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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: 01/17/2004 08:27 PM
 

Good Afternoon Philadelphia!   

    I'm glad to announce that these discussions will continue until at least the end of January! 
    I have the necessary server space (especially with missed days!).  

In short:  The worst of the cold is still moving on, being replaced by a limited warmup, but, after a period of snow, more cold will pour in. 

 In long: We'll start this time with the winter weather coded radar.  Using atmospheric temperatures (surface as well as aloft) we can see that the next snow event is well on its way. 

The satellite picture this afternoon shows snow to the right of the ragged red 'clearing' line, and clouds to the left. You can see the snow on the ground wherever you see lighter grays and 'whites'. 

In this case, the infrared view, showing surface or cloud top temperatures, is not as helpful telling clouds from snow since the very thin cirrus still obscures the state across Pennsylvania and into New Jersey. 

This snow report map is more believable. There actually is a bit of snow in all of Pennsylvania this time.  I can accept this. 

The upper air picture (the 500mb map show how winds blow mainly from west to east parallel to the dark black lines at about 18,000feet) shows the center of the cold polar air vanishing off the right side of the image and another bubble of cold air swinging out of the Canadian north. The precipitation and moisture moving in, though, is coming up from the south due to the trough visible over Arizona, New Mexico and Texas.

The surface map shows both upcoming features in different ways.  The system in the south has a Low over SW Oklahoma with rain and snow blanketing much of the Mississippi and Ohio Valleys.  The very cold air with the next polar piece is visible as a front sweeping into North Dakota (primarily). 

Ahead of that SW trough, winds are picking up from the south over the entire region in the red circle (almost into Pennsylvania). 

And the dew point map (showing an expression of actual water vapor in the atmosphere - higher dew points = more moisture) shows moisture flooding northward out of the Gulf of Mexico feeding all this rain and snow.  This is moving east with a warm-up coming with it. 

Looking into the near future, the interesting feature of the upper air flow is the merging of the southern jet stream trough with the new Canadian (northern Jet Stream) trough. I've marked the two troughs in red.  This map is for Sunday 7am. 

Sunday at 1am has snow falling over the city and a warm front brining a brief relief from the cold in. (It's those south winds I told you about). 

The lows Sunday morning will only be 27 F.  About 20F warmer than a few nights ago!

Sunday 7am, and we may actually see a period of rain as temperatures rise above freezing. Woo hoo!

The highs Sunday will climb to an incredible 40F!  Nice nice nice!

By Sunday night 7pm, the two troughs are merged but tilted southwest to northeast (called a positive tilt by meteorologists and not known for very severe weather). 

At that time, the cold front sweeps in and we may change back to snow overnight Sunday into Monday. 

Lows Monday morning return to the low 20'sF as the next piece of polar swings in. 

For Monday night, 7pm, there is just one big positively tilted trough over the east coast as that polar piece of cold air swings out to sea...

And with clearing skies and north winds, we drop to the middle teens for Tuesday morning. 

Wednesday morning looks the same... mid-teens for lows. 

And the same for Thursday morning. Darn north winds!

More on the snow tomorrow. See ya then!

Meteorologist  John Ensworth

Surface Station sky cover color key: