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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: 11/24/2003 10:08 AM
 

Good Morning Philadelphia!   

    I'm glad to announce that these discussions will continue until at least the end of this Year!.  I have the needed server space.

In short:   Rain tonight followed by a 20 degree drop in temperature.  brrr!

It won't be the shocker that the central and western parts of the US experienced, but a good cold front is headed our way right now. 

The visible satellite view show clouds increasing over southeastern Pennsylvania and a solid bank of clouds approaching from the west. The red arrow shows the direction of air flow at cloud level...note the small 'lanes' in the clouds. They are orientated perpendicular to the air flow due to wind shear.  Air below and air above this level is moving at different speeds, so rolls form in the atmosphere and where the air rises on the 'uphill' side of the roll, a cloud forms. Then the air rolls down, then back up again...repeat.  Neat. No?

Much of the cloudiness around the city is actually on the ground...Fog!  This is prompting a dense fog advisory this morning for Philadelphia. 

The next front, kicking off all these clouds, is just reaching the western side of Pennsylvania. The rain band is narrow and not particularly heavy at present.  Some frozen precipitation is falling as far south as Ohio in a narrow band. 

The main feature with this front is the gigantic pool of very cold air behind it.  Temperatures in North Dakota are only reaching the -0's so far this morning! Freezing temperatures are found in most states from the west coast to Texas to Ohio. We are in a tongue of warm air still flowing up from the south.  We are warmer than Phoenix and Houston and LA.  Wow. 

So we don't have much of a wind chill over the area with relatively warm air and light winds (enabling fog to form) but there are wind chills out west of the lakes that are in the -20F zone.  Oh, cold. 

The cold air is associated with the big trough just west of the Great Lakes and stretching down to the Gulf of Mexico (I didn't label it because it is obvious!).   This map shows how the air is moving at about 18,000feet above sea level parallel to the black lines from 'basically' west to east. 

So looking at this frontal passage in detail: 

By 1pm today, the front is a bit closer with most of the precipitation behind the frontal boundary itself.  With the air warming some, there won't be freezing precipitation this far south at that time. 

The 500mb map trough (the winds at 18,000feet) has rapidly swung to the east towards Philadelphia. 

Today's highs will still look and feel nice. We'll warm up to solid mid 60's.  Wow again. 

But the chance of rain will increase rapidly as the afternoon progresses.  Eventually a blast of cooler air will sweep by with a wall of gray rain sweeping in from the west. 

By 1am tonight, the front will be past us and temperatures will be dropping to about 35F (so no freezing stuff expected).  A cold rain will continue into the early morning hours. 

By dawn Tuesday (7am), the rain will be continuing with chilly, but above freezing temperatures. Just expect a nasty morning tomorrow. 

Lows this morning will be in the mid 30's near the Ocean and as low as the 20's out west (the higher elevations of central and western Pennsylvania will see snow.  You won't have to drive far to go play in it!).

At this same time, Tuesday morning, the chance of rain is high (80%) but beginning to end from the west. 

Tuesday night, 7pm, the upper level trough is already GONE toward Greenland, and a new broader trough is dominating the central US. 

The center of the cold high pressure system slides over the city and the rain is gone (again by Tuesday 7pm). 

Highs on Tuesday will be a bit of a shock after 60's today. We will hover in the lower 40's tomorrow. 

And the chance of rain is gone. 

With clear skies, light winds, and the center of all that cold air out west moving past us, we will see upper 20's and another hard freeze. 

But the temperature recovery (with the trough gone) will start immediately on Wednesday with highs slinking up to or getting above 50F again. 

I'll see you again Tuesday night and we'll look at who got snow out west and how much rain we received...and how cold it will get Wed AM. 

Find the rain coat (what? It's always by the door!), and umbrella (this is the fifth one I've bought this year!) and stay warm and dry.

Meteorologist  John Ensworth

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