Saturday, January 26, 2013

Revisiting Friday Night's Fluff

Snowfall last night varied between a half inch to three plus through the Delaware Valley, with the city generally ranging between 1.5 and 3.0 (higher end of the range above Market Street, lower totals below).  Despite the commuting fail that last night's snow presented, it also was an incredibly dry, fluffy snowfall as it moved through the region.

Liquid equivalent yesterday ranged between 0.05 and 0.15 around the region, with snowfall totals generally yielding ratios in the 20:1 to 25:1 category.  To put ratios in a better perspective, the average snowfall yields a ratio of 10" or 11" of snow for 1" of liquid (meaning if one inch of rain fell in the form of snow, you'd yield 10 or 11 inches of snow).  Given how dry the atmosphere was, ratios were bound to be enhanced somewhat yesterday but it's usually tough to pull a 20 or 25:1 ratio unless the atmosphere's cold enough aloft and there is just enough moisture present through the lower and mid atmosphere to generate snow.  The type of snow many of us saw yesterday was dendritic, which means you can sometimes see the individual flakes such as below.

Photo from Andrew Veres of yesterday's snow.
The bulk of yesterday's snow fell in a two hour window between 4 and 6 PM in Philly and Southeastern's a radar shot from 5 PM yesterday showing the heavier swath of snow extending from near Phoenixville east through Philadelphia, with a second steadier swath of snow extending from Pennsburg in Northwest Montgomery County southeast along the Bucks/Montgomery County line towards Lower Bucks County.  In these areas snowfall rates were close to an inch per hour and they ended up picking up the most snow in the region from this event -- between two and three inches of snow.

More: Snowfall totals (NWS storm report)