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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 02:40 PM
 

Good Afternoon Philadelphia!   

    Wow, I have a LOT to cover today with you, here we go...

One of the big stories is the tropics and how it figures into our weather.  It may seem strange with cloudy skies and temperatures in the 70's in Philadelphia this afternoon to worry about the tropics, but hold on...

First is the super sized hurricane Fabian that has remained a category 4 hurricane for quite a long time.  It is forecasted to re-curve before getting to the east coast now.  Almost everyone is comfortable with this, but until it does it... everyone on the east coast needs to keep an eye on it. (Bermuda is quaking in their boots as well). 

More at home, we have had a number of lines of showers move through as this next front pushed overhead.  In the last two days, as can be seen in the radar summary of fallen precipitation, we have had about 1.5 to 2inches with over two inches in spots. 

The cold front is now stationary (so it is now called a stationary front because of that) right over our heads. 

Looking at another surface map, you can see the remains of another tropical system - former Tropical Storm Grace - now in Arkansas.  Tremendous amounts of tropical moisture are arcing northeastward  (see my green arrow) south of the front.  Many many inches of rain will fall over a very large area because of this. 

And here come some!  These showers are moving along and to the south of the front, but we should see showers and light rain increase as the day goes on as the next blob approaches. 

The visible satellite picture - showing how reflective the earth and clouds are - shows the big thick band of clouds over our heads. The puffy looking surfaces are cumulus and cumulonimbus clouds (puffy clouds and raining puffy clouds). 

The infrared satellite picture (that shows how warm (red) or cold (blue) the earth and clouds are) shows warm bare ground in the Great Lakes region and to our distant south.  The highest and coldest clouds are out west where it is raining and north of there (where surface moisture is a bit thin due to the passage of the front yesterday - so no rain is forming there). 

Looking at the surface winds, you can see where the front is yourself.  Winds are colliding there (also called converging). Winds over Philadelphia are cool and from the northeast, winds over the Virginias are from the southwest.  That is a head on hit! So the air finds it easier to rise there and form storms and rain. 

The heat is far to the south again.  We get a day of heat, then much of a week of cool.  Weird Summer!

The 500mb map (showing winds blowing parallel to the black heavy lines mainly from west to east over the US at about 18,000 feet - the height that steers storms often) shows an interesting feature.  All across the nation we have troughs (dips south in the air flow - blue lines)  over ridges (northward bulges of air flow - red lines).   There are two flows right now each with the opposite feature opposite of what the other flow has.  We expect rain and clouds on the right side of troughs (big or small), so the Ohio Valley trough is what is kicking off this newest round of showers approaching us. 

The 500mb map for tomorrow shows the Canadian trough rotating down towards us.  (And Hurricane Fabian down in the lower right entering the frame). 

That trough and the nearby front = more rain showers for us. 

The southern trough and the northern trough line up (as do the ridges out west) making one large trough. It is this flow that will protect us (almost certainly) from Fabian. 

But with the trough here, and the front lying north south along the coast - we can expect more rain until it moves off to the east. 

Friday morning and Fabian is starting to curve to the northeast being influenced by this big trough on our head. 

And still we might have some coastal showers near us. 

Then Saturday morning, Fabian is making big waves along the east coast US but is beginning to move away (or WILL it?!).

And we are mostly out of the rain/shower chances... not entirely... but mostly. 

See ya Wed AM!  Until then, take care!

Meteorologist  John Ensworth

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