Tuesday, August 13, 2013

Stormy Start To Day (Flash Flood Watches Out)

Update, 9 AM: Up to 1.98" of rain in Philadelphia, 1.67" of which fell in the last hour. Coatesville, with 6.53" of rain, has been the region's rainfall winner so far today. Most of the region has picked up at least an inch of rain, with higher amounts in the heavy storm that traversed through South Jersey as well as the cell that impacted Chester County early this morning.  Flooding is common in various parts of the area, specifically along Route 42 (again) just above I-295. The line of storms is pushing east through South Jersey now, with rains across the northern burbs nudging northeast through Bucks.

We should see these storms pushes east through the next couple of hours -- gradually exiting the region around midday. A break in the action with, perhaps, a bit more shower and storm activity in the offing for the Delaware Valley later this afternoon as the cool front approaches from the west. Any afternoon activity will be a lot weaker than what moved through over the past few hours.

Flash flood watches are out for the northern two-thirds of the region today -- generally the Philly metro and points north -- as a cool front approaches the region and a stronger wave of energy along the front has triggered thunderstorm activity that has moved into the region during the pre-dawn hours. The interaction of the stronger disturbance and abundant atmospheric moisture (PWAT's near two inches) are providing the combination of ingredients that will produce heavy thunderstorms that will produce quite a bit of rain today through the Delaware Valley.

This morning's rain and thunder are in association with the upper level energy and disturbance that is ahead of the cool front itself and not tied to the cold front itself.  That feature (the cool front) is still back in Western and Central PA and producing its own thunderstorm activity across Upstate New York and Northwest Pennsylvania.  More on the front's part of the picture later.

Heavy rain and thunder this morning with the upper disturbance will push northeast through the morning hours.  Most of this will work through the region between now and midday, with the Shore and South Jersey getting in on this later in the morning while Southeast Pennsylvania gets the brunt of the thunderstorm activity over the next couple of hours as cells push northeast.

More rain and thunder will develop out ahead of the front later on today and push on through. However, those afternoon showers and storms will be a bit more scattered and less widespread as this morning's activity should do a pretty efficient job of churning out our atmospheric energy.  I don't foresee this afternoon's activity being as heavy or as nasty as what's working through Southeast Pennsylvania as of 6 AM (the time of this post). We're seeing some severe storms with the strongest cells out there -- and even have had a tornado warning down in Northern Maryland with the storms as they push in.

In terms of rainfall totals, some locations will pick up more than two inches of rain.  Modeling varies on exact placement of heavier rainfall totals this morning -- with the GFS nailing Southeast PA, the higher res modeling nailing Delaware, South Jersey, and the Philly metro.  I posted the NAM higher res graphic below for reference point followed by the GFS to show the slight geographic difference.

Given "climo" and the tendency for the Fall Line to enhance rainfall, the GFS probably has a bit more credence of accuracy in where the heaviest rains will end up falling overall...and some spots north and west are in line for more than three inches of rain with today's storms. However, as the system pushes east and northeast through the morning hours the axis of storms will gradually push east as well.

One last note, some of the stronger storms could produce gusty winds and there's a very slight chance of downbursts that develop from some of thunderstorms out there this morning -- potentially some of them could produce brief rotating downbursts (weak tornadoes). We're not in a slight risk of severe weather locally but the Storm Prediction Center notes there's a very slight possibility of damaging winds due to downbursts within some of the storms.

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