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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: 09/29/2003 09:30 AM
 

Good Morning Philadelphia!   

    I'm sorry to announce that these discussions will end at the end of September.  They are taking up a HUGE amount of server space.  If you REALLY want them to continue, email me.  Otherwise feel free to use the archives as teaching tools.  There are many good case studies in there - including 

As the nights get longer and the days shorter, the chill winds begin to blow (yes, right after the tropics dominated our weather). I'm starting out this fall discussion with the wind chill map to commemorate the chance of season that happened a week or so ago. Wind chill is a combination of wind speed and temperature.  The colder the temperature is, the colder you feel (of course), but the stronger the wind blows, the more energy leaves your exposed skin due to evaporation of water from your skin surface.  This wind cooling can freeze your flesh faster than just cold air can. 

The surface temperature map shows 30's creeping down through the Great Lakes.  We still have 60's on the coast, but the cold stuff isn't far away. 

The wind direction has air flowing out of the cold north and west towards us (pushing the front to our east out to sea... keeping the latest hurricane away). 

The radar map this morning shows a few showers out in the NW part of the state where the chance of rain will remain all week.  The pattern, as you will shortly see, isn't going to change much all week long nation wide.  

The big picture map shows the weekend's cold front off to our east and another, new shot of cold air sweeping down the Plains. AND there is SNOW falling in places around the Great Lakes.  Get those woolly clothing items out!

We're clear, dry and nice around Philadelphia this morning. You can see the clouds out west (where they plan to remain for days to come). 

On the infrared satellite picture (remember, this is NOT like a visible satellite shot...reds are WARM surfaces (ground or water usually) and blues are very cold (high clouds and deep convection like thunderstorms).  From this view, we can see that most of the clouds out west are fairly low (they are a similar temperature as the ground.  The atmosphere gets cooler as you go higher.) except for the thunder showers in the NW corner of PA where some colder 'green  colored' clouds are.

The reason for us feeling the season (change) is this BIG trough in the eastern US.  The red dashed lines show smaller troughs (called short waves) that are traveling along with the winds going down and up in this big trough (this is the wind flow from mainly west to east across the US at about 18,000 feet).  Each little ripple can kick of showers or thunderstorms if there is warm air at the surface, moisture at the surface, or air colliding at the surface (as at a front). 

So off we go into the uncertain future... there will be a hint of a front pushing through the lakes by 2pm today.  With the BIG trough associated with the cool air, and with the ripples of short waves coming by, more showers will pop up where they are already happening in NW PA. 

By 2am, little has changed.  There are showers up near the lakes. 

The temperature map at this time shows a wide swath of 30's and 40's all across the eastern 3/4ths of the US with this big trough.  Very cool air - EVERYWHERE!

Tuesday night 8pm and the Great Plains front is moving slowly slowly towards us. The upper level trough is hardly changed, so we see the same situation.  Most of the showers remain to our northwest. 

Here you can see the trough hasn't budged much. 

Thursday morning... yes, showers to our distant Northwest. 

And more cool air (look! 20's near Chicago!) still dug in over the US. 

Friday 8am and another front heads down across the Great Lakes (snow is a certainty out there!).

Then Saturday 8am we see that front approaching with rain almost reaching us. 

So enjoy this dry cool  morning / day.   Find those coats!

I'll see you again for one of the last discussions tomorrow!

  Remember, I plan to end these forecasts Sept 30th.  Email me if you wish them to continue.  Just a few loyal viewers can keep it going!

Meteorologist  John Ensworth

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