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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 02:27 PM

 Welcome to a new week!   The theme for this week is RAIN.  Previously, we've had very hit and miss showers with the city getting hit sometimes, and only scattered clouds other times (over the last 3 weeks to a month). But now, no one will be missed.  

The main player is the player that plagued us with rain/snow all spring and winter... the 500mb pattern.   Remember, the 500mb map is a good indication of the direction of airflow across the US (in this map). The wind blows nearly parallel to the dark black lines from west to east.  The height of these winds is about 18,000 feet. 

The big dips in the flow reflect cool air aloft and this is often above the air behind cold fronts.  The stormy region on the 500mb map is what ever is to the right of any dip (trough) in the airflow.  The new trough axis (in red) is further west than it has been the last two weeks (blue).  This puts us right in the storm laden region of the nation!

In the visible satellite, you can see the extensive cloudiness approaching from the west.  We have lower clouds near Philadelphia this morning with a wedge of clearing that must pass before the big storms come.  How can I tell these clouds are lower?  Take a look at the next satellite view...

The Infrared Satellite picture shows TEMPERATURE as seen by the satellite. The icy thunderstorm cloud tops are very cold and colored blue.  The hot bare ground (or warm clear ocean) is marked in red.  The clouds over Philadelphia are not very different in color than the ground, so they are low and made of liquid water (not ice). 

On the surface map, you can see the gathering surface storm in the Great Lakes with a trailing cold front. Ahead of this system is the first batch of thunderstorms forming.  Air flows counterclockwise around a Low Pressure system, so the winds here are nearly parallel to the white lines (like the black lines in the 500mb map above), so we have southwest to south winds over all of Pennsylvania.  This is bringing, momentarily, warmer moister air in (just in time to make storms for us!). 

The radar view shows these storms, some of them are getting quite strong in Ohio, and new cells are popping up in western PA moving to the east north east.  These might expand/form further south and hit us later this afternoon (especially as the daytime heating increases and destabilizes the afternoon more). 

The Severe Storms Prediction Center (SPC) in Norman, OK is calling for a chance of severe weather almost to the Philadelphia area...

And now we begin to look at the future. Tomorrow, the front is on top of us and the chance of severe weather is slight everywhere around us. 

Wednesday sees Philadelphia and eastern PA still under the gun for some storms to become severe.

Let's break it down a bit better...  By today at 2pm, we have rain spreading across the state ahead of the approaching frontal system. 

Thursday morning at 2am brings us Rain.  And some more rain. With little advancement of the front itself. 

Tuesday morning 8am and the front hasn't moved much at all (the upper air pattern is not driving it forward) and the storms reign supreme across all of Pennsylvania and the surrounding east coast states. 

Amazingly, Tuesday night at 8pm and the map looks almost the same! A front to our west and rain and thunderstorms ahead of it...

Now we'll switch to the computer model, the ETA, and see what Wednesday night 8pm looks like.  Very little change actually.  Still many thunderstorms and showers around. 

Then we need to change models again to the GFS (which forecasts further into the future) and look at what it says for the same time as the ETA above.  You can see that it has almost the same forecast.   A line of thunderstorms and heavy rain from Maine to Georgia and beyond with us under the gun. 

Saturday, I was calling for the rain to end by Thursday or so, but it now looks like this... the Thursday night map has still more rain falling over eastern and again western Pennsylvania.

Eastern Pennsylvania is still under a chance of rain by Friday night 8pm...

And we get to come up for some dry air and sunshine only by Saturday.  What an amazing run of rain... if it happens. 

I'll be here with the details as the week drips on.   See you tomorrow. 

Meteorologist  John Ensworth

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