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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: 09/15/2003 09:41 AM
 

Good Afternoon Philadelphia!   

    The rains are moving on, but the scenario currently is shown in the graphic below. With the high that moved over us earlier this week now sitting off to the north east and the circulation and moisture remains from T.S. Henri slipping towards, the circulation around these two centers feeds strong east winds over Philadelphia. 

The low center of Henri came onshore in Virginia (since it is for lovers...is Henri waiting for Isabel? oh, more on that later). 

Meanwhile, the central plains front is picking up some strength and is quite dumping a lot of water out there. It will get here eventually.

With the low to the south, we have east winds (meteorologists name winds by where the winds come FROM) across the state (and quite brisk, I may add). 

The general shield of rainfall has moved north and dissipated somewhat.  There are a few stronger shower/thunderstorms near the city, but little else around the region. 

Looking at the rainfall that has fallen, it is easy to see where the remains of Henri came onshore and moved north-northwest. 

Nearer to home, there has been widespread light rain and some heavier accumulations to our west (up to 1 to 1.5"). 

The satellite view shows an almost completely solid bank of clouds over the whole state (inland). The thick rich moisture filled tropical air of Henri easily cools as it is lifted up onto the land and makes these low clouds.  You can see a couple of dimples in the cloud fiend in central and northeast PA.  The sun is starting to get low in the west and the taller clouds have shadows on them on the east side. 

Comparing the infrared satellite view to the visible satellite picture above, you can see the taller clouds reach up to where the temperatures are the lowest.  The colder temperatures are colored blue and the warmer temperatures (usually the bare ground) are in deep red. 

But now lets look into the very serious future.  This is a visible satellite view of the US, Gulf and western Atlantic.  You can easily see the monster hurricane Isabel churning out there.  (The stars are added for effect and are not real). 

Hurricane Isabel has been a category 5 storm (the maximum on the Safford Simpson scale).  It is now just below that cat 5 level with winds at 150mph.  This is an incredibly dangerous storm! On this map, the great folks at www.accuweather.com have added the notes for the remains of Henri, and two tropical waves further in the south. 

The steering currents for this storm are found at the 500mb level (seen in the numerous discussions before).  A big ridge to the east and a deepening trough (shown) to our west should pull the storm northward pretty quickly once it approaches the US east coast. 

The www.accuweather.com projection brings the storm in anywhere from the Florida coast to (hopefully) recurving it out at sea to leave the US alone.  The most likely hit, though, looks like the Carolinas or Virginia (or northward). 

Another representation of this forecast shows it threatening the N.Carolina coast by Thursday this upcoming week.  The bad news is, if it hits anywhere to the south of Pennsylvania, the moisture will flood over us with copious amounts of rain.  And if the hit is closer, we could get damaging winds even this far inland.  This is something you MUST watch this week. 

Saturday morning (as we follow the GFS out a week into the future), we see the remains of Henri still showering over us. 

The upper level pattern shows that ridge to our east and the trough to the west.  The mini-trough to our south is the upper representation of Henri. 

And there is still some chance of showers around Sunday AM.

Monday AM and the Henri trough is still to our south... the trough out west is moving a bit closer. 

Because of the lingering trough (you often see showers and rising air making clouds on the right side of a trough in the northern hemisphere) we still have some chance of rain Monday AM.   Isabel is seen entering the frame down to the distant southeast.

Tuesday AM and the trough approaching us from the west is here but weakening.  The ridge to our east is holding very strong. The circular flow inside Isabel is visible to down in the Atlantic. 

We have the front passing with the arrival of the weakening trough and more rain (it has all that Henri moisture to work on!). 

Wed AM and the trough stalled over us is weakening further,  while a new trough begins to dig out over the Great Plains. Here comes Isabel east of Florida... very easy to see!

With the trough stalled, the rain and stalled front persists. Isabel is a big area of heavy rainfall down in the Atlantic. 

The trough over us is being replaced (sadly) by the strong ridge to the east.  The central US trough digs deeper and Isabel begins to turn to the north towards us. 

Amazing amounts of water are falling in and around Isabel with still some chance of showers (mainly due to daytime heating) and all the new moisture pushing into the area as the tropics are connected with us again. 

Friday AM.  Now comes the crunch time.  Isabel is now moving very close to the coast.  The central US trough digs and begins to move towards us as the ridge gets stronger to the east.  The hurricane will slide in between them (to eventually get caught by the trough and shuttled off to the northeast).  If the trough gets stronger faster and moves in closer earlier, it might do that out at sea and save us.... if it stays weaker or stays out west, then the southern US coast will get hit.  

Here we have a front too far to our west to save us from Isabel, and Isabel moving in. 

Friday night (according to this GFS run) hits the N. Carolina/Virginia coast. It's future then is up towards us just inland.  We would get A LOT of wind and torrential rainfall and horrendous flooding.  This is a BAD forecast for Philadelphia. But it IS a week away.  A LOT can change in that time. 

Here you see, Friday Night, the northern edge of the hurricane actually moving over Philadelphia. 

I'll see YOU Monday for more model/forecasting fun.

Meteorologist  John Ensworth

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