7:42 AM

I was really tempted to issue a Stage 3 Alert for locastions from Philadelphia through the New York City metro including central New Jersey and the southern Hudson Valley and Connecticut.  However, I think it would be wise to wait for 12Z guidance to confirm the trends and developments I am seeing this morning. 

My forecast has not changed, only my growing confidence of a significant snowfall over the forecast area.  All model guidance including the SREF means, GEFS means, GFS, NAM, UKMET, and Canadian have trended colder.  The ECMWF data in 12 hour intervals was not avialable to me for the time period in question, so I am leaving that model out of the equation. 

On of the issues that suggest a heavy snowfall is the development of mid level forcing over the forecast area.  The 850 MB low is expected to redevelop and intensify over the forecast area.  When this happens, the thermalgradient at the surface through 850 MB tightens significantly, producing impressive frontogenesis.  The latest average of guidance strongly points to this happening over the area I have high lighted on the snowfall map (see below), which suggest that some locations will hit 10 inches of accumulation when everthing is said and done.  Another interesting factor is that the water/snow ratio will be rising over northeastern Pennsylvania, the southern Hudson Valley, and Connecticut on early Sunday morning.  Instead of 1/10, those regions will rise to a 1/12 and 1/15 ratio.  Although the actual precipitation amount will be lower than over locations to the south like around New York City, the snowfall amounts will reach and exceed 4 inches due to the rising ratio. 

So with no changes expected in the forecast, here is a reposting of the snowfall forecast!

Map Descriptions and Details:

Light Blue:  This area mostly focuses on the central Hudson Valley.  At this time, the best dynamics remain to the south of this region as the storm track will be just too far south.  The precipitation will be all snow, however accumulations will range from 1 to 3 inches.

Blue: This area basically covers from Poughkeepsie, New York through extreme northern Pennsylvania.  The precipitation is still expected to be all snow, however like the central Hudson Valley, the heaviest precipitation will remain to the south.  I expect snowfall amounts of 2 to 4 inches in this location. 

Royal Blue:  This area includes much of the northern half of the Philadelphia metro including Philadelphia itself, central and northern New Jersey, the southern Hudson Valley, New York City metro, and all of Connecitcut.  It is this area that I think will be impacted by the one-two punch of first isentropic lifting snow followed by the strong dynamics of a developing coma head of the strengthening exiting low pressure system on early Sunday morning.  I expect a general 4 to 8 inches of snow in this region with some isolated locations reaching 10 inches at most under strong lifting parameters of a mesoscale nature.  Overall, I think most locations in this region will see an average of 5 to 6 inches from this storm.  However, the potential for significant influence of mesoscale processes and mid level forcing pushed me to a larger range in snowfall totals.  For example, should a band of heavier snow develop due to frontogenesis, then locations to the north and south of this band will get less snow than locations over that band of heavier snow.  This is due to air sinking to the north and south of the area of rising air.  What comes up, must come down.  I can’t forecast where that will happen, but I am betting it will.  Some locations will benefit greatly, others will be hurt by this.  The precipitation may mix at times with sleet late Saturday night, but I’m expecting an all snow event at this time.

Pink: Just to the south of the moderate snowfall will be an area of mixing of snow, sleet, and rain.  In this area, the influence of the redeveloping 850 MB low will cause mixng of the snow with sleet and rain.  This will significant hamper accumulation totals for locations in these area, which basically covers locations south of Philadelphia, south-central New Jersey, and coastal Monmouth County down through Ocean County, New Jersey and over southeastern Long Island.  I expect snowfall accumulations of 2 to 4 inches.  The precipitation will start as snow with an inch or two possible, mix with and possibly change to rain on Saturday night and then go back over to a light snow on Sunday mornign with another 1 to 2 inches possible. 

Green:  The push of warm air will be to strong to keep the cold air in these locations, which basically covers southern New Jersey.  The precipitation will start as snow but will change over to rain on Sunday night.  Accumulations of 1 to 2 inches will be possible on Saturday evening before the change over to rain.  The rain will change back over to snow on Sunday morning with up to an inch of snow possible.

Bust Potential:  There is still a lot that can go wrong in this forecast.  The 850 MB low can track a bit too far north and drag the rain/snow line towards New York City.  Given the model trends and the way the atmosphere is setting up, I don’t expect that.  There could also be a situation where the jet streak at 300 MB takes too long to develop and only light snow falls, which would limit accumulation totals significantly.  I will be keeping an eye on that over the next 24 hours as well!  Also, there are still boundary layer issues and the question of the amount of virga that will cut into potential accumulation that can impact this forecast.