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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: 11/10/2003 10:08 AM
 

Good Morning Philadelphia!   

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

In Short: Clear and warming up - rain mid week..

Starting this morning with the Weather Channel's surface map, the big chilly high that has been pulling eastward over the weekend is sitting on our heads this morning and will be leaving over the next couple of days.  The entire country is pretty quiet this morning.  That's refreshing!

With the high centered on Philadelphia, you can see on the surface winds map that winds are nearly calm because of it. Winds out to our west are picking up from the south (air flows outward and clockwise around centers of High Pressure). This will bring warmer (and moister) air into the region.

Temperatures are not THAT cold (and didn't get as cold as predicted due to winds and cirrus), 30's around Philadelphia and low 40's inside the city.

The winds, being nearly calm, do NOT add to the chilling of the air, so wind chills are about the same as the temperature (unless you are jogging or biking this morning). 

With the high pressure system on top of us, air is slowly sinking and skies are clear and cloud free. You can see valleys and ridges across most of Pennsylvania on this visible satellite view. 

Tonight, with warming south breezes returning, we'll only drop into the mid 30's at worst, in the city. 

This high is sitting on the right side of a shallow ridge that is moving eastward across the nation.  (And storms form most easily on the right side of troughs).  These dark lines show the air flow from west to east across the U.S. at about 18,000feet above sea level where the atmospheric pressure (500mb) is about half of sea level pressure (1013mb).   For the most part, the air is not going through very large ridges or troughs across the country...hence the quite weather. 

Looking to noon today (just an hour away) you'll see the ridge is a bit stronger and has moved eastward from when the above map was made.  The map above was made at 12Z which was 7am local time. You can also see the trough in the northern part of the upper atmospheric flow getting a bit deeper. Oh oh, the 'zonal' (straight west to east flow) is getting wavy again.

With that trough deepening, we see a cold front getting new life at the surface across the northern Great Plains. 

Tuesday 7am, the ridge on us has begun to move on, and the trough is getting closer and deeper. 

At the same time, the frontal system (cold front and warm front) are getting their act together and are beginning to kick off rain and showers across the upper Ohio Valley and Great Lakes. 

By Tuesday night 7pm, we are on the uphill side of what has formed as a broad trough in the northern part of the upper atmospheric flow.  

And rain is spreading across most if not all of the state.  More rain... yipiee!

Looking way out to Wednesday night, we are still on the up hill side of the trough, so nasty weather of some sort will continue...

But to find out about that, you'll have to come back tomorrow!

Keep enjoying the sunlight that bathes the city today...and I'll see you again Tuesday.

Meteorologist  John Ensworth

Surface Station sky cover color key: