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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 02:32 PM
 

Good Afternoon Philadelphia!  All the computer problems are solved, and I'm on live again! Hurray!

Here we go with an afternoon briefing.  Since it is now about 6pm, the daytime heating has kicked in and we have our afternoon round of showers in progress.  They are widely scattered but are dumping 1-2" of rain in localized places around the state. 

The age old front is sitting stagnant almost over our heads providing a focus for thunderstorms.  Air is lazily meeting nearly head on along this front.  The cooling air can't go into the ground, so it has to rise. Rising air cools as it goes up and expands. This helps get thunderstorms fired up!

You can see the directions of the showers around us.  One patch to our west is moving north.  At the moment, Philadelphia is only seeing the towering clouds to the west. We are not yet threatened by rain in the next 2-5 hours. 

The visible satellite view shows the same storms in a different light (pun intended).  The sun is now getting lower in the west and you can see the shadow on the right side of each batch of clouds/thunderstorms.  Neat eh?

To see why the weather service is so concerned about additional rain, look at this summary map of the rainfall nationwide for the last week.  The entire eastern US (on the right side of that very persistent upper level trough) has been hit HARD by a lot of water.  Pennsylvania and the Philadelphia area is no exception. We are in the 6-8" region.  Amazing. 

Winds are coming from the south now (as the upper level pattern switches (see below) and with the front to our west). 

The heat index map (a combination of temperature and humidity = stress on the human body) shows mid-80 heat stress coming up into the area.  Expect the heat and humidity to rapidly build as the weather changes over the next few days. 

The 500mb map, that shows the air flow at about 18,000 feet- the steering level for many surface storms, shows the jet stream (in red) moving up into Canada.  The big trough that was in the flow over the eastern US is getting quite isolated now.  It is marked in blue.  It has been cut off from the main jet flow and is expected to move to the south west away from us. 

The 500mb map for Wednesday afternoon shows the trough now inverted and sliding off to the south west into Texas.  We are in a strange place with a piece of a normal trough just to our north. 

But by Friday morning, the ridge takes ownership of the central and eastern parts of the country.  Expect rain to shut off and heat and humidity to build. 

By Saturday night, the ridge is even bigger and stronger.  The green circle shows the area to the right of the trough we'd expect storms and rain... it is far out into the ocean now. 

I'll be back tomorrow.  Until then, keep looking up and stay dry and cool!

Meteorologist  John Ensworth

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