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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: 02/09/2004 09:25 AM
 

Good Afternoon Philadelphia!   

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

In short: Still nice (for winter) and nice extending into the future.  Wow, what a break.  The central and southwest US will have all the winter fun for a while.   

In the Long: The surface map shows a high pressure system sliding off the coast with south winds on the west side of its clockwise air flow. These south winds bring in warmer southern air.  Nice! The cold front moving out of the central US doesn't have much moisture with it (the last system scoured the nation pretty well) and not much lift with it (you'll see the trough in the upper air is tilted the wrong way to really force air to raise and make clouds and rain). 

The visible satellite view shows what looks like wispy clouds (cirrus) overhead but some of the 'knurled' looking white forms below may actually be snow on the ridge lines in central and western Pennsylvania.   Let's look at the infrared satellite view to separate out the cloud types (and ground views). 

The wispy clouds ARE very cold (colored blue, warm surfaces are colored red - i.e. the ocean out east) so they are cirrus, and the knurled ground is visible through weak patches in the cirrus.  Yep, we're seeing the snow out there.

The 500mb map (showing winds as they blow parallel to the dark lines mainly west to east across the US at about 18,000feet where the atmospheric pressure is 1/2 what it is at sea level) shows a giant trough tilted in a "positive" direction over the nation.  The "positive" tilt comes from looking at the tilt of the words that are drawing a line up the bottom of the trough. If this were a line on a simple X-Y plot, then the rise and run of the line would both be positive, so we say it has a positive slope.   A positively tilted trough doesn't focus the divergence (spreading apart) of the air above, so the lift that makes clouds and rain/snow is much weaker.  You can see the result of this in the surface map.  The front and nation is quite quiet right now. 

So let's look at the beginning of our nice weather. The Lows Tuesday morning will only drop to freezing in Philadelphia (how nice is that after January/February 2004?!

As we drop only to freezing tonight, the little bit of precipitation that may form will definitely be in the form of rain (if anything hits!). 

By Tuesday 4am, we have the slightest chance of sprinkles, but not much --- and I said sprinkles not flurries.

Tuesday morning the cold front is almost on our heads and the precipitation (as already shown) is spotty and meager.

Tuesday night we climb to the upper 40'sF - it's a HEAT WAVE!

The 500mb map Tuesday night shows the smaller trough pushing this cold front ahead  of it. (Or is it just the cold air behind the front making the trough... chicken and egg?).

Tuesday night's map shows the front off the coast already and a region of snow showers out west, but we stay in the dry weather. 

Wednesday's highs will still rise into the 40'sF. 

And that is because the trough out west remains out west and we continue to have a mild ridge over the east coast. 

There isn't much to say with such a mild forecast. Enjoy the peaceful weather.  

Meteorologist  John Ensworth

Surface Station sky cover color key: