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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: 12/29/2003 09:32 PM
 

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:  We'll have a very late night, early morning hit of moderate to heavy cold rain, then a long gentle warmup begins and lasts into the weekend.  

 In long: The Infrared Satellite picture (which, unlike the visible satellite view that needs the sun shining, works by detecting the infrared energy coming from the ground, water, or clouds (which ever the satellite sees first)) shows high icy clouds moving in with the next cold front and rainfall.  Our mostly clear skies are being rapidly gobbled up by cirrus. 

This next system is coming in the form of a double barreled low pressure region (there are two Low's on the map near each other) and a cold front. 

The wind map shows strong south winds (as is normal) ahead of the front and the strong west (note it's not the very cold northwest or north winds behind the fronts).  I've drawn in the wind shift boundary in red (for contrast).  You can see the two 'kinks' in this line associated with the two lows.  The lows have a tighter counterclockwise rotation around them so they drag warm air (or let it lag) to the west on the north side of the lows. 

The temperatures are in the 50's!!! To our south (even into Virginia) and in the 40's and upper 30's across Pennsylvania. The air behind the front (far behind the precipitation) is only AT freezing, approximately. 

Zooming in you can see that most of the rain is at or ahead of this front (which is also common for a cold front). 

There is a line visible down through Ohio and southward of heavier showers. This is known as a pre-frontal trough and is a common but not a well understood phenomena. 

Checking the precipitation, we see only a mix (or partial changeover ) of snow and rain on the back side of the system. Our temperatures are expected to stay above freezing all night (even upper 30's like it is now), so no snow for us as this pulls though. 

By Tuesday 1pm, the front is off the coast, and the precipitation is wrapping around the now single low to our northeast.  With lake effect precipitation kicking in, there is more post-frontal precipitation expected than we see at present. . 

By Wednesday 7am, the upper air pattern (we look for storms on the right side of a trough and great weather on the right side of a ridge) shows flat conditions with some ridging hinted at to our southwest. This is the reason we enter a quiet warming trend. 

And the surface map shows high pressure building into the area and no rain or snow (this is, again, Wed 7am). 

Even way off to Thursday night, 7pm, we are on the right side of a building ridge... and the dry warm-up continues!

So long from, now, the mountains of central Arizona.  I'll see you here again tomorrow, then I'll take the 31st and Jan 1st off.  Stay dry and warm tonight!

Meteorologist  John Ensworth

Surface Station sky cover color key: