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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: 11/11/2003 08:01 PM
 

Good Morning Philadelphia!   

    I'm glad to announce that these discussions will continue until at least the end of November.  

In Short: After the rain passes Wednesday, we'll be really windy and cooler, briefly.  Some snow may fly as this storm passes in the higher elevations of central and northern Pennsylvania. Cool!

A large region of showers and embedded weak thunderstorm cells is sliding eastward with the next system towards it's destiny with Philadelphia for the overnight hours and much of the day on Wednesday. 

But on the Weather Channel's surface map...what is causing this?  I think this is an oversight. 

At the National Weather Service, their surface map (combined with an infrared satellite view - colder temperatures of ground or cloud are colored white and yellow and even blue, warmer temperatures are colored gray and even black (quite warm)) shows an Occluded front meeting up with a warm front and stationary front all to our west.   

Notice the streak of clouds from southern California up the frontal zone through New Mexico and all the way up to us.  This is the boundary between hot tropical air to the south and cool pacific air to the north. 

Taking a closer look at the Weather Underground's map, you can see the majority of the showers are in the wedge of warm air south of all the fronts. The purple 'occluded' front extending up into Canada is the boundary between cool air to the east and cooler air to the west.  Not much of a difference, not much of a front, not much weather there. 

You can see the high clouds in the Infrared Satellite view streaming towards us with the approaching system. Remember, on an infrared satellite view the warmer temperatures (in this color scheme) are deeper reds, cooler temperatures are the greens and blues.  The departing clouds to our northeast are part of a short wave that we'll look at on the upper level wind flow map below.  It kicked off a few showers in the area today (not entirely well forecasted!)

The interesting part of this system is the surface winds.  They are out of the south over the ENTIRE region!!!  Above and below the front the winds are blowing out of the same direction (normally fronts have air colliding on them... this supports the Weather Channels lack of any fronts being analyzed.  This is a screwy situation!). 

With the south winds, warm temperatures dominate the area as well. Temperatures are VERY mild for this time of year. You can see the temperature difference ahead and behind the occluded front is not worth mentioning.  Those fronts ARE hard to justify except for the rain part and maybe the north-south gradient of temeratures. 

Here is why the weather situation is so strange.  I've drawn in a blue dot/dashed line across the US that divides how the upper air flow is flowing right now. (Remember, on a 500mb map, you are seeing how the winds are blowing from (mainly) west to east parallel to the dark black lines at about 18,000 feet above sea level.)  So we have TWO jet stream flows.  The northern flow has a broad trough or almost zonal flow (zonal flow = air going from straight west to east) out west of us and a ridge over our heads.  The uphill side of the northern flow 'trough' going into our northern-ridge is where the showers are falling now. 

In a different story, we have a trough off the west coast and over California and zonal flow across the southern states sliding over a big ridge in the tropical ocean.  Two different weather patterns happening at once.  How much more fun could a  meteorologist have??? (Maybe 3 flows?). 

Jumping into the next 48 hours, a low begins to deepen on the uphill side of the zonal flow/trough over the northern plains and a new push of cold air starts to sweep down across the central US.  We are on the semi-frontal zone with a lingering chance of showers at 1pm Wednesday. 

By 7pm Wednesday, the front out to the west pushes towards us and gets it's act together spreading showers and thunderstorms in it's way.  (Note the tropical air moves north over Philadelphia, so we'll warm up even more before the cold front hits.). 

Then by 7am Thursday, the upper air pattern has switched with a BIG trough digging into the east and a big ridge forming in the west (this is in the northern flow, there still is a trough under the ridge out west, and a ridge in the Gulf of Mexico under our trough. 

Behind this front, significantly colder and windier air moves in.  Snow will fall in western Pennsylvania and northern parts of the state.  See how close together the lines are around the Low to our north?  The closer the white lines (the lines of equal pressure) the stronger the winds are (the stronger the pressure gradient force is).   Prepare for a shock of cold windy wetness on Thursday. 

Enjoy the warmth before it is gone.

 I'll see you again Wednesday!

Meteorologist  John Ensworth

Surface Station sky cover color key: