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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: 03/31/2004 09:31 AM
 

Good Afternoon Philadelphia!   

    I'm glad to announce that these discussions will continue until at least the end of February (it's a short month)! 
    I have the necessary server space (especially with missed days!).  

In short:  Rain light, rain heavy, rain continuous.   

 In long: We'll start today with the satellite views.  This first one is not used often on TV, but it is the water vapor satellite image.  It uses microwave 'eyes' and looks at water vapor in the mainly middle (or near 500mb) level of the atmosphere.  Whites are rising motion and moist air, reds are sinking motion and very dry air. Purples and blues are probably cirrus and ice clouds.   You can see the HUGE swirl of air over the eastern half of the country.  I've drawn red arrows showing the convergence of different air sources (cold dry northern air, warm humid southern air). 

The 500mb map (a similar view of the atmosphere but using wind and pressure data - the winds blowing parallel to the dark black lines west to east across the US at about 18,000 feet above sea level).  You can see the 'swirl' in the satellite view above as a deep trough that is almost cut-off from the northern flow.  It's a Big-un!

The visible satellite view shows thick clouds over the region.  It must be raining! Right?

The infrared satellite view shows the temperature of the ground and clouds. The blues are high clouds and are very cold, the reds are very warm.  The coldest clouds are in southeastern Pennsylvania.  We have warm (and therefore low) clouds over Philadelphia (even though they look the same as any other clouds in the visible satellite view above).  We'd expect the heaviest rain to be where the blue (the coldest and highest) clouds are. 

The radar view, though, shows only sprinkles down in the southwest!  I guess it's the heaviest rain in Pennsylvania, so you need more than just satellite views to determine rainfall sometimes. 

The surface map shows the front that has been pushed into the southeastern US by that northern flow (seen on the water vapor image at the top) and scattered showers are wheeling around the low in North Carolina. 

Now, into the future.  By this afternoon, little changes in the east.  The fronts are to the south and rain is rotating around the lows to our south and west. 

How about Thursday morning?  The low is a bit stronger and a more solid rain shield has formed. 

Um, Thursday morning... the rain becomes heavier as the low and another cold front inches in from the north. 

Friday... still more rain. 

Saturday?  Still more rain. 

The trough in the east begins to move off to the east on Saturday and the rain should taper off MAYBE giving us a break Sunday. We'll look at that tomrrow!

I'll see you again Thursday.

Meteorologist  John Ensworth

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