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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 02:30 PM

Good morning again!  Well, if you look back at yesterday's forecast discussion you'll note that we concluded that the rainfall would stay away from Philadelphia and if it threatened, it wouldn't be until Thursday.  Well, a day later we see the moisture from former Tropical Storm Bill racing towards us to hit tonight.

The protective front to our south is still there and the moisture hasn't crossed the line yet, but it now looks like the northern reaches of the precipitation will pass over our southern counties. 

The radar this morning shows the big arc of rainfall wrapping around the center of the low off the edge of the picture. Rainfall amounts have approached 4-5" in the higher elevations of the south and south eastern states. 

The visible satellite picture (which shows bright white, reflective things like clouds as white, and dark looking things like ground and water as black or dark) shows the first bands of high clouds (ice crystal / cirrus clouds) covering most of Pennsylvania. 

The infrared satellite view (which shows temperature of surfaces the satellite is seeing - blues are very cold ice, reds are warm ground or water), shows the warmest temperatures in an arc from southern Michigan across New York and off Mass. into the Atlantic.  There is a band of sinking air across here and when air sinks, it compresses.  Compressing air warms.  This air is sinking here because so much air is rising where the remains of the tropical system is, it has to go somewhere.  It can't go up into space, so it sinks all around the margin of the storm. We have some of this sinking air over us as well, but the cirrus shield is hiding it somewhat. 

The next satellite image is the water vapor image.  It shows microwave emissions from water about half way up in the atmosphere (17,000 feet  - give or take a few thousand feet). The whites to blues are very moist, the blacks are normal and the reds and oranges are very dry. You can see the huge plume of moisture from the southern Gulf of Mexico feeding into the storm to our south, and the thin band of dry (sinking air) from Michigan to New York to our north again. 

The 500mb map (showing winds as they blow parallel to the black lines from west to east across the US) shows the jet stream (the tightest bunching of those lines = the strongest height gradient (how high you have to go to find the pressure at 500mb; the surface is 1013mb on the average).   The jet stream is now taking up it's more traditional summer position far to the north of the majority of the US. 

So looking ahead to the path of the moisture from former T.S. Bill, we'll look at the ETA and GFS models.  They don't agree (what's new!).  Yesterday the ETA left us dry in Philadelphia and the GFS had us wet.  Today they have largely switched. 

The ETA at 8pm tonight shows the rain just entering all the southern counties of Pennsylvania. 

Thursday 8am and we are still under a bunch of precipitation!

By Thursday night 8pm we are STILL in the heavy rain bands!  (The NWS is not convinced this will happen). 

Friday morning at 8am in the ETA model (the 4th of July) we are finally in the clear. 

The GSF for Wednesday 8pm shows rain entering the southern counties much like the ETA did...

But by Thursday 2am, the rain is already heading off to the east and staying out of almost all of Pennsylvania. 

By Thursday 8am, the rain is avoiding us as if the state had Teflon on it. 

The weekend should be quite warm and steamy, with only a very slight chance of showers on Sunday afternoon as a weak front passes.  Tuesday next week we might see a slightly more potent front approach and get us wet, then a bit cooler and drier for a day or two.  More on that ... later.

I'll be updating this discussion via a dialup modem on the road through this weekend.  Have patience with me if the discussions are a bit... more... brief. 

Meteorologist  John Ensworth

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