Complicated storm may hold surprises

11/26/12   5:35 PM

So far in the Fall, storms that impact the Philadelphia and New York City metropolitan areas have been rather complicated and tend to hold surprises for those forecasting for them.  The previous storm that produced 13″ of snow in Freehold that certainly was not forecasted by any model guidance.  Now, here we are again with another complicated storm.

As I had warned this morning, the disturbances over the southern Plains appeared to be more enhanced and closer to phasing than what any model guidance had suggested.  Now many of the 12Z models are starting to pick up on this threat and as such the models are starting to enhance the precipitation potential over southeastern Pennsylvania, central New Jersey, and southern New Jersey.

Right now, when we look at the water vapor satellite picture, you can clearly see the Sub Tropical Disturbance  is far more enhanced and carrying more moisture than what is being modeled.

When considering this aspect I decided to increase the snowfall totals for all locations from the Delaware River Valley on east towards the coast, but not in a significant manor as there are still significant boundary layer temperature questions.  In short, how warm will temperatures get before the precipitation enters the region and how much will dry air be a factor in helping with cooling of the atmosphere?  At this time, this aspect of the forecast clearly has to be NOW-Casted.

So here is what I’m thinking this evening.

1. This storm is going to be stronger than modeled.  I think we can clearly see that this evening as the models are attempting to play catch up to what we are seeing on the radar and satellite this evening.  The interaction of this storm with the growing thermal gradient along the coast is going to create a dynamic that currently is not being modeled well.  As such, the threat for a rapid explosion of the surface low on Tuesday before a rapid exit on Wednesday morning is clearly a threat.

2. While currently not forecasted, the potential for mesoscale banding from the Monmouth/Ocean County coastal waters of central New Jersey to the Philadelphia metropolitan area is going to be a threat.  IF this banding sets up on Tuesday night driven by enhancement of 500 MB lifting via a negatively tilted trough, than temperatures will crash and snow accumulation will be possible.  How much snow would be determined by how much moisture is driven into the coastal line from the Atlantic AND how fast temperatures can fall due to evaporational cooling.  This aspect of the forecast CLEARLY has to be NOW-Casted but only has a 15% chance of happening, thus not shown on the current snow forecast.

So given the thoughts above, the threat is clearly there for a moderate snowfall for central New Jersey, southern New Jersey, and southeastern Pennsylvania IF all aspects of this storm evolves perfectly.  That evolution is currently not expected by most model guidance but given the surface trends, the threat is clearly there.  For now, the current snow map has most locations in eastern Pennsylvania and western New Jersey in the 1 to 2 inch snowfall range with portions of eastern Pennsylvania to the northwest of Philadelphia pushing to 2 to 4 inches.  The coastal locations are expected to have a trace to an inch of snow.

This storm will be NOW-CASTED and watched closely over the next 36 hours.