Sunday, December 23, 2012

Boxing Night Storm Uncertainties

The storm that will spawn the Boxing Day rain/snow/ice event for the Mid Atlantic is moving onshore to the West Coast today, poised to move across the country over the next three to four days and cause a litany of travel and precipitation headaches throughout the nation. Besides rain and snow, severe weather looms as a threat along the Gulf Coast for Christmas Day and early Wednesday, and icy precipitation seems to be a potential hazard for some parts of the country.

For Philadelphia, this storm looks as of now to be a mainly rainy one as it moves in Wednesday.  How warm we end up getting varies based on the outcome of the storm and its track, which is still uncertain even within 72 hours of its arrival.  The two main computer models are hung up on how low pressure tracks through and whether or not it transfers to the coast.

The GFS is the warmer, by far, of the two main models at this point.  It is enhancing the surface low pressure, strengthening it as it lifts north through the Appalachians.  This is a generally rainy, warm track for much of the region.  It may start as snow or ice briefly in the usual cold spots but the strength of this low would bring milder air north, resulting in a rainy outcome for most of the Boxing Night event.

The Euro has waffled a fair amount between a warmer track similar to the GFS and one that we highlighted a couple of days ago -- the situation where the primary low was weaker and transfers its energy to a secondary low that develops across the Delmarva, moving offshore. This would be a "less mild" scenario where the city would likely remain rainy but snow and icy rain would likely fall north and west of town before an end as rain scenario.  The farther north and west one is the more frozen and less rain one would see but there would be enough mild air from the dying primary nudging in to change most everyone outside of the Poconos over to rain at some point.  The Poconos and Central PA mountains would stay frozen longest and would probably get several inches of snow out of the Euro scenario.

Details aren't set in stone but it doesn't look good for snowlovers around Philadelphia on this storm at this point.  I do tend to think the Euro's track may be a bit closer to reality in the end so the northern burbs and Lehigh Valley should keep an eye out for the possibility snow or sleet on the front end of this storm before precipitation changes over later on Wednesday.  We're not going to get into specifics today on how much falls where but note that the Lehigh Valley, the Poconos, Central Pennsylvania stand a better chance of seeing at least some frozen precipitation from this than Philly, Millville, Wilmington, or the Shore will.

We'll provide another update on the storm in more detail tomorrow on the site.