Tuesday, October 28, 2008

What A Storm!

All of the buzz today here at Phillyweather.net and across emails and chat rooms around the web was about the surprise snow that fell across the region. While it was a surprise to everyone, the reasoning behind it is pretty simple. It's all about dynamics.

This coastal storm was stronger for a few reasons. I posted the above graphic last night to show you how the sharper upper level trough was feeding more energy into this storm and allowed for a stronger storm to take shape. The amazing bombing of precipitation last night across the Delaware Valley was a sign of this energy explosion taking place. Check out this 32 hour radar loop from Weather Underground that shows the storm's genesis and evolution across the region late Monday afternoon through earlier Tuesday night. The stronger-than-advertised upper level trough, combined with the trough moving slightly slower than advertised in computer guidance, lead to the bustogenesis that was last night's rain.

This same upper trough and its stronger dynamics also lead to today's surprise snows across the northern burbs (congrats Montgomeryville, Ivyland, Richboro, and other parts of the area on your snow!). The strong coastal storm was being fed and nurished with upper level energy diving into the storm from Canada. This helped enhance precipitation across the Mid Atlantic as the low deepened over the coastal sections of Long Island early this morning. As the precipitation intensified and became heavy, colder air was being pulled down from higher levels of the atmosphere.

As this colder air was being pulled down to the surface by the heavier rainfall, temperatures in the lower atmosphere cooled to below freezing throughout much of the precipitating atmosphere except right at the surface. This is called dynamic cooling. As a result, heavy rain turned to snow where the precipitation was the heaviest.

In the higher elevations of the Poconos, cooling took place sooner as subfreezing air was closer to the surface than it was down here in Philadelphia. The Poconos received much higher snowfall amounts and in some places received over a foot of snow. I'll talk tomorrow about the winters where we have seen an October snowfall (even a trace or more) and how those winters fared.

More: Snowfall totals and wind reports through 8 PM (NWS)