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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 02:31 PM

Welcome to another great Tuesday!  We have showers to our west, but they are not very large in extent, nor are they very heavy at the latitude of Philadelphia. 

Moving to a larger scale picture, you can see that the area of rain and moderate thunderstorms is rather isolated and mainly to our south. 

For the day, though, we are under a slight risk of severe weather as the front we'll look at in a moment, approaches, and as the short waves I'll point out to you pass overhead. 

Skies are mostly clear this morning, but the thick cirrus and some lower clouds will be approaching and mucking up the skies for us. Even if we don't get any rain, we won't see much sun after mid-day in Philadelphia.  This will limit daytime heating (hurray!) and reduce the widespread nature of severe weather across the eastern half of the state. 

A quick glance at the infrared shows that the coldest, highest, most icy clouds (the deepest blue) are to the south of us where the heaviest precipitation is falling.  The area circled in black in the upper left is the surface front!  You can actually see a cold front with you eyes! (Can you find it in the visible picture above?)

The surface map this morning shows the advance of the cold front toward New England with areas of rain scatted along and behind (to the west) of it.  Winds going around the low up in Canada are out of the west across Pennsylvania, which will keep the sea breeze at bay (no pun intended). 

In fact, speaking of the areas of rain scattered along the front, we can see four areas (circled in red) on the water vapor satellite view where a lot of air is rising and forming clouds.   We can see these lift areas as the right hand side of short waves traveling along in the upper air stream (see the next map). 

I've drawn in the trough axis of the short waves on the upper air map this morning and you can see a perfect correlation between these small troughs and those areas of cloudiness and rain.  Pretty fascinating- no?

Traveling into the future, we see the front (by 2pm) not far from where it is presently and no real rain in the Philadelphia area. 

Come Wednesday 2am, we see the area of lift in the atmosphere and convergence along the approaching front getting closer, but no widespread rain is effecting Philadelphia yet...

By Wednesday 8am, we finally have rain moving in and the front almost parked on our heads.  

The front is not moving very rapidly (now that it is orientated west-east and the short waves are traveling relatively rapidly off to the east and no longer kicking the front ahead) so we will still have showers and rain with some thunderstorms hitting Philadelphia by Wednesday 8pm and beyond.  

Enjoy the break from the heat that is coming!

Meteorologist  John Ensworth

Surface Station sky cover color key: