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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: 10/03/2003 08:25 AM
 

Good Morning Philadelphia!   

    I'm glad to announce that these discussions will continue until at least the end of October.  

Overnight, the High pressure system (with clockwise airflow around it at the surface) center moved almost on top of Philadelphia bringing the core of the coldest air to us, and changing the wind direction. 

The winds near the center of the High are nearly calm and the air flow that was out of the northwest from Canada is now from the South out there! What a change. 

Temperatures are in the 30's and 40's as predicted and the winds and temperatures together make...

 ...really brisk wind chills down near 20 out west (since winds are nearly calm here, we don't get much of a wind chill...if you ride a bicycle or go running though...you'll feel it). 

The infrared satellite shows a very similar picture as the temperature map two up from here does. The warmer/red colors are, indeed, warmer temperatures. The blues/greens are colder (usually clouds). The Atlantic Ocean and the Great Lakes (see them peeking out through the leading edge of the clouds?) are quite warm compared to the land.  The Appalachian Mountains are visible as a colder ribbon stretching through central PA and down to the southwest. 

The 500mb map, showing the winds as they blow parallel to the black lines from west to east across the US at about 18,000 feet above sea level, shows two 'lobes' to the big (and I mean BIG) trough in the eastern US.  The one that kicked the last front through and brought a scattering of showers is off shore now, and the one driving the next front is pin wheeling around into the Northern Plains. 

So with no Low, no front, and no lake effect wind flow (the south winds are kicking off those showers lake effecting northward into Canada), the radar is clear over Philadelphia. 

So the future is in the next dip (short wave trough) coming through the BIG trough flow. Showers and a new front take shape by 2pm today over the Great Lakes (with some snow in side!). 

Tonight 2am, and the rain is entering the western part of the state...

Saturday morning and the cold showers are pouring into the city as the front touches western PA.

Then the rain and front are all but here by Saturday night (8pm).  Snow remains up north of the lakes and far up north in Canada. 

Enjoy all this great, relatively uneventful, Fall weather!

Meteorologist  John Ensworth

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