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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: 01/08/2004 10:26 PM
 

Good Afternoon Philadelphia!   

    I'm glad to announce that these discussions will continue until at least the end of January! 
    I have the necessary server space (especially with missed days!).  

In short:  As clouds linger and snow flurries come and go, BITTER cold will dominate the northeastern states through the weekend. .  

 In long: The Infrared Satellite picture, shows the temperature of things the satellite sees (via how much infrared (IR) light is emitted from clouds or the ground - cold high icy clouds emit little IR light and are colored blue, warm ground or ocean surfaces emit a lot of IR light and are colored red/orange).  We can see that a band of cold high clouds are streaming over the area at this hour and the underlying ground is very cold (colored green!). 

The actual temperatures tonight show temperatures in the core of the cold air to our north only in the -20's!!!  We are hovering around below freezing in the late afternoon (as the sun goes down). That's cold. 

The neat thing is the jet stream.  I've talked in earlier discussions that the jet stream form as a result of the difference between the cold air to the north and warm air to the south.  The upper level winds blow stronger where over the boundary between the temperatures. You can see this happening right now with >100mph winds aloft right in the right place (go back to the temperature map and look for the places where the temperature contrast (gradient - technically) is the greatest). Neat no?

The surface winds are MUCH calmer with a switch in the wind direction notable under the jet's location.  North winds (cold!) dominate north of the jet belt (stream) and southwest winds dominate over Pennsylvania south of the jet. 

(The temperatures + surface winds = wind chill, and we have 10's to 20's F wind chills out there right now. Bundle up!).

This switch in the wind directions under the jet stream is visible on the surface map as occurring where the purple surface trough is drawn to our north.  This is not much of a front, but it IS colder up there!

And the cold air is slipping in.  The highs Friday afternoon will be only about 28 F in Philadelphia (see the single digits HIGHS in New York State? Wow!). 

With all this cold air in place, we have a system sliding by to the south that might kick off snow flurries across the area by Friday 1pm. 

And the National Weather Service is acknowledging the chance of snow over the area by Friday morning. 

For Saturday 7am, we still have a tiny chance of precipitation as the system moves out into the ocean. 

Unfortunately, for Saturday, the highs will ONLY rise (or drop) to near 20 F. Holy Cow this is cold air. 

By Sunday, we warm up a bit... if you call returning to the upper 20's for Highs warming.  Note the extreme cold to our north is pulling out though.  That can only be good news. 

And for Sunday, another westerly flow moves in kicking off Lake Effect snow and some snow showers into Philadelphia. 

This is a very serious cold event.  Get ready and stay warm.  At least ice won't be taking down power lines!  Whew!.

From my home base again, I'll see you Friday, then Monday after the cold. 

Meteorologist  John Ensworth

Surface Station sky cover color key: