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The Story Behind the Weather - By Forecaster John Ensworth M.S. - 

The Discussion of Weather Events Daily for Philadelphia and Pennsylvania

 Last updated: 09/14/2003 02:28 PM

Good Morning!  Today's forecast discussion is virtually identical to yesterdays.  You should include it in your reading this morning for the whole story!  We are just a day forward in the pattern.

The frontal system has moved a bit further east than was forecast, but for the Philadelphia area, it still means rain and south winds. 

The radar image this morning shows the rain HAS cleared out, but there are spotty showers in the states around us.  Just wait for the daytime heating to destabilize the atmosphere again.  There is more precipitation coming. 

Because of that, the National Weather Service has issued a Flash Flood Watch for eastern Pennsylvania counties including the Philadelphia area. The ground is wet and if more rain falls, it will easily run off. 

We still have a slight chance of severe weather this afternoon, but with the front advancing and the trough (see below) becoming less deep (great phrase- no?) the risk has slid off to the east and lessoned.

There is no elevated risk of tornadoes this afternoon.  This map shows the slightly elevated risk of hail today for eastern Pennsylvania...

...and the risk of strong thunderstorm winds are slightly elevated as well (though more of a risk down south). 

Cloudiness is thick across the state today except right to the west of the front. Any clear skies will allow warming of the ground and increase the chance of strong weather this afternoon. 

The infrared satellite picture below shows cloud top (or ground where clear) temperatures.  The deeper blues are the colder cloud tops.  Air is rising faster and further there and these bluer areas are the regions you'd expect the precipitation to be falling.  (Scroll back up to the radar view and compare the blues here to the rain there... is there a match?).

I've drawn in the switch line between north-ish winds and south-ish winds (the winds are nearly calm along this blue dotted line). This is the present location of the front!  The temperatures (Red numbers to the upper left of the station circles) are in the upper 60's to the west of the front and in the lower 70's ahead of it.  The dew points are quite different across this boundary.  The green numbers are the dew points. Dew Points express the amount of water vapor available in the atmosphere - the higher the dew point, the more water there is. Below 40F is quite dry, above 60F is quite moist, for a reference.  Ahead of the front the dew points are in the 70's, behind it, they are in the low 60's.   The best location for thunderstorms will be where the temperatures and moisture contents are highest (ahead of the front). 

Please refer to my example wind direction diagrams in the ocean, then compare the wind directions on both sides of the front. See?  You could find the front yourself!

[A key to the station sky condition code is at the very bottom of this web page.]

So for today we see rain and thunderstorms across the state with the chance of severe weather to the east of the front including the Philadelphia area.

The pattern of air flow at 500mb (the wind barbs are the same as they are on the surface map above) or at 18,000feet above sea level, show the big trough we've been watching for a week (see yesterday's discussion for more on this). 

This trough is quite large today and we are on the uphill : inclement weather side of the trough. 

And we still have some showers in the area tomorrow because of it (the front will be passing us and heading off to the east). 

By Friday am, the trough is getting less extreme and moving eastward.

The rain is gone, but the cooler and drier air is still in place. 

By Saturday morning, the trough is really shallow, and it's central axis is on our heads.  Warming is starting, but it won't be really bad yet. 

And the rain is still avoiding us. 

The next chance of rain comes with a much less powerful front speeding towards us on Sunday. 

So keep your eyes out for severe weather one more day, and get ready to enjoy a mostly nice weekend. 

Meteorologist  John Ensworth

Surface Station sky cover color key: