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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 02:16 PM

As is our want, we'll start with the overall picture, the surface map.  The eastern seaboard high pressure is parked just to the south of the state.  In a high pressure system, there is a sinking motion in the atmosphere overhead.  This sinking motion inhibits clouds and warms the air (as air compresses, it warms... feel the bicycle tire when you inflate it).  The clear skies, light southwest winds and this sinking air is the reason we are topping 80 today.  Nice!
 Out over Minnesota, we can see the Wednesday - Friday storm sneaking up on us. 

The satellite pictures shows these clear skies from our high pressure and the cirrus over Ohio associated with the next storm. The sky is so clear today that you can see the patterns of mountains and valleys and the flatter lands in the southeast part of the state if you look at the picture below close enough. This is a visible picture, so bright is visually bright, and dark is visually dark. 

The map below shows the winds (parallel to the dark lines and blowing generally from west to east across the US) half way up in the atmosphere (500mb height level) is much flatter (straight west to east) than it has been for almost a month. The ripple (trough) in this flow associated with the approaching storm is visible inn Wisconsin and Illinois. 

The next storm system is going to track from it's current position east and slightly north through the southern Lakes.  The center of the low will remain to the north, so the precipitation will be the stronger thunderstorm variety.  The following LONG sequence of maps are the ETA computer model output stretching out 84 hours into the future. 

They come in two's with a surface map on top of a precipitation map for different periods between now and then. 

The first is a 30 hour forecast valid Wednesday 2am. The low is over Lake Huron and...

the precipitation at Wednesday 2am is still clearly west of the state. 

By 8am on Wednesday, the low is at the NW corner of PA with a warm front north of us, and the cold front to the west.  It will be quite warm and sticky. 

The precipitation is moving into the western reaches of the state with a LOT of precipitation north of the warm front and low. 

By Wednesday at 2pm, the Low is far to the northeast already and the trailing cold front is now entering the western and northwestern part of PA. 

The rainfall by Wednesday 2pm drapes all but the southeastern part of PA (Philadelphia will be just seeing a chance of rain now). 

By Wednesday night at 8pm, the low is to the northeast of Main and the cold front cuts the state in half (east from west). 

Widespread rain, the heavier storms now in eastern PA, cover the state. 

Then taking a big jump forward (Friday 8am), we see that the front hasn't drifted far and another low pressure system is oozing its way toward us. 

The precipitation Friday morning shows renewed showers covering the state. 

So soak up the sun and heat for today and much of Wednesday (in the eastern part of the state).  It won't get cold and clammy with this storm system, but the rain and clouds will keep temperatures cooled to the 70's. 

Meteorologist John Ensworth, M.S. 

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