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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: 01/14/2004 12:10 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 are thickening and temperatures are dropping fast heading for the coldest three day period in at least a decade for Philadelphia and New England. And the warm up promised for the weekend, is probably out.  

 In long: This forecast again builds on yesterday's forecast discussion, (and therefore the day before forecast) so look them over before heading on... 

Errata: I stated yesterday that the lowest temperature records may be set (as per accuweather.com) Wednesday night pre midnight and Thursday morning post midnight.  I mis-spoke.  The real event may happen 24hours later... Thursday night into Friday morning with single digits to below zero F temperatures possibly happening in Philadelphia and breaking both day's record lows. 

In fact, the numbers Accuweather.com is calling for are much colder than those from the Franklin Institute on the front page of this forecast site. They are as follows (with snow forecasts).  Note that the weekend and even on into Monday are not, in any way, warm.  They are just warmer than the next 3 days. 

Today	Tonight	Tomorrow	Friday	Saturday	Sunday	Monday	Tuesday
21	16	18/6	16/13	23/22	32/23	32/20	38/26

Next, we'll hit the 500mb map.  This is a map of the air flow at about 18,000 feet above sea level.  Air is moving at the 'half surface pressure' map parallel to the dark black lines mainly from west to east (through the roller coaster pattern).  That pattern is caused by temperature differences across the map region. Over the Northeastern part of the North American continent there is a piece of the air usually found and born at the North Pole!  This polar vortex is extremely cold air. We'll watch it rotate around before it moves off (and allows us to warm up next week) in the maps below. The troughs, marked in red dashed lines, are areas of added lift.  You would expect storms to get more of a punch on their right side. 

You can see this same pattern by looking at the forecasted maximum temperatures today from the National Weather Service. This has the same sloping look as the map above (if you ignore the colder higher elevation mountains in the western US). 

The surface map shows a Low in the Great Lakes following the edge of the very cold air in the map above (and the mid-atmospheric wind flow in the 500mb map) headed right for us.  With THIS MUCH cold air around, of course all that is falling is snow. 

Looking at the color coded radar data at the time of this update, you can see a wide swath of moderate to heavy snow moving in quickly. You should see the snow begin by the rush-hour tonight. 

The visible satellite view, showing what is white and reflective vs. what is dark and absorptive of visible light, shows the clouds in place as the system approaches rapidly.  But which clouds mean business (snow) and which are just low clouds? To answer that, we need another view of the clouds and earth...

The view that works is the Infrared Satellite image.  It shows the temperature of the surface the satellite first sees by seeing how much Infrared Light (IR) (sort of like a heat lamp) is emitted.  Very cold clouds or ground/snow don't emit much IR energy and are colored blue, warm land or ocean currents emit a lot of IR energy and are colored red/orange (see the ocean to our distant southeast). 

The coldest clouds are cirrus clouds sweeping out ahead of the system.  The 500mb winds are carrying these from the regions of heaviest snowfall over our heads.  This supports the forecast of a direct hit coming from this storm.

Also from the NWS, the snow will have begun by 7pm tonight (probably by 4pm) in Philadelphia ...

And will continue until it tapers off around this time Thursday.  24 hours of very cold snowfall. Powder anyone?

Then, on the 500mb map, we see the Polar Vortex rotate around nudging almost to Philadelphia...

And Thursdays Highs will struggle to rise to 20F.  (See the below 0F HIGHS up in Vermont? Wow.)

For Friday 7am, the vortex begins to pull out to sea, but we have a strong NORTH flow of air into the area...

Bringing below zero temperatures deep into Pennsylvania and New Jersey.  Will be dip below 0F and set records?  We'll see!

Friday's highs don't recover after that (with snow on the ground and north winds?) and we do stay in the 10's F for highs  then.  Nasty nasty. 

Get home before the day long snow storm hits, then STAY WARM whatever you do. 

I'll see you here again Thursday, after the snow and as the cold locks in. 

Meteorologist  John Ensworth

Surface Station sky cover color key: