Back To Home
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 02:31 PM

I hope you had a great vacation weekend.  Our forecast discussion today will center around a tale of three short waves, and one long wave.  The surface map this morning (below) shows no frontal systems near Pennsylvania.  There is a front out on the northern great plains, but that is not our concern right away.   I've drawn in three purple dashes on the map showing three short wave locations (ripples in the upper air flow that bring in cool air aloft, making thunderstorms/rain easier to form).  You can see that to the right of each line there is some precipitation (it is pretty far ahead of the Montana short wave, but it's there!). 

You can see these ripples directly on the 500mb map showing air flow aloft. 

And on the infrared satellite picture (showing temperatures - reds are warm, blues are cold (ice clouds usually)). The short waves are marked again and you can see clouds to the right side of each. 

The water vapor satellite picture shows the path of water vapor in the middle parts of the atmosphere.  You can visually pick out the short waves here too!

Where the short waves are approaching we have a slight chance of severe weather across the northern US states as marked.  Where there IS a surface front, the two elements together are giving us a moderate risk of severe weather in Nebraska and Iowa. 

Tomorrow, with the short waves just sliding eastward replacing each other in position, we see little change in the chance of severe weather across the nation (including PA and almost Philadelphia).   The western states aren't included in the risk areas because there is so little surface moisture. 

Now we'll go in small steps out a number of days and watch the progression of waves (and the front).  We see at 2pm this afternoon showers entering the western part of the state. 

That wave ripples over us and gives us rain and thunderstorms in the area (and a brief relief from the heat and humidity) by 8pm tonight. 

That wave passes us (for the most part) by 8am on Tuesday, but the effect of the front is now approaching. 

Aloft, you can see the first shortwave just off the coast to the east, the second just to our west, and the third out in the middle of the country at this same time. 

By 2pm on Tuesday, we are in a break, while rain falls in the extreme western part of PA still. 

Little change yet by 8pm Tuesday...

Then the front really starts to move in by 2am Thursday...

And the next shortwave gives it a kick and brings it to life by Wednesday 8am (still to our west though). 

The upper air map shows the (previously mostly flat ; i.e. west to east) air flow now developing a larger dip over us.  This is our famous (this year) east coast trough that will cool temperatures down to the 80's by this time. 

Then the rain is here again by Wednesday 8pm.  With the trough overhead and rain and a front passing, things ought to be *relatively* nice for the end of the week.

By Thursday 8am, that front is off shore and we have slightly cooler and drier around.  Nice!

The jet stream (and our trough) is taking on a split flow feature (marked in big fat blue arrows).  There is a northern branch going to the north, and a southern branch south of us.  This will be an interesting forecast for the weekend!

But to find out what it means, you'll have to come back all week!  See you next time. 

Meteorologist  John Ensworth

Surface Station sky cover color key: