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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: 12/22/2003 10:19 AM
 

Good Morning Philadelphia!   

    I'm glad to announce that these discussions will continue until at least the end of this Year!. 
    I have the necessary server space.  

BUT I may have to switch to an approximately every-other-day schedule as Christmas year-end work piles up.  I'll do my best to stick with ya! 

In short:  Partly cloudy skies will increase to cloudy skies over the next day.  Christmas Eve will see rain, while the rain may switch over to snow by Christmas Night. 
Please Note: I'm logging in via dial-up modem from Corpus Christi, TX where I will be spending the Christmas break.  
Due to the slow connection, I'll keep it brief when I can log on and update this site. 

 In long: The Lake Effect snow is not quite gone yet. But with temperatures rising to the 40's today, the snow melt is underway again. New Jersey is becoming snow-free again.  But don't give up entirely...

The winter snowfall radar map shows nothing really noticeable in the way of winter precipitation, and the few showers falling up north are coming down in the form of rain. 

In advance of the next system, we have a transport of middle atmospheric and lower atmospheric warm air (streaming north then east out of Texas). 

On the surface map, even through the winds don't show a continuous flow, you can see 

THIS is a new map for these discussions. This is like the 500mb map, but it is at a different level...it is the 850mb level and these are the winds blowing mainly from west to east, mostly parallel to the dark black lines at under 5000 feet above sea level (as opposed to 18,000 feet above sea level at the 500mb level).   

I've marked in red arrows the air flow that is bringing in the warm (and moist) air from the Gulf of Mexico. 

At the surface, nothing stands out as a big storm, but we have a Low in the lakes and a Low in western Oklahoma/Texas Panhandle.  But this forecast is all about potential...more on that later...

Clouds are getting thicker across the state though they are mostly clear at this hour.  But are they high clouds or clouds associated with rain?  The radar view above shows us that the clouds in north central PA are more meaningful, but they don't look very different in a visible view. 

The infrared satellite view shows where clouds are colder (since it is showing temperature  NOT how reflective things are like the visible satellite picture).  The clouds over the rain shower areas are getting lifted higher into the atmosphere (giving them time to make rain) and they have the colder tops (so they are colored blue). 

So NOW to the 500mb level, we can see the cause of these showers... the mini-troughs are marked with thin red dashed lines.  No matter how big or small a trough, you can expect some lift of the atmosphere on the right-hand side. Which brings us to the BIG trough out west.  On it's right side you'd expect a big system to develop out of nearly nothing (which is about to happen!). 

So by 1am Tuesday, the trough will intensify the Low over TX/OK and begin to shape the next big system over Arkansas throwing moisture along a front to it's northeast into western PA. 

For Thursday 7pm, the trough out west is charging in for the kill (not a real kill, just for rocky weather). 

And on it's right side you can see the rain getting stronger and the system organizing nicely (again by Tue 7pm).  The rain isn't in Philadelphia yet, but it'll be here for Christmas Eve. 

For Christmas Eve into Christmas Day the snow pack from the GFS model (from the pro side of www.accuweather.com (a subscription to this site would be a GREAT Christmas present to yourself of those in your life that like to follow the weather). 

The 'snow would fall IF precipitation falls' map shows snow maybe falling to our west on Christmas Day, but the weather service is holding onto the chance that the cold air behind the front will mix with the departing precipitation by Christmas Night and maybe....just MAYBE you'll see a new dusting on the grass out front. 

Get your shopping done and keep watching the skies.

Meteorologist  John Ensworth

Surface Station sky cover color key: