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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: 01/12/2004 11:37 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 warm (for this time of year) right now, but the bottom will drop out in the next 48 hours. Then the weekend will slowly warm again.  

 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 fairly thick clumping of clouds now flowing out of the west and northwest.  You can't see the warm lakes as red since they are blocked by cold clouds.

The warm ocean is very visible as reds and deep oranges to our southeast.  The Gulf Stream is seen as a strong boundary line of warm and cool water streaming southwest to northeast (north is up, of course, in the picture). 

The current map shows the waves of cool, cold and bitterly cold air.  We are behind one cold front with another moving down.  Some lake effect snow is forming out to the west, but nothing is falling in Philadelphia yet. 

This map, the 500mb map, is showing how the winds are blowing at the 'half pressure' level in the atmosphere, or about 18,000 feet above sea level.  Winds here are steering most storm systems and are flowing mainly west to east over the continent parallel to the dark black lines.

We have a trough with the cold air in the east coast, and warm air with the ridge out west.  Cold air is dense and lower so there is a dip, or trough, in the atmosphere.  Warm air expands and extends higher in the atmosphere (pressure-wise) and builds a 'ridge' in the atmosphere.

We expect bad weather to the right of the trough (out at sea right now) and good weather to the right of a ridge (over the Midwest and east coast right now). 

Expect for the cold air at high elevations, the coldest air in Canada is associated with the trough. 

If you look at the forecasted highs tomorrow, again ignoring the colder higher elevations of the western mountains, you see the same ridge/trough pattern in the temperature map as you see in the upper level, 500mb, map above. Neat. No?

By Tuesday night, this pattern gets a bit more extreme (according to the computer model, the ETA). 

And you can see that front to our north will have passed by then bringing in colder air (this is Tuesday night again). 

For Wednesday morning, the trough slides off the east coast a bit, but a new bump begins to roll around from the west.  You can see this shorter wave to the west of Lake Superior over northern Minnesota. 

At this time, VERY cold air is pouring down behind the front and into the trough.  The whites here are morning temperatures below -20F!!!

We are only in the lower 20's F by Wed morning. 

Some snow is forming along the boundary through the lakes with that shorter wave (this is still Wednesday morning). 

The surface map shows the front by Wednesday morning way down in the deep southeastern US. 

Wednesday morning's local low temperatures will be about 24-26F with incredibly cold single digits in New York State again!

Our chance of snow by Wednesday EVENING picks up as that wave approaches along the edge of the very cold air. 

By Thursday morning, the trough is deepest again over us and the northeastern states. (Brrrrr!). 

The -20's and below are pushing into Maine. 

And by Thursday evening, we are only at 23 F (still). What afternoon warming???  There won't be any. 

For Thursday morning, the bands of snow continue. 

And by Friday morning, the cold begins to back off and moderate (the -20F's retreat north a bit). 

And we pass the bottom of the cold with a morning low of only about 10F. Oh cold!

So hang on and prepare for a bitter cold rest of the week and look for the moderating that will return this weekend. What a winter!

I'll see you here again Tuesday night! 

Meteorologist  John Ensworth

Surface Station sky cover color key: