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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 02:26 PM

A great good morning to you!  How can you not like the weather that's out there today?  We are cooler and drier than normal for this time of year, and are enjoying vaulted blue skies.  Wow.  And this will probably last for days to come with only brief interruptions by a chance of rain bringing another shot of relatively cool dry air. 

This morning's forecast discussion will be briefer than most due to the tranquil weather pattern. That will give you more time to go play outside.

The surface map shows the reason for the clear skies...the high pressure behind the front that just passed us is dominating the weather.  Air sinks slightly in high pressure inhibiting clouds from forming. 

There is a small bit of nocturnal clouds that formed in the north flow behind the front to our north, but the Philadelphia area is GREAT.

You can see the beginnings of some more moisture leaking back towards us in the light south winds area on the surfaced observation map below.  The green numbers are the dew point, which describes how much water vapor there is in the atmosphere.  We have mid 50's dew points coming in from the west first as the north winds and mid 60's dew points over Philadelphia and south eastern Pennsylvania follow the front away to the east.   

Dry air feels better because your bodies natural cooling mechanism, sweat, works well and you don't remain as wet when you sweat.  It evaporates into the drier air easier than it can in moist air. 

[A key to the sky conditions codes at the station locations is located at the very bottom of this page.]

A quick glance at the upper air maps, at 500mb (or about 18,000 feet), shows a big trough just to our west.  According the the excellent home state forecasters, Joe Bastardi at www.accuweather.com, this mean trough position should keep reforming a bit further to the west week after week all summer long.  If this happens, we'll eventually (near summer's end) be in a big ridge and baking in sweltering hot air.  Enjoy what we have while we have it!

By 2am Friday, we are still in the clear with the next front just touching NW PA.

By Friday 8am, there is a chance of showers north and west of us, but it won't really be effecting Philadelphia yet...

The forecast 500mb map shows the trough widening and deepening a bit as that front approaches. The jet stream (in oranges and yellows) is just north of the state. 

By Friday 8pm, the front is just hitting Philadelphia and we've had rain and a few thunderstorms possible over the area.  There is some trailing rain chances behind the front (but remember, the last two frontal passages have been drier than forecast due to the lack of an extra kick of upper air cool temperatures (short waves) timing right for making storms in eastern PA. 

The 500mb map at that time shows a kink in the bottom of the trough that might suggest a short wave will be available ast the front passes this time.  We might get some more significant (if brief <12 hours long) rainfall. 

But don't forget... we'll have more nice weather than we will either rain or heat/humidity.   Get out and throw a Frisbee around this weekend!

Meteorologist  John Ensworth

Surface Station sky cover color key: