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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: 11/03/2003 10:09 AM
 

Good Morning Philadelphia!   

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

In Short: Sunny and Warm/Hot... with showers and cooler air sneaking as the week continues.

Philadelphia is now the edge of the collision zone between the cold air to the north and warm tropical air to the south... let's look at it some more!  It is even MORE evident now.

Today's highs are expected to approach or hit 80F!!!  Amazing.  This is what a ridge can do.  What do I mean by that?

If you look at the expected high temperatures across the US today, you see an amazing pattern.  Cold in the northwest US and hit in the southeast US.

This is causing and caused by the trough and ridge pattern in the upper atmosphere.  The flow aloft helps bring cold air southward into the western US, and the cold temperatures compress the air molecules and cause the atmosphere to sit closer to the earth.  In the southeast, the upper level flow brings warm air up from the southwest, and the warm air expands and makes the atmosphere extend higher off the earth.

You can see the shape of the black lines (parallel to the wind flow at this height, about 18,000feet above sea level) exactly match the warm/cold boundary in the temperature map above. 

So the cold air sits under the trough, the warm air under the ridge, the interesting weather happens where the two collide. One neat note... look at the black circle in the eastern US.  I've circled the cooler temperatures there due to the higher elevation land there. 

Winds are weak and from the southwest (just like the flow aloft) over the state, and stronger and from the northwest up in the cold air over the Great Lakes. 

And a close-up look at the temperatures shows the amazing gradient (change over distance) in temperatures.  We're in the 70's F  in Philadelphia, and temperatures are only in the 30's F just a few hundred miles to the north. 

Where the cold air is and where the jet stream is... (look back up at the 500mb map for the places near us where the black lines (lines of equal height of the 500mb pressure level, which is about 1/2 of the surface air pressure)) is where the clouds are thick.

On the combination surface fronts and infrared satellite picture (on this satellite view, the image is of Temperatures NOT visible clouds, the blacks or dark grays are warm, the whites are cold, the yellows/blues are extremely cold)  you can see the white shading north of the front across the entire country.  Neat - no?

The air north of the front (tilted just like the trough/ridge pattern in the 500mb map) is cold enough for the precipitation falling to be coming down as snow across the northern Plains. Brrr. 

Monday night, we see the front has sagged a bit further south, but we'll stay warm and dry. 

by Tuesday night more warm air starts pushing northward and snow and ice are falling in Canada to our nearby north. 

By Wednesday night,  the ridge to the south has shifted a bit east and the trough out west has shifted east as well.  A new low is forming on the front over the Great Lakes. 

This will push that front southeastward into our neighborhood. The low will pass to the north, and rain will hit followed by colder temperatures all week. Today will be the warmest you'll feel, probably for the rest of the winter.  Soak it up!

Have a great Monday! I'll see you again tomorrow.

Meteorologist  John Ensworth

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