Monday, November 26, 2012

An Update On Tuesday's Event

The overall consensus on the models, except for the NAM, is for Tuesday morning's event to be rather modest and for many, a non-event overall.  Precipitation amounts on the GFS and Euro in last night's run generally top out in the one-quarter of an inch or less department if it fell as liquid, which in snowfall terms means less than a couple of inches if precipitation were to fall as snow.

The operative word, in the city, is IF.  Temperatures on the usually-cold GFS are projected to be above 32 for the duration of the event so it may be a mixed bag of snow and light rain.  The Euro runs similar in scope for Philadelphia during the event so it would be a rain/light snow type event in the city (Airport).  It may very well be a different story in Chestnut Hill, Mount Airy, and Overbrook (it could snow on the north and west edge of the city) but for Center City, I-95, and along the river this could be a mixed bag event.

South Jersey (generally below Route 70) looks to be more rain than not as well from the event.  There could be some wet snowflakes mixing in on the front end but odds favor more wet than white in South Jersey.  It could be a 50-50 split between 195 and 70, with light snow more common north of 195.

Southeast and Eastern Pennsylvania should be solidly light snow in this event.  Both the GFS and Euro do paint snow in your backyards but with precipitation amounts rather modest on both models, accumulations from the GFS and Euro are generally on the inch to maybe two.

The exception, as pointed out earlier, is the NAM.  The NAM, compared to the GFS, is closing off an area of low pressure and enhancing moisture on the northwest side of the "low".  I'm showing two 925 mb maps (think a couple of thousand feet up) to show you the difference between the flat GFS wave and the more pronounced NAM wave.

The GFS shows a flatter surface and low level wave -- there's no "closed off" low unlike the NAM (see the 81 circle that extends north to near Allentown, south to near Norfolk, VA on the bottom map).  The resulting difference is that the models are showing two different results for precipitation for the region.

The GFS's wave of precipitation is less pronounced, weaker in intensity, and generally a modest event overall as it moves through the region on Tuesday morning and midday.  The NAM is more robust as the low is closed off above the surface to our south and precipitation is enhanced to its northwest.  This means a Southeast PA "jackpot" scenario would play out on the NAM depiction.

The NAM's jackpot generally resides in a swath from Gettysburg to New Hope, from Tamaqua south to the PA border (it's borderline rain/snow along the PA/MD border on the NAM) with a three to six inch potential snowfall in this area. Philly would be on the fence for this and shift between rain and snow, perhaps picking up an inch or two of accumulation, but the "better" snows would be to the north/west of the city.  This is in line with the models' thinking on precip type but is much more aggressive on precipitation amounts.  Keep in mind that this is the only model showing such a scenario playing out -- the GFS and Euro are much more modest in comparison.  However, if the wave that fires this precipitation up is a bit stronger, we could see that swath of snow tomorrow morning and early afternoon overperform the GFS and Euro.

What does this mean for you all?  For those southeast of I-95, not much.  Snow might be a bit tough to accumulate and odds favor South Jersey and most of Delaware not getting accumulations of note on this.  Some flakes are possible but precipitation seems to be more liquid and modest.  For those in Southeast PA, you'll get at least some snow and while odds favor modest accumulations of an inch or two, the NAM is hinting at the potential for more.  The city looks to be in line for about an inch of snow on this.

Timing-wise, snow (and rain) should spread in from the west after 4 AM.

I'll provide another update on this later this evening and we'll update snowfall totals for the region then.