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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 02:15 PM

"Here comes the rain again. Falling on my head like a memory"... ahem.  Ok, I started out this discussion with a song.  How about a map?  As we saw yesterday the low is heading to the north of the state with rainfall starting across the northwest half of the state.  The big sequence of forecast maps yesterday still look good.  The rainfall that will bolster this cold front/low pressure system for later Thursday and Friday (and with lingering chances of showers into Saturday) just dumped huge amounts of rainfall across the southern plains.  You can see the rainfall from Arkansas up to Indiana. 

This is caused by a ripple in the 500mb map that is a pool of cooler air aloft.  It is caused a short wave and can be seen in the next map (circled in red).  Big troughs have persistent storminess on their right side (when looking at a map like this) and small troughs do the same thing!

The satellite picture shows clouds rapidly spreading statewide. There is one small, relatively, clear area to our west that might move over Philadelphia late this morning. But for the most part, we've seen the last of the sun until the weekend. :(
The area circled in red shows where the polar jet is and a streak of cirrus clouds.  In a visible satellite it is hard to see the difference between cirrus clouds (very cold ice crystals and at a high altitude) and lower (warmer) water clouds. So lets look at the infrared site. 

In this picture I've shown the jet and associated (blue colored) high clouds.  As this jet approaches, that will help pull air upward from the surface and kick off the thunderstorms that are to come from this afternoon through Friday (the showers later Friday into the weekend will be coming from the short wave shown above. 

Keep the umbrella handy again, and I'll see you here tomorrow. 

Meteorologist John Ensworth, M.S. 

(This page sometimes contains sources from AccuWeather.com   www.accuweather.com ).