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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: 10/21/2003 09:42 AM
 

Good Morning Philadelphia!   

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

A FANTASTIC week continues...

We start off this morning with clouds on the increase.  The skies over Philadelphia are mostly cloudy, but the clouds aren't THAT thick or continuous across the state.  You can see, just to the northwest of the lakes, a second layer of small puffy clouds over a lower continuous deck of clouds near ground level.  Can you see the shadows they cast on the lower clouds?  The even lanes of clouds in central Pennsylvania are perpendicular to the wind flow - which is from the west-northwest. 

The nearby Low pressure system stayed to the north (it was forecasted on Saturday to head more toward Philadelphia), but it's front is swinging in. 

You can see the white lines on the map above getting closer together around the low.  These are lines of equal pressure.  The closer they get the greater the change in pressure over distance occurs.  This makes the wind blow stronger. 

On the surface wind map, you can see these strong winds sweeping up out of the west southwest at ground level.  The winds are northwest behind the front.

The southwest winds are, briefly, bringing in warm air (upper 60's and lower 70's F) from that direction...

AND moisture.  We now have mid 50 dew points in place, which is pretty moist!

All this is happening because the winds at about 18,000 feet above sea level are changing direction and a new trough is deepening over our heads.  We are on the 'uphill' or right side of the trough again, so we are in the area where storms and clouds more easily form. 

Looking into the future, the front makes it to the city by 2pm today with some showers breaking out along it (but not a great mass of rain, just scattered showers). 

By 2am Wednesday, the front is off at sea, but another ripple of low pressure (a surface trough) sweeps out of Canada with a slightly cooler shot of air coming in behind it.

By 8am Wednesday, more showers are breaking out around the lakes out west (this may be some lake effect rain). 

Then, for Wednesday 8pm, we have a continued chance of showers as that trough sweeps through looking like a front that isn't marked as a front. 

At this time, the bottom of the trough is just to our east again.  It is harder for showers to hit us Wed 8pm than it was this afternoon with this change in the upper air flow. 

Then, taking a quick glance into the beginning of the weekend (Friday 8pm) we are under a small ridge and should be warming up nicely (before the next front sweeps through early next week). 

Stay dry and warm!  I'll see you again tomorrow morning.

Meteorologist  John Ensworth

Surface Station sky cover color key: