7:23 AM

This morning an energetic disturbance over the Ohio Valley is racing east, slightly faster than most model guidance have projected, and is producing light snow showers over parts of western Pennsylvania.  This disturbance will continue to move east, but will run into the surface high pressure currently sitting over the Philadelphia and New York City metropolitan areas.  As a result, the majority of the precipitation with this disturbance will dry up before reaching the northern Mid Atlantic, however I can’t rule out a few isolated snow showers as this disturbance moves through this afternoon and evening.  Otherwise quiet conditions can be expected through tomorrow afternoon before a complicated low pressure system reaches the Ohio Valley on Tuesday night.

The water vapor satellite picture this morning shows a strong Sub Tropical disturbance loaded with moisture moving into California and pushing into the Southwest.  This disturbance will eventually interact with the Polar Jet Stream and the Polar Vortex over central Canada.  The result is a storm track that cuts up towards the Great Lakes.  Of course, in the winter, no forecast is ever easy and this is no exception.

A strong Polar high pressure system will be locked over New England Tuesday night through Wednesday morning.  This high pressure will produce cold air damning.  Cold air damning is when cold air, usually forced by high pressure nosing down the eastern side of a mountain chain, is locked in or damned up against the eastern slop of the mountains and the coastal plain.  The higher pressure produces a north to northeasterly wind, thus slowly the erosion of cold air from the surface.  It is that type of set up that will be evolving on Tuesday night.

Locations around Philadelphia, the southern half of New Jersey, and central/eastern Long Island have little to worry about with this storm.  Despite the seasonably chilly conditions ahead of this storm, a persistent easterly wind from Tuesday afternoon through Wednesday morning will ensure temperatures at all levels of the atmosphere will support only rainfall.  However, the forecast becomes more complicated just to the north and west.

On Tuesday night, the primary low pressure system will move towards the western Ohio Valley.  A long warm front will extend eastward through West Virginia and off the Virginia coast.  The CAD development at the surface to 850 MB will lead to a weak area of low pressure to develop along the Virginia coast, however I can’t stress enough that this is a secondary low.  With the primary low well to the west, the forecast area will be under the assault of strong warm air advection and here is where this becomes a tricky forecast.  Strong isentropic lifting will develop over the northern Mid Atlantic on Tuesday evening and continue on through Wednesday morning.  This strong lifting can be seen very well on the GFS and ECMWF where strong Omega values (changes in vertical pressure) can be observed.  The stronger the positive Omega values, the stronger the lifting.  All model guidance also suggest that plenty of moisture will be moving into the region by Tuesday evening.  Given this data, I think precipitation will begin to break out on Tuesday afternoon and many locations, given the cold temperatures, will start out as snow for this storm.

The map to the left is a preliminary snow/ice map for this upcoming storm.  I expect some changes as I get more data on the boundary layers.  However, given the snow pack in place over the interior, the stubborn high pressure system, and the strong isentropic lifting; I felt confident enough to present these ideas.

As I said earlier; eastern and central Long Island, the southern half of New Jersey, and much of the Philadelphia metropolitan area will start out and remain as rain. A few isolated snow flakes may mix in at the start, but this will be brief.

The Connecticut coast through the New York City metropolitan area, north-central New Jersey, and the northern suburbs of Philadelphia will start out slightly colder with a brief period of light snow accumulating a trace to an inch at most.  The snow will quickly change over to rain after midnight with no further concerns thereafter.

I broke up the interior locations of northeastern Pennsylvania, northern New Jersey, the Hudson Valley, and interior Connecticut into two tiers based on the timing of the cold air retreating.  Given the strong warm air advection at 850 MB, I don’t foresee this storm as producing a significant amount of snow.  Basically a period of front end snowfall before the precipitation change over begins.  By late Tuesday evening through Wednesday morning, I expect precipitation to fall in the form of sleet and freezing rain.  The model guidance for this period projects up to an inch of precipitation will fall in this time period.  I took that value and decreased it some as the GFS/NAM tend to be a bit aggressive with the QPF of late.  However, the threat is pretty clear that a significant ice storm is possible on Tuesday night into Wednesday morning in these locations.  This forecast is strongly dependent on the boundary layer in these locations, however given the 3 to 8 inches of snow on the ground (accounting for some melting, lets assume 1 to 4 inches on the ground by Tuesday night) and the high pressure to the north, there is strong support for a boundary layer remaining below freezing on Tuesday night.

On Wednesday, as the primary low races towards the St. Lawrence River Valley, the warm front will move into New England and all precipitation will change over to rain.  A strong cold front will follow on Wednesday night, ushering in a much colder air mass for this weekend.

Strong Polar/Arctic high pressure will build into the Mid Atlantic on Thursday morning with much colder conditions.  Temperatures will struggle in the 30′s for highs on Thursday afternoon.  Friday will be even colder with many locations in the lower to mid 30′s for highs.  This drastic change in the air mass may catch some off guard so don’t be fooled with the warm conditions on Wednesday afternoon as the cold air is on the way.

As for this weekend, there is a growing threat for a moderate to significant snow storm between Saturday afternoon and Sunday.  A strong Sub Tropical disturbance will be racing from the Gulf of Mexico into the Mid Atlantic.  One lesson I learned from Saturday night’s minor snow storm was that the model guidance significantly under forecasted the precipitation potential of the low pressure system until within 66 hours where QPF guidance increased.  I took an average of the model guidance for this period and included the potential for more moisture being involved from the Gulf of Mexico.  The structure of the 500 MB pattern does not suggest a storm coming up the coast towards the 40/70 bench mark, but rather exiting east-northeast off the Virginia or northern North Carolina coast.  This type of storm track usually favors an accumulating snowfall for the Philadelphia metropolitan area, much of central/southern New Jersey, and the southern half of the New York City metropolitan area.  As far as details of boundary layers and accumulation rates, I would prefer to wait until the air mass supporting this potential snow enters the region before speculating on that.

It is clear the winter pattern is going to become very active and guidance is going to become less reliable.  My rule of going 5 days at a time would be wise, and I’m going to stick with it.