ANOTHER METEOROLOGICAL WINTER HAS BEGUN. ITS TIME FOR OUR ANNUAL 
ANALOG EXERCISE OF COMPARING AUTUMN WEATHER AND ENSUING WINTER 
TEMPERATURES. UNLIKE THE TWO PREVIOUS WINTERS, THE EL NINO/SOUTHERN 
OSCILLATION (ENSO) INFLUENCE ON OUR WEATHER IS NOT EXPECTED TO BE AS 
DOMINATING. THE WINTER OF 2006-7 HAD A MODERATE EL NINO, THE WINTER 
OF 2007-8 HAD A STRONG LA NINA. THIS WINTER THE MOST LIKELY OUTCOME 
SHOULD BE EITHER ENSO NEUTRAL, (LA NADA), OR POSSIBLY A WEAK LA 
NINA. 

WHEN THE ENSO GETS STRONG IN EITHER DIRECTION, IT BECOMES THE 
DOMINATING PLAYER IN OUR WINTER WEATHER REGIME. THIS WINTER WITH 
ENSO RATHER MUTED, OTHER LESS PREDICTABLE OCEANIC INFLUENCES WILL 
PLAY A GREATER PART. NEITHER THE PACIFIC DECADAL OSCILLATION (PDO) 
OR THE NORTH ATLANTIC OSCILLATION (NAO) ARE AS WELL FORECAST. THE 
FORMER AS THE NAME IMPLIES VARIES VERY SLOWLY. IT IS CURRENTLY IN A 
NEGATIVE STATE WHICH MEANS RELATIVE TO NORMAL WARMER WATER IS IN THE 
WESTERN PACIFIC OCEAN. THIS NORMALLY WOULD FAVOR A NEGATIVE OR 
MILDER PACIFIC NORTH AMERICAN (PNA) TELECONNECTION. BUT THE PDO WAS 
PREDOMINATELY NEGATIVE DURING THE 1960S WHICH WAS ONE OF THE COLDEST 
AND SNOWIEST DECADES FOR THE NORTHEAST. A NEGATIVE PDO IS NOT THE 
DEATH KNELL FOR SNOW. ONE OF OUR COOPERATIVE OBSERVERS STRATIFIED 
WINTER SNOWFALL IN PHILADELPHIA BASED ON PDO AND ENSO STATES. 
WINTERS WITH A NEGATIVE PDO AND WEAK LA NINA OR LA NADA STATES 
TENDED TO HAVE AVERAGE OR NEAR NORMAL SNOWFALL. 

FOR THE OTHER OCEAN AND THE NORTH ATLANTIC OSCILLATION (NAO). UNLIKE 
THE PDO WHILE THE NAO MAY PREDOMINATE IN ONE POSITIVE OR NEGATIVE 
DIRECTION, IT RARELY STAYS THAT WAY FOR MORE THAN A FEW WEEKS AT A 
TIME. WHEN THE NAO IS NEGATIVE OUR WEATHER GETS COLDER AND OR 
STORMIER AS THE LAST HALF OF NOVEMBER WAS. WHEN IT IS POSITIVE, WE 
TEND TO BE WARMER AND LESS SNOWY LIKE LAST WINTER. THERE IS A VERY 
INTERESTING RESEARCH PAPER DONE BY DR LUKE OMAN FROM RUTGERS 
UNIVERSITY ABOUT THE RELATIONSHIP BETWEEN THE NAO AND SNOW IN 
PHILADELPHIA (OTHER PLACES IN THE NORTHEAST ALSO). THIS PAPER SHOWS 
A 44 PERCENT INCREASE IN SNOWFALL OVER ABOUT A 50 YEAR PERIOD WHEN 
THE NAO WAS NEGATIVE VS WHEN IT WAS POSITIVE. IF WE EXPAND THAT 
DIFFERENCE TO 1 STANDARD DEVIATION IN BOTH DIRECTIONS, THAT 
DIFFERENTIAL INCREASES TO A WHOPPING 198 PERCENT.
 
WHEN THE LA NINA OR EL NINO IS STRONG, IT CAN OVERWHELM THE NAO AND 
REDUCE ITS INFLUENCE ON OUR WEATHER. BUT WITHOUT A STRONG ENSO, THE 
FATE OF THIS WINTER WILL BE INFLUENCED MORE BY THIS LESS PREDICTABLE 
PARAMETER. THERE ARE A COUPLE OF METEOROLOGICAL AGENCIES THAT 
FORECAST THE NAO FOR THE WINTER: THE UKMET MET OFFICE IS EXPECTING 
ON AVERAGE A SLIGHTLY POSITIVE NAO OVERALL, THE GERMAN MET OFFICE IS 
EXPECTING A NEGATIVE NAO FOR THE MONTH OF DECEMBER AND A POSITIVE 
NAO FOR THE MONTH OF JANUARY.
  
SO THIS TAKES US BACK TO WHAT HAVE NEAR NORMAL AUTUMN TEMPERATURES 
AND ABOUT NEUTRAL ENSO CONDITIONS PRODUCED IN THE PAST IN 
PHILADELPHIA. ALL OF THESE ANALOGS HAD AN OCTOBER AND NOVEMBER THAT 
FELL WITHIN THE NORMAL THIRD OR TERCILE OF AVERAGE TEMPERATURES 
ALONG WITH AN ENSO STATE THAT WAS EITHER NEUTRAL OR A WEAK LA NINA.  
ANOTHER MEASUREMENT OF ENSO WHICH RELIES ON OTHER METEOROLOGICAL 
PARAMETERS IN ADDITION TO SEA SURFACE TEMPERATURE ANOMALIES IS 
CALLED THE MULTIVARIATE ENSO INDEX (MEI). THE MEI IS CURRENTLY IN A 
WEAK LA NINA STATE. OTHER METEOROLOGICAL AGENCIES AND THE CLIMATE 
PREDICTION CENTER'S CLIMATE FORECAST SYSTEM MODEL ARE PREDOMINATELY 
FORECASTING EITHER A WEAK LA NINA OR ENSO NEUTRAL CONDITIONS FOR 
THIS WINTER.  THE ANALOGS MARKED WITH A *** HAD A STRONG LA NINA THE 
PREVIOUS WINTER AS WAS THE CASE IN THE WINTER OF 2007-8.  

HERE ARE THE LUCKY SEVEN OF WHAT OCCURRED IN PAST ENSO NEUTRAL OR 
WEAK NINA WINTERS THAT HAD AN AVERAGE OCTOBER AND NOVEMBER. THIS IS 
BASED ON THE 136 YEARS OF RECORD IN PHILADELPHIA.
  
 
YEAR           ENSUING   ENSUING   ENSUING  AVG    ENSUING ENSUING
               DEC       JAN       FEB      TEMP   PCPN    SNOW


1897-8         38.1      35.4      35.6     36.4   11.42   19.4    
1906-7         35.3      34.2      27.0     32.2    8.47   38.6
1921-2         35.6      31.2      36.7     34.5    7.88   28.0 
1926-7         32.3      32.6      40.6     35.2    9.04   11.8  
1956-7***      41.7      29.2      37.4     36.1    8.18    7.9
1983-4         33.2      26.2      38.7     32.7   12.40   21.6
2000-1***      31.3      32.5      37.4     33.7    8.63   26.1 
 
AVERAGE        35.4      31.6      36.2     34.4    9.43   21.9 
NORMAL         37.4      32.3      34.8     34.8    9.57   19.3

THE OFFICIAL CPC OUTLOOK FOR THIS WINTER IS FOR EQUAL CHANCES OF
IT BEING WARMER OR COOLER THAN NORMAL. THE PRECIPITATION OUTLOOK HAS 
EQUAL CHANCES OF BEING EITHER WETTER OR DRIER THAN NORMAL. 
ENSO NEUTRAL AND WEAK LA NINA WINTERS BRING BACK TO CLIMATOLOGICAL
NORMAL LEVELS THE CHANCES OF PHILADELPHIA HAVING EITHER A 6 INCH OR 
GREATER OR 10 INCH OR GREATER SINGLE SNOW EVENT THIS WINTER. 
SINCE THE WINTER OF 1949-50, THE CHANCES OF A 6 INCH OR GREATER 
SINGLE SNOW EVENT IN PHILADELPHIA IN ANY GIVEN WINTER IS ABOUT 75 
PERCENT, A 10 INCH OR GREATER SINGLE SNOW EVENT ABOUT 25 PERCENT. AN 
INTERESTING ARTICLE ABOUT ENSO CLIMATOLOGY AND LARGE SNOW EVENTS CAN 
BE FOUND IN THE OCTOBER 2003 ISSUE OF WEATHER AND FORECASTING.
  
AS ALWAYS PAST PERFORMANCE IS NOT ALWAYS AN INDICATION OF FUTURE 
TRENDS. ENJOY THE WINTER OF 2008-9, ITS THE ONLY ONE OF ITS KIND WE 
WILL HAVE.