Friday, February 03, 2012

Weather History: The Long Road Home

February 5th has been taken over in our lore with the big 28.5 inch snowstorm that pummeled Philadelphia just two years ago.  However, in 2001, a rain-to-snow event caught many by surprise as commuters went to work with rain, left work with inches of snow on roads, and lead to one of the more painstaking commutes home for many in the Delaware Valley as "normal" commutes turned into ordeals that lasted hours on end thanks to a rapidly intensifying low pressure center.

The February 5th event was progged to be a rain ending as snow event across the Delaware Valley.  The storm intensified more than expected, with the intensification allowing for the storm to generate and draw down additional cold air from aloft towards the surface.   What started as a general rain event in the morning in Philadelphia and a mixed bag event in the outer Philadelphia suburbs turned to snow from northwest to southeast through the region as the morning progressed.

By the time the storm was done late in the day, three inches accumulated in Philadelphia at the Airport and much more accumulated to the northwest of the city.  Chalfont picked up over fourteen inches of snow, with the Valley Forge area getting ten inches of snow and Pottstown picking up a foot.  A foot was common from the 422 corridor north into the Lehigh Valley, with accumulations as high as seventeen inches in Lehigh County and twenty inches in Warren County in New Jersey.

The storm intensified more rapidly than anticipated -- strengthening by 25 millibars in a 24 hour period, with this type of rapid intensification helping to increase precipitation rates and thereby drawing down colder air from aloft.  While precipitation started as snow in the Lehigh Valley, the expectation was for the event to be a modest event...and not one where over a foot of snow would fall up there.  Snow increased in intensity up there, becoming heavy by late morning.   The rain/snow/slop mix in Pottstown shifted to all snow by 10 AM, becoming heavy in intensity by Noon.  Rain changed to snow in the city during the midday timeframe, becoming heavy for a time in the early afternoon.  In Philly, heavy snow fell with temperatures above freezing, which was indicative of the dynamics at play above the ground as this storm moved up the East Coast.

This storm was one that I missed out on -- I was still in Minneapolis at the time but my days there were numbered as I was due to move east in mid to late April 2001.  However, I heard the tales of three hour commutes from Bryn Mawr to Ridley, five hour commutes from Conshy to Pottstown, and other such tales of commuting woe.

What makes this storm a bit more interesting is the major forecasting bust that occurred just a month later.  Perhaps this is the ultimate yin-yang of meteorology at play?   One overperforming storm outdone by an equally impressive underperformer.  Opposing powers at play -- snowstarved getting their wishes in one shot, snowloathers in another.

More:  Ray Martin's NJ Snowstorm Archive.