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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: 11/15/2003 03:58 PM
 

Good Afternoon Philadelphia!   

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

In short: Mild temperatures (very close to normal, actually) and rain every other day. 

We are on the north side of the High pressure system that has oozed out of the southern Great Plains over the last few days.  It was the pressure difference between this big high and the low now to our distant northeast (just off this map in the upper right corner) that gave us the wind problems that folks will be talking about for quite a while to come. 

Looking at the situation with help from graphics from www.accuweather.com (you SHOULD pay for a 'pro' site membership to this site...it is GREAT!), you can see this situation again.  The winds here are dying down as the high approaches and the tightest change in pressure over distance region slides to the northeast. 

Zooming in, you can see we don't really have any frontal system nearby, and the tight pressure contours are up in the 'tiny' states and Main to our northeast. The radar is showing us some start of the rain that will next affect us out over Ohio. 

In the light of the setting sun, we are clear, with some high clouds spreading ahead of the showers to the west. 

The satellite picture above is all you can see using visible light.  In an hour, there won't be anything but darkness overhead. So we can turn to an infrared satellite view shows us the temperatures 'visible' from satellite.  Cloud tops are cold and are colored light yellows and greens. Very tall clouds are very cold and icy - and are colored blue.  The warm ground is colored orange and really warm land or ocean/lake surfaces are deeper reds.

You can see how warm the ocean still is to the east (and how warm the Great Lake still are). The coldest clouds over Ohio are where the rain showers are located...that's the way it works!

Winds move around a High pressure system (over N. Carolina) clockwise.  You can see this pattern over the entire US.  The calmer southwest (read:warmer) winds are approaching. 

Air on the east side of the Appalachians is warmer than out west. This is keeping Philadelphia more pleasant than the rest of Pennsylvania. 

With the warmer temperatures and claming winds, we have wind chills only in the upper 40's. Not 'much' of a chill if you dress nicely. 

The upper level winds (moving parallel to the black lines west to east over the US) at about 18,000feet (above mean sea level) are shown on the 500mb map. The interesting features have been labeled.

Our wind maker is departing to the east and northeast of us while the next shot of showers is going to be caused by the small trough rolling out of the northern plains towards us under a big Ridge visible in Canada.  Below this 'split flow' we have winds moving straight west to east (zonal flow) across the lower latitudes. 

Sunday afternoon arrives and the high has slipped to our east (giving us moist south winds again). The next system shapes up as a warm front (the leading edge of this returning warm moist air) and a cool front out in the Mississippi Valley. 

By Sunday night (7pm) and the mini-trough is almost upon us (hard to pick out on this map).  The broad ridge in Canada has also shifted eastward a bit. 

At this time, we'll see more rain spread over (primarily overnight Sunday) as the system to our west approaches from the west and southwest. 

Monday morning, 7am, and the warm front / Low pressure system slides over Philadelphia bringing a bunch of water with it. 

Monday night, 7pm, and a relatively narrow ridge is approaching warming us up again and shutting off the rain. 

The surface map at this time shows the dry weather (and new storms popping up over the southern plains ahead of the new trough seen above). 

And the, oh oh!  Tuesday night, 7pm, and that new trough is approaching the east coast...

And rain and thunderstorms spread eastward hitting the entire east coast for some more rain overnight Tuesday into Wednesday. 

I'll see you back here Monday morning!

Remember, it could be snow hitting every other day. 

Meteorologist  John Ensworth

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