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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: 12/02/2003 11:15 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 necessary server space.

In short:  After some snow flurries or even a dusting of snow today, temperatures will continue to stay very chilly with a slow recovery as the week goes on.  We will also remain dry and quite until a possible BIG storm for the Weekend (Fri PM/Sat/Sun). 

 In long: We have seen a fair amount of water hit over the last week - around 2 inches in the Philadelphia area. But now it's time for the cold to come.  The good news is wet soil is harder to cool than dry soil.   If and when snow starts, it will have a tough time sticking and making roads very hazardous. 

Looking at the visible satellite view, you can see clouds sweeping across the state.  They look thin and unimpressive, but they may make today's weather interesting!  Notice how the skies are clear over some of the Great Lakes to our northwest.  That Lake Effect trick is working again (see yesterday's forecast discussion for more on that). 

To back up that lake effect claim, I present to you the winds.  They are still flowing briskly out of the northwest with winds around Philadelphia singing along at 20mph with wind gusts to 30mph!!

This is caused by the trough visible at the surface on the surface map below.  This is a 'quasi' front that is making the pressure gradient tighten up (remember, when the white lines are closer together on the surface map, the pressure is changing faster over a smaller distance which makes the wind blow harder). This trough is also making air collide at the surface. Since colliding air can't push into the ground, it must get lifted into the sky, helping clouds and snow (or rain) to form. 

So, on the radar, we see light showers popping up all over the place. 

If we take into account the temperatures at the surface and aloft in a computer program, we can color the snow blue, and the mix of snow and rain as pink (and, if it were on the map, rain would be green). 

Because of the cold temperatures, the trough, and the lake effect snow bands drifting towards us, the National Weather Service has issued the following special weather statement for a dusting of snow in the city. 

SPECIAL WEATHER STATEMENT NATIONAL WEATHER SERVICE
 MOUNT HOLLY NJ 1138 AM EST TUE DEC 2 2003 ...
SCATTERED SNOW SHOWERS WILL BE SPREADING THROUGHOUT EASTERN PENNSYLVANIA...CENTRAL AND NORTHERN NEW JERSEY...NORTHERN DELAWARE AND EXTREME NORTHEAST MARYLAND... AN ARCTIC COLD FRONT DROPPING SOUTH THROUGH THE REGION HAS BEEN ACCOMPANIED BY SNOW SHOWERS. ISOLATED AREAS IN NORTHERN NEW JERSEY AND NORTHEAST PENNSYLVANIA HAVE HAD AN INCH OF SNOW. HOWEVER...MOST AREAS HAVE ONLY RECEIVED A DUSTING OF SNOW OR FLURRIES. TEMPERATURES WILL DROP TO NEAR FREEZING AS THE SNOW SHOWERS SPREAD THROUGH THE PHILADELPHIA METROPOLITAN AND NEARBY AREAS. A COATING OF SNOW COULD OCCUR...ESPECIALLY ON BRIDGES AND OVERPASSES. THIS WOULD MAKE ROADWAYS SLIPPERY AND MOTORISTS SHOULD EXERCISE CAUTION. TO THE SOUTH OF PHILADELPHIA...THE SNOW SHOWERS SHOULD DIMINISH TO SCATTERED FLURRIES. STAY TUNED TO NOAA WEATHER RADIO...YOUR LOCAL RADIO OR TELEVISION STATIONS...OR YOUR CABLE TELEVISION OUTLET FOR FURTHER DETAILS OR UPDATES.

The upper air pattern (this map shows the winds at about 18,000feet above sea level as they blow mainly from west to east across the US parallel to the dark black lines) shows a trough pulling out and an Overall Western Ridge dominating the weather.  On the right side of a Ridge, we would expect clear weather, and our snow flurries are more of a local effect than a major weather event. The Trough Under the Ridge (marked below) in the southern Rockies will be part our weekend weather maker. 

So for 7am Wednesday, we see Lake effect snow/rain continuing out west as the High center moves towards us.  With air moving clockwise around a High center, we STILL have the necessary NW flow over the lakes. 

With the High coming, the center of the coldest air is right with it (the cold air IS dense and is seen as High pressure on the map)- we drop to the lower 20's upper 10's in Philadelphia by morning.  Wow, this IS cold air!

Wednesday night 7pm, the High begins to pass us and winds get light, greatly diminishing the lake effect precipitation  (but not eliminating it yet, the flow is still out of the northwest up there, but it won't effect Philadelphia anymore). 

Now we jump way to the end of the week as the next storm sets up.  This *MIGHT* be a very big storm over the weekend with POWERFUL winds, bitter wind chills, and maybe a SIGNIFICANT snow storm / blizzard for central Pennsylvania extending into  Philadelphia by the end of the storm. 

A mix of snow and rain begins to approach Philadelphia by Thursday night 7pm. 

Friday morning, 7am, and precipitation is gathering.  You will see a Low that forms over the Great Plains (with the trough under the ridge) heading towards the east coast as a tropical low (read juicy with moisture) moves up east of Florida to join with it. 

The upper air pattern (at 18,000feet above sea level again) shows a BIG trough rolling up over the Appalachians by Friday night 7pm. 

Friday night,7pm's weather map shows this big low off the Virginia coast.  This low can tap a LOT of moisture from the Ocean and the tropics and pump it to the west. The black lines are lines of equal pressure.  With the High north of Maine and the Low to the south, we have the lines bunching up over Philadelphia. This means powerful onshore winds will begin to form.  The moisture lifted inland into and over the very cold surface air means snows and blizzard conditions (at least for central Pennsylvania). 

The model (ETA model) run for this time shows heavy precipitation wrapping around the low into our state for Friday night. 

So the weather conditions show snow falling over most of the state (we are just on the snow/rain border due to our low elevation and proximity with the coast). 

Then the cold air moves eastward (getting pulled into the eastward moving low) by Saturday night, 7pm.  We change to snow at this time. 

The GFS model shows, by Sunday 7am, the big trough rolling off the east coast. 

And temperatures (minimum) for Sunday morning are right near 32F in the city.  Rain on the coast, snow inland. 

By 1pm Sunday, snow will still be falling in or near Philadelphia. 

How much snow will actually fall on Saturday?  It's too hard to call this far out.  Check back as the week continues!

Stay warm and get some early Christmas shopping done.  I'll see you again tomorrow morning!

Meteorologist  John Ensworth

Surface Station sky cover color key: