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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: 09/14/2003 02:37 PM
 

Good Afternoon Philadelphia!   

Storms are moving through this afternoon!  And another front invades our brief touch of heat and humidity (hidden by clouds for the most part). 

The 500mb map (showing how winds blow across the nation at the 1/2 sea level pressure height in the atmosphere - which is roughly at 18,000 feet - the winds are blowing west to east parallel to the dark black lines). The dotted blue line shows the newly reborn 'east coast trough' sinking down over the Great Lakes and the east central US. This IS happening further west each time it happens.  This gives us a longer period of hot humid return (from the south) air.   

Just to our south is the western extent of the dangerous Bermuda High pressure system.  When it gets reestablished overhead, we'll get warm again... and incoming tropical systems MAY hit the east coast.  Keep an eye on this situation... I know I will!

You can see the high off the right side of the frame below (on the US surface map). There is our cold front just clearing the area to the south. 

Surface Winds behind the front are from the northwest, and winds to our south are from the southwest.  You can see the front visually here!

And you can see the front in the temperature map as well.  The current surface temperatures are sultry 90's just a couple states to our south (and they aren't that large of a state). 

You can see the lines of thunderstorm cells over our heads.  Fair weather cumulus are popping up behind the front.  These will diminish after dark when the sun isn't warming the ground anymore. 

The infrared satellite view (showing temperatures of whatever the satellite is seeing) show the coldest (bluest) and therefore the highest clouds are the thunderstorms that are NOT as easy to see in the visible satellite above. 

The line of thunderstorms pushing through is thin, but fairly strong (as you can tell from the reds up the line.  Air is colliding along this line (see the wind map above) and since air can't go into the ground, it rises making the storms form.

The SPC (Storm Prediction Center) even has us slated for a Slight Risk of severe out of these storms.  But they are almost gone... the threat will be over in an hour or so. 

The rainfall totals on the Doppler Radar network show rainfall near Philadelphia totaling up to an inch or so.  That's a good line of storms!

Jumping ahead a bit, we have the tropics firing up!  Hurricane Fabian will become a BIG monster (Category 2 at least) and will threaten the east coast in the next week to ten days.  As I said, we'll watch this.

The forecast track and intensity has it getting close to the Caribbean by the end of next week. We should have PLENTY of warning if this does come our way. 

The 500mb map by Sunday night shows a trough on the east coast and a split off trough down in the central US.  The ridge is still just to our south waiting...waiting...

We have the High on our heads by tomorrow night with cooler dry air in place (the Gulf coast depression (above) will be feeding moisture into the trough over Oklahoma/Kansas.  This will matter for us later. 

Monday morning and the rain is increasing over the central US. 

The central US trough begins to merge with our departing trough...

And rain is pouring north and east ahead of it.  Yep, more rain may slip back in here by Monday night.  

I'll see you back here on Tuesday (I'll be taking Labor Day off UNLESS something scary forms tropically or severe weather wise). 

Until then, take care!

Meteorologist  John Ensworth

Surface Station sky cover color key: