PROGNOSTIC DISCUSSION FOR MONTHLY OUTLOOK
NWS CLIMATE PREDICTION CENTER CAMP SPRINGS MD
300PM EST TUESDAY NOV 30 2010

30-DAY OUTLOOK DISCUSSION FOR DECEMBER 2010

LA NINA CONDITIONS CONTINUE ACROSS THE TROPICAL PACIFIC OCEAN WITH THE 
INTENSITY ABOUT THE SAME AS OBSERVED WHEN THE MONTHLY OUTLOOK WAS ISSUED IN 
MID-NOVEMBER.  SEA SURFACE TEMPERATURES (SSTS) NEAR THE EQUATOR IN THE EASTERN 
AND CENTRAL PACIFIC OCEAN IN NOVEMBER ARE BETWEEN 1 AND 2 DEG. C BELOW NORMAL 
NEAR THE EQUATOR FROM THE SOUTH AMERICAN COAST TO ABOUT 160 E.  BELOW NORMAL 
SSTS EXTEND FROM THE EQUATOR TO 20N AND 20S LATITUDE THROUGHOUT THE EASTERN 
PACIFIC EAST OF AROUND 150W.  BELOW NORMAL OCEAN TEMPERATURES EXTEND FROM THE 
SURFACE TO OVER 200 M DEPTH ALONG THE EQUATOR EAST OF AROUND 160W, LEADING TO 
THE LOWEST UPPER OCEAN HEAT CONTENT VALUES SINCE LATE 1998.  LARGE SCALE 
ATMOSPHERIC CIRCULATION OVER THE TROPICAL PACIFIC OCEAN IS CONSISTENT WITH LA 
NINA AND SHOWS SUBSTANTIALLY REDUCED CONVECTIVE ACTIVITY OVER THE WESTERN AND 
CENTRAL EQUATORAL PACIFIC OCEAN AND ENHANCED CONVECTION OVER INDONESIA.  LOW 
LEVEL EASTERLY TRADE WINDS ARE ENHANCED AS ARE WESTERLY WINDS AT UPPER LEVELS 
OVER THE WEST AND CENTRAL PACIFIC.  THESE INDICATORS SUGGEST LA NINA 
CONDITIONS. 
 
ALL NUMERICAL AND STATISTICAL ENSO FORECAST MODELS MAINTAIN THE SUBSTANTIALLY 
NEGATIVE SST ANOMALIES THROUGH THE END OF THE YEAR.  MJO ACTIVITY REMAINS WEAK 
SO THERE ARE NO CONFIDENT INDICATIONS AT THIS TIME THAT TROPICAL INTRASEASONAL 
ACTIVITY WILL COMPLICATE THE FORECAST. THE SST ANOMALIES IN THE NINO 3.4 REGION 
ARE FORECAST TO AVERAGE SOMEWHERE BETWEEN -1.5 AND -2 C IN THE UPCOMING WINTER 
SEASON, SUGGESTING BORDERLINE MODERATE OR STRONG LA NINA CONDITIONS FOR 
DECEMBER.  THE MOST RECENT CFS RUNS FROM LATE NOVEMBER SHOW NO SIGNIFICANT 
DIFFERENCES FROM THE FORECASTS AVAILABLE FOR THE OUTLOOK ISSUED IN 
MID-NOVEMBER.  

THIS UPDATED OUTLOOK FOR DECEMBER, 2010 REFLECTS THE MEAN ATMOSPHERIC 
CIRCULATION PATTERNS EXPECTED IN THE FIRST TWO WEEKS OF DECEMBER AS PREDICTED 
BY SHORT, MEDIUM, AND EXTENDED RANGE WEATHER FORECAST MODELS.  THIS SHORT TERM 
CIRCULATION PATTERN ALTERS THE TEMPERATURE AND PRECIPITATION FORECAST ISSUED AT 
MID-MONTH, WHICH WAS PRIMARILY INFLUENCED BY LA NINA COMPOSITES. 

THERE IS A BROAD CONSENSUS AMONG WEATHER PREDICTION MODELS FOR BELOW NORMAL 
500-HPA HEIGHTS AT MID-LATITUDES FROM EASTERN NORTH AMERICA TO WESTERN EUROPE.  
ABOVE NORMAL 500-HPA HEIGHTS ARE PREDICTED TO PREDOMINATE AT HIGH LATITUDES 
OVER EASTERN NORTH AMERICA AND GREENLAND. FOR MUCH OF THE FIRST TWO WEEKS OF 
DECEMBER.  THIS FITS THE NEGATIVE ARCTIC OSCILLATION (AO) AND NEGATIVE NORTH 
ATLANTIC OSCILLATION (NAO) PATTERNS QUITE WELL.  THE GFS IS PREDICTING AN 
UPWARD TREND IN THE AO AND NAO TOWARD MID-MONTH, HOWEVER THE MAJORITY OF CFS 
ENSEMBLE PREDICTIONS FOR WEEK 3 AND 4 FORECASTS PERSIST A NEGATIVE AO 
CIRCULATION PATTERN INTO WEEKS 3 AND 4, SO THE FORECAST HAS BEEN REVISED TO 
REFLECT LA NINA COMPOSITES DURING NEGATIVE AO EPISODES.  THE CHANCES OF ABOVE 
NORMAL TEMPERATURES IN THE SOUTHEASTERN CONUS ARE GREATLY REDUCED COMPARED TO 
LA NINA COMPOSITES THAT ARE NOT STRATIFIED BY THE AO.  HOWEVER, THESE 
COMPOSITES DO NOT SUGGEST ELEVATION OF THE CHANCES FOR BELOW NORMAL 
TEMPERATURES ABOVE CLIMATOLOGICAL VALUES, SO THE FORECAST FOR THE SOUTHEASTERN 
CONUS IS FOR EQUAL CHANCE FOR ABOVE, NEAR, OR BELOW NORMAL MONTHLY MEAN 
TEMPERATURES.  AN EARLY AND ABNORMALLY HEAVY SNOW COVER IS EXPECTED TO REDUCE 
THE TEMPERATURES IN THE CENTRAL GREAT BASIN REGION FROM WHAT LA NINA COMPOSITES 
AND THE EXTENDED RANGE WEATHER FORECASTS FOR THE FIRST TWO WEEKS IN DECEMBER 
SUGGEST.  ELEVATED CHANCES FOR ABOVE NORMAL TEMPERATURES ARE THEREFORE CONFINED 
TO TEXAS, PARTS OF THE SOUTHERN GREAT PLAINS, THE SOUTHERN ROCKIES AND THE 
SOUTHWEST.   BELOW NORMAL TEMPERATURES IN ALASKA PREDICTED IN THE FIRST HALF OF 
DECEMBER ARE CONSISTENT WITH LA NINA COMPOSITES SO CONFIDENCE IN THE CHANCES OF 
BELOW NORMAL MONTHLY MEAN TEMPERATURES HAS BEEN INCREASED FOR MOST OF ALASKA.  

THE PRECIPITATION FORECAST IS QUITE SIMILAR TO THE FORECAST ISSUED IN 
MID-NOVEMBER AND REFLECT COMPOSITE PRECIPITATION AMOUNTS FOR NEGATIVE AO 
CIRCULATION PATTERNS DURING EARLY LA NINA WINTERS.  A STORM SYSTEM PREDICTED 
FOR EARLY DECEMBER WILL BRING PRECIPITATION TO THE NORTHEASTERN CONUS, 
INCREASING THE CHANCES THAT MONTHLY TOTALS FOR DECEMBER WILL BE IN THE ABOVE 
MEDIAN CATEGORY.  COMPOSITES FOR NEGATIVE AO AND LA NINA CONDITIONS ALSO FAVOR 
ABOVE MEDIAN PRECIPITATION FROM THE OHIO RIVER VALLEY TO NEW ENGLAND.  THE 
CHANCES FOR DRY CONDITIONS HAVE BEEN DECREASED FOR THE WESTERN GULF COAST IN 
COMPARISON TO THE OUTLOOK ISSUED EARLIER IN THE MONTH BASED ON NEGATIVE AO/LA 
NINA COMPOSITES.   


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 JAN ... WILL BE ISSUED ON THU DEC 16 2010


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

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