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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 02:24 PM

(The Weekend Update)

    The sogginess continues for the eastern half of the state this morning.  From yesterday's discussion (see the archives) it looks as if the GFS model was more correct in what was to come than the ETA (which was the favorite model among forecasters yesterday).  The rainfall (at least in the north) is further to the east already.  We'll see and end to this round of rain earlier than expected. Note also that the center of the low is further south than either model showed.  See?  The weather is not always THAT predictable.  But we do the best we can. 

Just for fun, let's look at a comparison between two surface maps.  The maps were made about 2 hours apart. The above is (obviously) from the Weather Channel  and shows a single low over eastern Tennessee. The map below from the National Weather Service (and including the background infrared satellite picture) shows two lows analyzed, one in eastern Kentucky and one in southwestern Tennessee.  That isn't very different, but it can matter when making a forecast.  (It dosen't relate to our local storm, but the front that the NWS finds in the Great Plains is shown as a mere trough with (again) an extra low found in Kansas when compared to the Weather Channel. 

For us in Philadelphia, the rain is at it strongest this mid-day.  The current radar shows heavy rain over the entire area.

At this time the rainfall accumulation to our west is already up to around an inch to an inch and a half of water. These rainfall amounts will soon be adding up in the Philadelphia area.  Since it is still raining out there, even more rain will accumulate before the event is over - and the story is the same for our city.  Expect 1.5 to 2 inches of rain on already very wet ground. (Drought? What Drought?) 

So isn't a surprise that there are the current flash flood watches in the state:

It's just plain wet.  The overall upper air (steering) pattern for the US should become more zonal (flowing directly from west to east) then reverse from what it has been for the last few weeks (a trough or southward dip of the air flow over the eastern US and a ridge or northward bulge in the air flow over the western US). to the opposite.  We will (in about 6-7 days) have a ridge in the east and a trough in the west. This should warm up temperatures to normal values (or a bit above). But the rain isn't going away (look for more thunderstorms).  In the output from the GFS model, I've circled the next rain events for Pennsylvania.  This is predicting rain for Monday night, Wednesday night, Friday night, Saturday and Sunday.   

Sorry to be a bringer of bad news, but your lawn should look GREAT by now!

I'll probably take Sundays off unless big weather happens.  See ya Monday!

Meteorologist John Ensworth, M.S. 

(This page sometimes contains sources from AccuWeather.com   www.accuweather.com ).