Saturday, February 16, 2013

Last Night's Rain & Elevated Snow

Those looking for snow last night were a bit disappointed when they woke up this morning in the city. North and west, especially across elevated areas, was a different story. Snowfall totals last night ranged from trace amounts along 422 to as much as four inches in parts of Northampton County and farther north into the Poconos, the winners in an elevation snowfall that arrived a bit earlier than modeled and tracked a bit more northwest than modeled, which yielded more snow farther north over the Poconos and Lehigh Valley than locally.

I posted three radar and temperature overlays -- 9 PM, 11 PM, and 1 AM.  You can see the band of precipitation moving east across the region from west to east across the region.  However, the main focus of precipitation shifted from Lancaster County (9 PM) to across Upper Bucks and Northampton Counties by 11 PM and into Central and North Jersey by 1 AM.  Temperatures cooled quite effectively once precipitation started -- you can see Quakertown drop from 41 to 32 between 9 PM and 11 PM as heavier precipitation moved overhead.  Spots that were in the band of heavier precipitation tended to do pretty well from a rain changing to snow standpoint and were able to make the changeover.  However, the city and immediate burbs were not as lucky as the best forcing and dynamics with this band of precipitation tracked to the north.  However, rain did end up changing to snow (although briefly) in places where this band went through.

This event was the wintertime equivalent of a summer thunderstorm -- mesoscale components were the driver in intensity of precipitation and those are hard to pin down in specifics until the day of although the general idea can often be picked up in advance.  Modeling had a pretty good idea this was going to move through, with the Euro and GFS showing something like this for a few days, but it arrived in a couple of hours earlier than modeled to our west and those couple of hours, on a day where temperatures managed to jump into the middle and upper 50's through much of the region, made the difference in lower elevated areas. However, those spots with elevation and in that heavier band of precipitation were able to overcome those time and temperature obstacles and coax a nice snowfall.