Sunday, January 27, 2013

Family of Precipitation Types on Monday

We're likely going to see winter weather advisories out for portions of the region tomorrow as the end of this wave of cold begins with a yo-yo of temperatures over the next 96 hours.  A warm front aloft will be pushing towards the region tonight into tomorrow morning, with warm air aloft pushing over a cold surface.  The result is light overrunning precipitation that will fall, with a wide variety of precipitation types expected around the region.

This won't be an all snow event -- nor will it be an all rain event.  A little bit of everything is in the cards for the Delaware Valley, with longer periods of the frozen end of the spectrum the farther north and northwest you are.

I have two views of the NAM -- the one immediately below shows the higher resolution variant that Penn State's E-Wall has a view of.  I like showing the precip type maps although the high-res isn't always the most accurate with details...the graphics do provide people a decent idea of what to expect.  We have a litany of precipitation types falling across Pennsylvania on Monday morning -- light snow over Eastern Pennsylvania, freezing rain and sleet across Central PA, with light rain out by Pittsburgh.  We're going to see an onset of light snow on the front end, transitioning to sleet and/or freezing rain as warmer air aloft pushes in.

The temperature profile on the "operational" or main version of the NAM (below) shows a decent chunk of the region, generally from the city on northwest, below freezing on the front end of the event (left graphic, everything above the purple line).  The graphic on the right shows precipitation in general with temperatures at 5000'.  The solid black line that goes through Philadelphia and Southeast PA shows the freezing line aloft, marking the "snow/no snow" line.  While the surface is still below freezing, rain is falling.  This suggests that precipitation will quickly transition from snow to sleet to icy rain or icy drizzle before ending as drizzle or light rain here in the city.  The farther north and northwest you are, the longer the frozen end of this event.  May not be all snow as warm air pushes in quickly but you will have frozen or freezing precipitation for the balance of the event.

This is not a "huge" system but there's enough overrunning pop with it that a few tenths of an inch of precipitation could fall.  Enough for perhaps an inch or so of snow around Southeast Pennsylvania away from the city, perhaps two in the Poconos before the changeover to sleet and then freezing rain.  The city could get a coating of snow before the transition takes place.  However, with the system's timing suggesting a start around Monday morning's commute in the western burbs, we could have another nasty go of it to the office.

We'll have another update on this later this evening once advisories are issued and we'll update the transition timing this evening as well.  There will be at least winter weather advisories out for a good chunk of the guess would be the 295 corridor on northwest.