PROGNOSTIC DISCUSSION FOR MONTHLY OUTLOOK
NWS CLIMATE PREDICTION CENTER CAMP SPRINGS MD
300 PM EST MON NOV 30 2009

30-DAY OUTLOOK DISCUSSION FOR DECEMBER 2009

THIS UPDATED OUTLOOK FOR DECEMBER COMBINES INFORMATION FROM WEATHER FORECASTS 
OUT TO 14 DAYS, WITH EXPECTED EARLY WINTER CLIMATIC IMPACTS ASSOCIATED WITH EL 
NINO.  THE LONG WAVE UPPER LEVEL CIRCULATION PATTERN PREDICTED BY THE GFS, 
EUROPEAN CENTRE (ECMWF) AND THE CANADIAN MEDIUM RANGE WEATHER FORECAST MODELS 
INDICATE A NEGATIVE ARCTIC OSCILLATION (AO), A NEGATIVE NORTH ATLANTIC 
OSCILLATION (NAO) AND A POSITIVE PACIFIC-NORTH AMERICAN OSCILLATION (PNA) 
PATTERN.  THESE FEATURES ARE ASSOCIATED WITH BELOW NORMAL TEMPERATURES 
THROUGHOUT MUCH OF THE CENTRAL AND EASTERN PORTIONS OF THE CONUS.  THIS IS 
EXPECTED TO OVERWHELM THE USUAL ENSO TEMPERATURE TELECONNECTION PATTERN FAVORED 
IN THE EARLY PART OF AN EL NINO WINTER, AND THEREFORE THE TEMPERATURE OUTLOOK 
IS SUBSTANTIALLY ALTERED FROM THE OUTLOOK ISSUED IN MID-NOVEMBER.  ABOVE NORMAL 
PRECIPITATION AMOUNTS ARE FAVORED IN THE EARLY PART OF DECEMBER FOR MOST OF THE 
SOUTHEASTERN CONUS, INCREASING THE CONFIDENCE FOR ELEVATED CHANCES OF ABOVE 
MEDIAN PRECIPITATION FROM THE EARLIER OUTLOOK WHICH WAS BASED ONLY ON THE EL 
NINO TELECONNECTION.     
  
THE EL NINO CONDITIONS HAVE MAINTAINED THEIR STRENGTH INTO LATE NOVEMBER, AND 
FORECASTS CONTINUE TO INDICATE THAT SEA SURFACE TEMPERATURES (SSTS) IN THE 
PACIFIC OCEAN WILL REMAIN WELL ABOVE AVERAGE FOR DECEMBER.  SSTS ARE CURRENTLY 
MORE THAN 1.5 DEG. C ABOVE NORMAL ALONG THE EQUATOR FROM 170 E TO 130 W 
LONGITUDE, WITH ANOMALIES CLOSE TO 2 DEG. C  JUST EAST OF THE DATE LINE.  
SUB-SURFACE OCEAN TEMPERATURES IN THE EASTERN EQUATORIAL PACIFIC OCEAN ARE WELL 
ABOVE NORMAL.  THE AVERAGE HEAT CONTENT OF THE UPPER 300 METERS HAS BEEN STEADY 
IN RECENT MONTHS AFTER HAVING RETREATED SLIGHTLY FROM THEIR PEAK VALUES IN LATE 
SUMMER.  ATMOSPHERIC CONDITIONS OVER THE TROPICAL PACIFIC ARE CONSISTENT WITH 
EL NINO, WITH LOW LEVEL WESTERLY WIND ANOMALIES AND UPPER LEVEL EASTERLY WIND 
ANOMALIES OBSERVED IN OCTOBER AND EARLY NOVEMBER.  THESE INDICATORS ARE 
SUGGESTIVE OF A MODERATE EL NINO EVENT.  MOST MODELS PREDICT THAT THE EL NINO 
INTENSITY WILL REMAIN MODERATE THROUGH DECEMBER.  

THE MADDEN-JULIAN OSCILLATION (MJO) HAS WEAKENED SINCE MID-NOVEMBER BUT IS 
EXPECTED TO HAVE SOME INFLUENCE ON THE CIRCULATION OVER NORTH AMERICA IN EARLY 
TO MID-DECEMBER.   

THE TEMPERATURE OUTLOOK FOR DECEMBER IS MOSTLY BASED ON THE CFS MODEL FORECAST 
AND CALLS FOR ELEVATED CHANCES OF BELOW NORMAL TEMPERATURES IN MUCH OF THE 
EASTERN AND CENTRAL PORTIONS OF THE COUNTRY.  THE CFS FORECAST FACTORS BOTH THE 
SHORT-TERM CIRCULATION PATTERNS AND EL NINO TELECONNECTIONS IN ITS FORECAST AND 
AGREES WELL WITH ANALOGS BASED ON NEGATIVE AO, NAO, AND POSTIVE PNA PATTERNS 
MODERATED BY EL NINO TELECONNECTIONS.  INCREASED CHANCES OF ABOVE NORMAL 
TEMPERATURES ARE INDICATED FOR THE PORTIONS OF THE SOUTHWESTERN U.S. AND FOR 
MOST OF ALASKA.  

THE PRECIPITATION FORECAST IS CONSISTENT WITH EL NINO COMPOSITES FOR THE 
SOUTHEASTERN CONUS.  THE CONFIDENCE IN THE FORECAST FOR ABOVE MEDIAN 
PRECIPITATION AMOUNTS FOR THE GULF COAST AND THE SOUTHEAST IS INCREASED BY 
SHORT TERM FORECASTS OF SUBSTANTIAL PRECIPITATION ASSOCIATED WITH A STORM 
SYSTEM EXPECTED IN THE FIRST WEEK OF DECEMBER.  SHORT TERM FORECASTS INDICATE 
ENOUGH PRECIPITATION IN THE EARLY PART OF THE MONTH TO NEGATE THE EL NINO 
RELATED DRY SIGNAL OVER THE NORTHERN ROCKY MOUNTAINS, SO THE AREA OF BELOW 
NORMAL PRECIPITATION INDICATED THERE ON THE EARLIER OUTLOOK FOR DECEMBER HAS 
BEEN REMOVED.  EL NINO USUALLY INCREASES THE CHANCES OF ABOVE MEDIAN 
PRECIPITATION AMOUNTS FOR SOUTHERN CALIFORNIA, AND MJO ACTIVITY FURTHER 
INCREASES CHANCES FOR ENHANCED PRECIPITATION AMOUNTS.  HOWEVER THE AREA OF 
EXPECTED ABOVE MEDIAN PRECIPITATION CHANCES HAS BEEN REDUCED SOMEWHAT FROM THE 
EARLIER OUTLOOK DUE TO INDICATIONS OF DRY CONDITIONS BY THE CFS.  A STRONG 
UPPER LEVEL RIDGE PREDICTED TO BE OVER ALASKA IN EARLY DECEMBER IS EXPECTED TO 
BRING RELATIVELY DRY CONDITIONS ALONG THE ALASKAN PANHANDLE IN THE EARLY PART 
OF THE MONTH, RESULTING IN A REVISION TO EQUAL CHANCES FOR ABOVE, NEAR, OR 
BELOW MEDIAN PRECIPITATION AMOUNTS.  ENHANCED STORMINESS PREDICTED FOR EARLY 
DECEMBER RESULTS IN ELEVATED CHANCES FOR ABOVE MEDIAN PRECIPITATION FOR 
PORTIONS OF WESTERN ALASKA.  
 

FORECASTER: DAVID UNGER

NOTES: AREAS OF UNCERTAINTY THAT FORMERLY WERE LABELED CL (CLIMATOLOGICAL 
PROBABILITIES) ARE NOW LABELED AS EC (EQUAL CHANCES).

THE NEXT MONTHLY OUTLOOK...FOR JANUARY 2010...WILL BE ISSUED ON THU DEC 17 2009

THESE OUTLOOKS ARE BASED ON DEPARTURES FROM THE 1971-2000 BASE PERIOD.

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