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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: 12/25/2003 10:29 AM
 

Good Morning 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:  Say goodbye to the rain, hello to a few snow flurries, then a very quiet, peaceful gentle warm-up through the weekend.  
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. I may have to switch to an every-other-day schedule as I travel.

 In long: Over the last week, more rain has drenched eastern Pennsylvania, and the last two days only added to it. According to the radar estimated rainfall totals, we've had a wide area of one inch amounts fall. (Remember, radar can be fooled by ice crystals melting as they fall into warmer air.  Ice pellets and sleet can also look like a lot more water is falling than actually is). 

The Low is moving north into Canada so we have the standard northwest flow across the area picking up moisture from the Great Lakes forming Lake Effect snow across the area.  A few ripples of much colder air aloft (not shown here today, since I want to go open presents!) are kicking off other areas of snow in the cold air behind this cold front. 

Because of this pattern, the lake effect snow is causing flight delays in Detroit and Buffalo and Boston, but only Buffalo is greatly effected. 

So what about our snow flurries today? Behind the front, we have straight west winds bringing in colder air...

Freeing or near freezing air is filtering down behind the Appalachians as we speak.  We are hovering in the upper 30's, the borderline for getting snow!

So on the radar map that includes temperature information (both at the surface and aloft),  you can see that everything falling in and around Pennsylvania is snow or a snow/rain mix.  But can we see more detail near Philadelphia?

When significant rainfall or precipitation is NOT happening near a Doppler Radar, it is put into what is called clear air mode.  In this setting, the radar is EXTRA sensitive to small water droplets,  bugs, and snow flakes. Philadelphia's radar (marked by a + to the left of the word Philadelphia) is in clear air mode and is picking up light snow flakes falling almost in the city.  The colored speckles around Philadelphia and Trenton are called ground clutter and are reflections of the radar signal off buildings, hills and trees (they are always there so they are usually ignored by the computer, but not as much in clear air mode). 

So if you run out side RIGHT NOW and look up, you should see some snow flurries coming down in the next hour or so this morning before it warms up too much.  Is that good enough for a white Christmas?  Maybe not, but it's snow nonetheless.

Merry Christmas!  I'll take a Christmas break for a couple days.  I'll see you again Monday Dec 29th and we'll see how hard it is to get home from wherever you are... and the weather will be GREAT through the weekend. Enjoy!

Meteorologist  John Ensworth

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