FIRST WEEK OF DEC FOR PHILLY HAS BEEN INTERESTING..

The much anticipated ..BY SOME.. Winter of 2010/2011 season is upon us as we round out the last few days in November.  December 1st marks Meteorological Winter to us weather folk while the official start to winter waits 22 days later.  December 1st through the 5th time period has been an interesting week going back in history. In 2002 7.0" of snow fell at PHL for one of the earliest snowstorms producing 4" or more.  This storm now ranks as producing the most snow for so early in the meteorological Winter season, beating out Nov 27th, 1949 and 1938, which had 6.9" of snowfall.  In 1957 7.0" of snow fell, beginning on the 3rd and ending on the 4th, for one of the earliest  snowstorms producing 4" or more.  Finally, In 1960...
14.6" of snow fell at PHL, beginning on the 11th and ending on the 12th, for one of the
earliest snowstorms producing 4" or more, and the greatest Fall storm in PHL records.  


So The idea is there that storms can be formed and enough cold could become available to produce such snow events!


So where do we sit today??!! and .. Where do we sit come the first week of December.


I have always been a believer that this early is the game.. The Pacific Ocean holds the key. 
For this I turn to what is known as the Pacific North American Oscillation or short for "PNA" .  PNA is a climatological term for a large-scale weather pattern with two modes, denoted positive and negative, and which relates the atmospheric circulation pattern over the North Pacific Ocean with the one over the North American continent.  Essentially we use this signal as a "balancer" and like Algebra or Calculus.. What you do to one side you must do to the other.  So to cite an example; You have a ridge over the west... You will have a trough over the east... and Vice Versa.  


The PNA has been very negitive according to the chart below which denotes more troughiness over the western states.  In fact we had a series of major winter storms over the southern plains, pacific northwest and the western slopes of the rockies.  The cold air itself has been bottled up over the west while the east has endured cool and at times mild readings.  

2 long range models I use to predict such PNA fluctuations in addition to ensemble members from NCEP confirm the idea of the next 365 hours or 8-10 days of continued storminess over the west with cross polar flows situated out west.

HOWEVER... THERE IS AN EXCEPTION TO THIS RULE!
WHILE I DO USE THE PNA TO TEST WINTER EVENTS.. I ALSO LOOK AT 2 OTHER SIGNALS THAT COULD SUPPORT THE COLD AIR SHOULD A STORM LOCK ITS GRIP WITH THE SOUTHERN JET STREAM.  THESE TWO SIGNALS ARE THE NAO AND AO.  
NAO= NORTH ATLANTIC OCILLATION 
AO= ARCTIC OSCILLATION.   

THE NAO index is used on a NEG vs POS scale like the PNA scale is.  However, instead of testing pressure's on the left coast.. It uses pressure readings on the north Atlantic... Hence North Atlantic Oscillation.   When strong ridging (or High Pressure) develops over the southern tip of Greenland, A block develops (Negative NAO) which slows the southern and northern jet stream allowing for both to phase thus allowing for the slowing of storms as they move up the eastern seaboard.  Since storms look to gain latitude due to the Coriolis forces this type of BLOCK would allow for the re-curvature to occur much faster and much closer to the coast. Secondly, on the backside of this block you gain a strong North Westerly flow which is a "Cross Polar Flow" bringing super cold air into the eastern US.  
The Arctic Oscillation is a little more complex and explaining it will probably confuse you all.  Just remember this.. A Negitive AO helps to lock such NAO and even PNA patterns in place that favors storms.  
Last year for example we had a HISTORIC negative AO signal which reached below a -5 on the scale.  Below is where we are today with the AO.. and NOTE how it is trying to reach such severely negative levels.  


POSTIVE AO = BLOW TORCH (WARM WEATHER)
NEGITIVE AO= COLD BLASTS (COLD WEATHER)
NEUTRAL =    PNA AND NAO BECOME BIG DECIDING FACTORS IN WHICH WAY TO GO.

 




SO HERE IS THE MASCO EQUATION OF THE DAY...

NEGATIVE NAO + NEGATIVE AO = SNOW & COLD

HOWEVER, 

NEGATIVE NAO + NAGATIVE AO + POSITIVE PNA = ICE/SLEET/ELEVATION SNOW....

PRETTY BIG DIFFERENCE WHEN YOU FACTOR IN THE PACIFIC! 

SO WHAT KIND OF PATTERN COULD WE BE SEEING ENTERING INTO THE FIRST WEEK OF DECEMBER????

HERE IT IS.. THE DECEMBER EXPRESS!  A STORM TRYING TO RIDE THE SOUTHERN JET STREAM WITH ENOUGH ELEVATION COLD AIR TO GIVE A WIDESPREAD SNOW EVENT FOR WESTERN VA, WV, WESTERN MD, POCONO'S, LEHIGH VALLEY (ICE) , BERKSHIRE MTNS, GREEN MTNS, AND WHITE MTNS.  FUTURE MODEL RUNS WILL CONFIRM OR DENY SUCH STORMS.. BUT IF HISTORY DICTATES THE FUTURE .. THINGS COULD GET MIGHTY INTERESTING!!! 

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