A Thin Line

A tricky forecast for parts of the area as we move into Wednesday -- one that features the prospect for more snow in parts of the Mid Atlantic and one that features the prospect for more rain as well.   The question is what falls when, how much snow falls where, and temperatures.

This system will have a similar arrival point -- early morning hours -- as today's system and has the potential as it works into a dry airmass of causing some evaporational cooling at the onset despite temperatures at the surface that will be above freezing.  This will result in a mixed bag of rain and snow as close to the city as the northern suburbs, perhaps even into the city itself if the colder modeling from the Euro (below) and NAM are correct.   As we transition into the daylight hours on Wednesday, chilly precipitation will fall off and on during the day...with any mix of snow/rain transitioning over to rain from south to north.

The question is just how far north that transition zone gets -- especially for areas north of I-78 where the GFS depiction warms you sufficiently above freezing to make this an event that's more like Monday and less like a snowstorm.   The NAM is colder than the Euro even, with potentially a foot of snow for the Poconos, a few inches across the Lehigh Valley, and even an inch or two in the northern burbs in a longer duration event that would last through Wednesday night before ending.    The NAM also brings cold air back in during the evening and turns the rain for folks north of the city back into snow.  The GFS cuts those totals down by 60-70%, which just a few inches in the Poconos, an inch or two in the Lehigh Valley, and maybe an inch into Bucks County.

The low's track in this storm will be a bit farther south than in today's storm -- computer guidance generally sends the low along or just south of the Mason-Dixon line.  The reason why the GFS is warmer is because it has a stronger push of warmth aloft and at the surface plus it is the most northern of the tracks, with the Euro south of the GFS and the NAM a bit south of both.  The NAM also "skips" the storm to the coast from Western PA, similar to the Euro but a touch south whereas the GFS doesn't have this skip taking place.

Historically, the NAM does "well" in temperature profiles within 48 hours of an event -- the operative word is within 48 hours.   Based on the midday runs of the NAM that were initialized on data at 8 AM this morning, 48 hours would be Wednesday morning.  After that point, its reliability begins to decline.    I'm not so certain the NAM ends up more right on this event in terms of its final outcome.   However, I do think we'll see some snow at times, perhaps closer than many of us would like.

When it doesn't snow on Wednesday upwards of an inch of rain can't be ruled out across the Philadelphia area, with less to the south.   It's not going to be a "nice" day no matter how you slice it and no matter what ends up falling.