Friday, January 20, 2012

Updated Projection For Saturday's Precipitation

Saturday's storm has trended ever-so-milder in the computer guidance, particularly in the middle levels of the atmosphere.   This storm is not an easy one to nail down on specifics because of the hint...and in the GFS and NAM it's a rather loud hint...of icy travel in parts of the region on Saturday morning.   For those in South Jersey -- generally south of Route 70 and east of Route 295 -- going to be tough to get much in the way of snow outside of a coating to an inch on the front end.   The transition to rain should be a relatively quick one...and probably will take place before daybreak in most of South Jersey.   However, that transition to rain might be accompanied by a period of icy rain or sleet.

The storm is still positioned to start between 1 and 4 AM from west to east -- that really hasn't changed.  Precipitation is moving through Ohio and will shoot into Pennsylvania later this evening, hitting Philadelphia after midnight.

First, the Euro...which is painting a snowier and less icy picture than rest of the models.  The model is showing more snow because it is bringing more moisture, having it fall heavier, and is just a smidge colder at the surface and aloft.  The result is a snowier solution for most of the region.  Those generally above 295 and in Pennsylvania would pick up a few inches of snow -- generally on the order of 2-5", with some 3-6" totals across the Poconos and upper portions of the Lehigh Valley.  Even the city, on the Euro side of the fence, stands to pick up 3" or so of snow.  This is the best case scenario...not only for snow starved but also for those who aren't fans of sleet and freezing rain.

However, the GFS and the NAM have nudged themselves back into an icy direction for the region...especially for Philly on north/west.   That transition to icy rain and sleet could begin in the city by 7 AM, nudging quickly north into the Pennsylvania suburbs shortly thereafter.   The middle levels of atmosphere will moderate above freezing while the surface stays below freezing or right at the freezing mark.  This will result in icy travel generally along and northwest of I-295 up into the outer Philadelphia suburbs to the north and west.   Whether it falls as sleet or freezing rain is does look like a variety of slop will far during Saturday morning and the roads will likely be pretty slick for a time with the myriad of precipitation types that will fall.

The "good" news is that most precipitation will taper off by midday -- precipitation should taper to drizzle or light sleet or light rain by Noon from the city on west, with New Jersey seeing precipitation continue into the early afternoon before ending, generally in the form of rain south/east of 295.  

In terms of snowfall totals -- the city is still poised for 1-3" of accumulation, with 2-5" generally north of the Pennsylvania Turnpike and Interstate 195.  3-7" of snow is in the cards for the Lehigh Valley and the northwestern half of Berks County.   The red shaded zone that generally bisects most of the 2-5" zone and 1-3" zone from Philly on north/west into the outer burbs is the likely zone where icy travel is most likely.  We could see a good bit of freezing rain and/or sleet in these regions, making travel less than ideal on Saturday morning.  In the 3-7" zone, the best chances for 7" totals will be in the Poconos and Northwest Jersey -- not everyone will see that amount of zone but it's possible in isolated spots.