Wednesday, January 16, 2013

A Note About Thursday PM

Computer modeling is hinting at a third wave along the frontal zone that's been nearby for the past couple of days.  This wave of energy is developing across East Texas in the mid levels and will spin up low pressure over the next day or so that will move east and northeast across the South.  If you're along the Jersey Shore, Delaware coast, and south of Wilmington the odds are there for a third round of precipitation on Thursday afternoon and evening.  The city rides on the fence but as of now may avoid getting in on this system.  It is something that we will watch though since trends are nudging this system a bit closer to Philadelphia as we close in.

Below is the GFS, followed by the Euro's depiction of this storm.  Both show a wave of precipitation to the city's south moving in on Thursday afternoon, continuing into the evening before tapering off.  The mid-shade blue line on the GFS map (first map below) is the "850" line where temperatures at 5000 feet are at freezing.  The darker blue solid line is surface of 32.  Temperatures on Thursday evening are marginal for snow south of the city but it's doable if precipitation if heavy enough...and on the GFS, it's showing a pretty decent trend of not only strengthening the storm system pretty quickly, but also sending a pretty decent shot of precipitation across the region...perhaps even grazing the city.  As of this morning, the GFS does not put anything into Philadelphia.  Comes darn close to doing so though.

The Euro is a bit less aggressive on the intensity of this system as it moves through -- it generally shows precipitation light and modest with a weaker low pressure center.  Temperatures are similar but precipitation is lighter...might be a sloppy mess at best, a cold rain at worst that could fall as snow across Virginia but not along the coast.

There are a couple of ensemble variants of the GFS and Euro, along with a couple of SREF variants that do nudge precipitation into Philadelphia and Southeast Pennsylvania.  Right now, the major models don't show this on the operational.  Odds favor anyone south of a Wilmington-Long Beach Island line getting some precipitation.  The key for snow, given the marginal nature of this setup, is intensity.  If the GFS' stronger setup is right, a rain to snow scenario is the probable result and some accumulation is definitely doable for most parts of the Delmarva and coastal NJ and DE.   A few inches is certainly in the offing down there.

If the weaker Euro setup pans out, a cold rain that could mix with or change to snow in some "steadier" areas of precipitation will result.

Another tricky forecast, enough to help turn forecasters' hairs gray (or simply lead to hairs falling out).

As always, we'll keep an eye on it over the next 24-36 hours.