7:40 AM

Clouds continue to increase this morning as two disturbances begin to impact the northern Mid Atlantic and much of New England.  Although all is quiet in terms of the interaction of these two disturbances, it is interesting to note just how close this set up is to a potential winter storm if all the pieces present on the water vapor satellite picture had interacted just right.  However, the disturbance over the Ohio Valley producing light snow will weaken under strong shear from the upper low developing to the north.  The majority of the precipitation will weaken and dissipate over the Ohio Valley, however a few widely scattered snow showers are possible in the evening and overnight hours tonight.

As the upper low forms over northwestern New York tonight, a shot of Arctic air will drive towards the coast to reinforce the cold boundary layer in place.  While temperatures will break the freezing mark once again over most locations this afternoon, a colder regime will take hold by tomorrow afternoon with most locations at or below freezing for highs.  The push of the Arctic air will be supported by a strengthening high pressure system over southeastern Canada which will provide dry and cold conditions over the Philadelphia and New York City metropolitan areas through Thursday.

I discussed yesterday my concern for the period of Friday through Saturday, the upper level set up for this period is a near carbon copy of 500 MB patterns of previous significant ice storms.  Model trends illustrate that the guidance is playing catch up here.  Let’s consider the ECMWF 2 M temperatures of the past three runs.  Along the coast, the high temperatures of the event have been dropped from the mid 50′s to now the mid to upper 40′s while the interior thermal forecast has also trended colder with temperatures in the mid 30′s rather than mid 40′s at the height of the warm advection of this storm.  Further, the strength of the high pressure system over Quebec on Friday night and Saturday morning has trended significantly stronger from 1030 MB from yesterday’s 00Z guidance to now a 1040 MB high, an increase of 10 MB in the guidance is not something to take lightly.

At this time I feel I have a confident handle on some aspects of this storm.  We know for the time that the storm impacts the Mid Atlantic, that the primary low pressure system is over the Great Lakes, not the coast.  We also know that a fresh Arctic air mass is in place ahead of this storm and there is strong 500 MB support for a strong CAD to develop by Thursday night and continuing in through Saturday morning.  That’s where we stand right now.

The map to the left is a preliminary threat map based on the data I have available and historical data from similar past events with a similar 500 MB configuration.  Although guidance is not showing this threat yet, but is trending in that direction, I have the highest threat for significant ice over much of eastern Pennsylvania through the Hudson Valley.  My hypothesis is that this Arctic cold air is not going to exit very quickly, especially with a 1040 MB high ridge down the coastal plain ahead of this warm front.  While 850 MB temperatures will have no problem warming above freezing, I have my doubts that temperatures from 950 MB to the surface will ever break freezing over extreme northern Pennsylvania.

Closer to the coast, the power of the coastal front will eventually overwhelm the cold air at the surface, but not without a fight.  The Delaware Valley should have a period of snow, sleet, and freezing rain before an eventual change over to rain on Friday night into Saturday morning.  Meanwhile, snow may start out briefly along the New Jersey coast and New York City, however a change over to rain should be pretty quick given the expected east-northeasterly winds and then easterly wind behind the coastal front.  I should note though that given the snow pack in place, temperatures will be hard press over most locations to break past 45 degrees at most and likely by the time the event is 24 hours away, most locations will be stuck in the upper 30′s along the coast rather than the warm 50′s seen forecasted for this period yesterday.

The structure of this storm will be a key factor to keep an eye on.  A secondary low at a triple point (where the cold front, occluded front, and warm front meets) is expected to form, however the storm track basically takes this low right over the Delaware River Valley and into the Hudson Valley.  In a way, what we have is a large occluded front driving from southwest to northeast followed by a return to a much colder regime.  The “warm period” basically lasts for roughly 12 to 18 hours along the coast as the pattern reloads.

Changes To Watch For:

There are a few issues I’m keeping an eye on for this period.

1. The position and track of the secondary low.  The secondary low will eventually become the primary low and this process begins while this low pressure system moves over the northern Mid Atlantic.  The transition develops due to the primary low over the Great Lakes becoming vertically stacked while the strongest PVA drives towards the New England coast.  If this transition occurs sooner or the coastal low is further south and east, this would have a significant impact on the forecast.  For one, the coastal front is slower to move east into the coastal plain, thus supporting a colder solution.  Currently, I don’t think this will happen however it is something to keep an eye on.

2. The impact of the snow pack on the boundary layer gradient.  This aspect of the forecast is going to have to wait until Wednesday night at the earliest.  The snow pack influence on the boundary layer combined with a fresh Arctic air mass will set up the coastal front that will eventually drive west into Long Island and the New Jersey coast.  The intensity of this boundary layer will not only determine the strength of low level lifting, but also the timing of change over to various precipitation types.

3. Dry air.  Yes, that culprit will have to be monitored very carefully.  The high pressure to the north is rather strong and last time the guidance and this forecaster under forecasted the impact of dry air on precipitation totals.  The strong CAD can also support a persistent dry layer of air at 950 MB as well.  The impact of such a layer would have two impacts.  The first would be to support strong evaporational cooling leading to colder temperatures from the mid levels to the surface.  The second is a delay in the start of precipitation, which could lead to a shorter period of precipitation.

4. The high pressure moves to the east faster and thus the warm air simply overwhelms the Arctic air at the surface.  The models and historical analysis would suggest a trend away from this solution, but it is something to keep an eye on.

5. The high pressure is slower to exit.  This aspect has me worried the most, especially for those in northeastern Pennsylvania and much of eastern/central New York.  Already the 00Z ECMWF is much slower in the exit of the high pressure system over Quebec with a 1040 MB high over northern Maine on Sunday morning.  This is one of the reasons why the ECMWF has trended colder.  IF this high is in place for that amount of time, then one could make the argument that many locations just away from the coastal plain would be impacted with snow, sleet, and freezing rain for an extended period of time.

After this storm, a new cold regime sets in as the negative NAO is reestablished and a ridge builds over the Canadian west coast.  A strong disturbance driven by the Sub Tropical jet stream is expected to cut through southern California and race towards the coast by the end of next week.  With cold air in place and some minor blocking over the Atlantic, there is potential for a light to moderate accumulating snow event for the northern Mid Atlantic on New Year’s Eve.  However, the details are unknown at this time.

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