.  This by the way is again developing because of the slightly displaced negative NAO.  The first disturbance knocks down the ridge and carves a trough in the Sub Tropical jet stream for the second disturbance to dive into.  This first disturbance has many important impacts.  For one, this disturbance will set up the initial trough axis, which thereby will give us a good indication of the position and location of the second disturbance on Thursday night.  Second, the first disturbance will attempt to bring the colder air back into the coastal plain ahead of the second disturbance.  This forecast is rather tricky here as some guidance still sees only one 500 MB disturbance in this time period, thus slows the progression of the cold air advection after 00Z Friday.  The first disturbance will spawn a weak coastal low that will likely produce rain, possibly mixing with snow at times, over much of the northern Mid Atlantic on New Years Eve.  As the low exits, the strength and development of this feature will be very important.  The weaker this first low is, the longer the cold air will take to move back towards the coast.

Now we enter the period of the real storm!  By Thursday evening, pressure falls should start to develop off the North Carolina coast.  The pressure falls are due to increased lifting as disturbance two approaches the Southeast.  Meanwhile, the Polar disturbance is going to be diving into the central Plains and driving towards the southern Ohio and Tennessee Valley.

Now, the type of storm we have developing is called a Miller B storm.  A Miller B storm is basically when a low pressure system or two low pressure systems redevelop along the Mid Atlantic coast and moves in a northeasterly direction.  The development of these low pressure systems are rather complicated.  The majority of these storms have the worst impacts over New England while the southern Mid Atlantic completely misses out, and guidance certainly points to that solution.  As for the northern Mid Atlantic which is basically Philadelphia and New York City metropolitan areas, the impacts are historically mixed.  In some cases, the intensification occurs over the New Jersey coastal waters leading to a rapid explosion of the precipitation shield, a fast drop in temperatures as the mid level and low level thermal gradient tightens, and rapidly deteriorating conditions.  The precipitation amounts exhibit a sharp cut off in these cases with locations west of Philadelphia at times having no precipitation at all while locations along the coast can receive a very heavy precipitation event.  In other cases, the intensification happens too late and that sharp cut off occurs over Connecticut and Long Island, leaving the brunt of the storm over New England.  The question is which scenario will play out on early Friday morning.

What do we know?

We know that the second Sub Tropical disturbance and Polar disturbance will phase.  We know that the initial thermal environment would likely support more liquid than frozen precipitation.  As such, we know that precipitation type issues will have to be resolved regardless of the storm track.  We know that this low pressure system will rapidly deepen and then will be forced to stall over New England.

What do we not know?

There is still uncertainty that the model guidance, whether the GFS, ECMWF, or ensemble guidance has a strong handle on the second Sub Tropical disturbance and the Polar disturbance.  This is key to the forecast on early Friday morning.  As I’ve been saying for days, the key for this entire forecast is the timing of these two features.  If a faster phase unfolds, then this is a major hit for the northern Mid Atlantic as well as New England.  Is this possible?  Absolutely.  We are talking about a shift in time of roughly 6 hours or less.  And if this scenario does play out then Friday morning is going to be pretty wild to ring in 2010 with falling temperatures into the 20′s and 30′s along with a mix of heavy rain to sleet to snow.  Boundary layer conditions may also be an issue in terms of accumulations, but I’ll get to that forecast once I get a handle on what is happening in the attic.

For now, I’m going with a forecast similar to most model guidance in terms of a rain to snow mix on Thursday night into Friday morning followed by light snow and snow showers through Friday and continuing on through the weekend.  The threat is there, which I’m illustrating, but one still has to respect the overwhelming support by the guidance suggesting the worst is going to be over New England.  Very windy and cold conditions will follow through early next week as the upper low basically sits over New England.  Disturbances will likely support periods of light snow or snow showers through early next week, which could support minor on and off accumulations.

I’ll be sure to continue to keep an eye on this storm.  The 12Z and 18Z guidance is going to be very interesting to pick apart and I plan on another Premium Update tonight (likely after 9 PM).