7:44 AM

Last night I commented on my concerns about cold air holding on and the potential for more ice than what guidance was suggesting.  The pattern appeared very familiar to me, but I couldn’t put my finger on it last night.  This morning however, after some extensive research of the Northeast Snow Storms by Kocin and Uccellini section on ice storms, the potential could not be more clear to me.

This morning, the Pre-Christmas Blizzard of 2009 is exiting into the Canadian Maritimes.  The brisk northwesterly winds still impacting much of Connecticut, Long Island, and the immediate New York City metropolitan area will continue to weaken and back to the west as the day continues.  However, cold conditions will remain with temperatures struggling in the upper 20′s to lower 30′s away from the coast and lower to mid 30′s along the coast.

Strong high pressure anchored over central Ontario will generally keep the New York City and Philadelphia metropolitan areas dry and cold.  A few scattered snow showers will be possible each day this week as weak disturbances rotate through.  However, I do not expect any significant precipitation through Thursday morning.  Temperatures will remain below normal with highs generally in the 20′s and 30′s and lows in the 10′s and 20′s.

Now we get to the main event.  For many in the region, Thursday night through Saturday morning is a special holiday, Christmas!  It is clear that a white Christmas in terms of snow on the ground will not be in doubt as many locations will have a foot to two feet along the coast and at least 4 inches over the interior if not more.  The question is if Santa Claus is going to have to use his skates rather than boots to get around on the night before Christmas, because this year besides some mouses that will steering there may be precipitation falling.

History can teach us a lot of things, one of them is to recognize a threat even when the models are not displaying the potential correctly.  Now, I am by no means taking a past storm and simply pasting it over to what I think will happen on Friday.  However, what I am doing is looking at how the models of that time handled this similar situation and what features supported a potential ice event.  Boundary layer conditions have many similarities to what is in place in several events,which can be read in Chapter 11 of Northeast Snowstorm: Volume 2.

To the left is the forecasted 500 MB map for this Friday night.  There are several features to take note of.  The developments in the Pacific, a redeveloping negative EPO, briefly support a disturbance diving into the Southwest and then tracking towards the central Great Lakes.  The upper level pattern is in a state of reloading, and thus the solution is reasonable.  Take special care to note the ridge that builds up into Quebec AND the Polar Vortex dropping south from central Canada.  Initially, I was a bit perplex with this solution.  Have I seen this before?  Is this physically reasonable?  So I dove into research mode!

This 500 MB map is a warning sign of much colder solutions than one might think.  When most see a large ridge over the Mid Atlantic, the first thought is WARM.  For the most part, that thought would be correct however there is more than meets the eye with this pattern.  The interaction of the Polar Vortex dropping south against the ridge sliding east will support strong convergence and confluence over Ontario, Quebec, and eventually the Canadian Maritimes on Saturday morning.  This feature seen on all model guidance as this level is a classic signal for a significant ice event for theMid Atlantic.  The closest case that matches this 500 MB pattern is the ice storm of January 28, 1994 which is seen to the right.

In this case, a Polar Vortex was also dropping south while a strong storm in the Sub Tropical jet stream was moving north towards the Great lakes.  This was a time of a pattern reload, which by the way lead to a period of snowfall not seen in some time for the first fifteen days of February.  Anyway, note the ridge building over the Mid Atlantic in this situation and the strong convergence that developed.  The primary low pressure track was clearly over the interior and a coastal low was barely registered in this scenario, however the thermal gradient that developed lead to dangerous ice storm for many.  At one point, interior eastern Pennsylvania was experiencing heavy freezing rain while Philadelphia was at 40.

All of the examples of ice storms in the past 50 years had this similar set up.  A ridge building ahead of a deep low pressure over the Great Lakes, while a new Polar Vortex was diving south from central Canada.  The set up leads to a strong high pressure system over Ontario and Quebec, which supports the development of a strong CAD.

Now, the boundary layer in this situation is a bit more unique than in previous cases.  In this case, a deep fresh snow pack that will not melt all that much is in place ahead of this storm.  In fact, the magnitude of the cold air can be picked up in the last ECMWF 2M temperature forecasts were temperatures in the 10′s and 20′s are common place up until Thursday morning.

This forecast for Friday is a combination of the information discussed above and the boundary layer considerations in place now and through this week.  We know that a strong Polar/Arctic air mass is in place and will be supported all week by strong high pressure over Ontario.  The snow pack in place will not allow for moderation through this week and actually enhances the cold air mass in terms of morning lows.  For example, through this week I am going to go slightly below forecasted MOS guidance to enhance the impacts of this new snow pack.  However, one interesting difference in comparison to other set ups is that the deepest and freshest snow pack is clearly along the coast, not the interior.  While there is clearly snow present over the interior, the ability for a fresh snow pack to allow for more cooling is well documented.

The strength and influence of the snow pack brings into question the evolution of mesoscale boundary layer interactions, which at this time I can not accurately forecast, but I can speculate that the potential for an enhanced coastal front up to 950 MB is going to have some merit.  This leads to the potential for secondary coastal cyclogenesis.  Let me be clear, I am confident the primary low remains over the Great Lakes.  However, the combination of a coastal low around North Carolina to Delaware and the strong high pressure over Ontario on Friday morning leads to enhancement of the CAD in strength and duration.

To the left is my threat analysis for Friday through Saturday morning.  Note the strong high pressure system over Quebec by Friday night that ridges down the coastal plain, this is supportive of Cold Air Damning (CAD).  I expect the CAD to be enhanced by a weak surface low pressure system around the southern Mid Atlantic due to the strong frontogenesis in place from 950 MB down to the surface.  This set up also produces a strong vertical thermal gradient as temperatures rapidly warm above freezing from 850 MB to 500 MB.  I am also concerned that the moisture advection will end up much stronger than currently modeled at this time.  This has been a theme this December and we only need to look at the past event of the model guidance not picking up this potential until 2 or 3 days out.

At this time, I can in no way forecast accumulation potential for any location.  However, I do believe all locations will have a period of snow, sleet, freezing rain, and rain for the period of Friday afternoon through Saturday morning.  The question to me is not if, but for how long.  The impacts of the snow pack should not be underestimated, even if the guidance does.  A 12 inch snow pack can alter a forecast in this situation from 35 and rain to 32 and freezing rain, and that is my concern for locations along the coast.

By Saturday evening, the primary low pressure system and upper low will lift into the Canadian Maritimes.  After looking at the state of the MJO and the trends of the high latitude blocking features, I am confident that a negative EPO/positive PNA/negative NAO/negative AO pattern will return in full strength for the end of this month and continuing into at least the first 15 days of January.  I suspect this time period will keep many snow lovers happy as conditions will be favorable for more moderate to significant snow events.

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