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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 02:33 PM
 

Good Morning Philadelphia!  

    Hey! We have our electricity on still.  Whoopee!

Anyway, to the weather...it's a good thing because the heat is here.  Under very clear skies (as seen in the visible satellite picture below) we are getting quite warm. 

There is no organized surface feature marked on the surface map in our area (just a few patches of vanishing valley fog elsewhere in the state). The dominating story is the big High parked over the Ohio Valley.  (and well, the tropical storm in the Gulf of Mexico, but that won't matter to us). 

Winds are mainly out of the west over the top of the High pressure center. The green lines I've added show the flow of moisture towards us. 

The dew point map is a map of the moisture in the air. The higher the dew point, the more actual water vapor there is in air.  We are getting our moisture from the Gulf of Mexico, across many states before it gets to us. 

The temperatures are toasty even at 10am, but they are extra toasty here on the coastal plains. That 'offshore' wind from above, is sinking off the higher terrain to the west again. Sinking air compresses and warms. So we are a bit warmer and as humid as Pittsburg and other locals to our west.  (Good thing the air conditioners are on!). 

The US is, for the moment, under the influence of a HUGE ridge bringing hotter than normal temperatures and dry weather over much of the nation.  The east coast is no different.

This map shows the path of air at 500mb pressure (about 1/2 sea level pressure) or at about 18,000 feet above sea level. The winds blow parallel to the dark lines roughly from west to east at this range of latitudes. 

Here we go with the glimpse of the future.  Yesterday the question was if the GFS model was right about what it did with the east coast trough in the near future.  It has backed off, but the other models slanted a bit toward it's more westward trough position than they originally forecasted.   A skewed (non GFS) solution seems to be favored now. 

The ridge, by Saturday early morning, is growing in the west and central part of the US giving us more troughiness overhead (but still quite warm). 

This ridging west, troughing east starts a front moving down the plains. 

Later Saturday morning, we - again- seem to be being placed under the old standby - the east coast trough. 

And the cold front is rapidly approaching  - increasing our chance of showers ahead of the trough. 

Saturday night and the trough has noticeably deepened. 

And rain and chances of rain hit everyone from Ohio to the coast and up into New England. 

Monday morning and the western US is under a Mega ridge.  We are under a quite deep trough again.  We'll be warm, but cooler air than most of the nation. 

The front is already gone (unlike previous fronts) off to the east.   This won't be a prolonged rain event. It will be fairly clear and a bit drier after the front passes... it would be a good night to take out the telescopes and look at Mars. 

By Tuesday night, the trough is already weakening and pulling away to the north as a new trough begins to bud way out west. This is a transient big trough this time, not a big permanent feature.

And Tuesday continues dry and nice with the front stalled to our south. 

That's all for today, I'll see you again here tomorrow.  Until then, keep looking up and stay cool!

Meteorologist  John Ensworth

Surface Station sky cover color key: