657  
FXUS07 KWBC 161332  
PMD30D  
PROGNOSTIC DISCUSSION FOR MONTHLY OUTLOOK  
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD  
830 AM EST THU NOV 16 2017  
 
30-DAY OUTLOOK DISCUSSION FOR DECEMBER 2017  
 
THE DECEMBER 2017 TEMPERATURE AND PRECIPITATION OUTLOOKS ARE BASED ON DYNAMICAL  
MODEL GUIDANCE, THE TYPICAL INFLUENCE ASSOCIATED WITH LA NINA, AND LONG-TERM  
TRENDS. DURING THE PAST MONTH, LA NINA CONDITIONS DEVELOPED AS NEGATIVE SEA  
SURFACE TEMPERATURE ANOMALIES CONTINUED ACROSS THE EASTERN AND CENTRAL  
EQUATORIAL PACIFIC ALONG WITH SUPPRESSED (ENHANCED) CONVECTION ACROSS THE  
CENTRAL TROPICAL PACIFIC (MARITIME CONTINENT AND PHILIPPINES). CLIMATE SIGNALS  
TYPICALLY ASSOCIATED WITH LA NINA WERE CONSIDERED IN MAKING THE DECEMBER 2017  
OUTLOOK, GIVEN THESE CURRENT ATMOSPHERIC AND OCEANIC STATES.  
 
A ROBUST MJO OCCURRED DURING OCTOBER, BUT IT WEAKENED SINCE THE BEGINNING OF  
NOVEMBER. THE DYNAMICAL MODELS INDICATE A CONTINUED WEAK OR INCOHERENT SIGNAL  
DURING THE NEXT TWO WEEKS. THEREFORE, THE MJO IS NOT EXPECTED TO PROVIDE ANY  
FORCING TO THE EXTRATROPICS. ALTHOUGH THE MJO DID NOT FACTOR INTO THE DECEMBER  
OUTLOOK, ITS EVOLUTION WILL BE CLOSELY MONITORED FOR THE REVISED OUTLOOK AT THE  
END OF THE MONTH.  
 
THE ARCTIC OSCILLATION (AO) INDEX IS FORECAST TO BECOME LARGELY NEGATIVE DURING  
MID-NOVEMBER AS A BLOCKING RIDGE PERSISTS OVER THE BERING SEA WITH ANOTHER  
UPPER-LEVEL RIDGE BUILDING OVER THE DAVIS STRAIT. THE DURATION OF THIS NEGATIVE  
AO INDEX IS UNCERTAIN AND COULD BE TRANSIENT WITH MANY GFS ENSEMBLE MEMBERS  
INDICATING THAT THE AO INDEX BECOMES MORE NEUTRAL BY THE END OF NOVEMBER. MOST  
INPUTS TO THE NMME AGREE ON ENHANCED ODDS FOR ABOVE-NORMAL TEMPERATURES ACROSS  
THE EASTERN CONUS. SINCE THE ECMWF MODEL AT WEEKS 3-4 (WHICH INCLUDES DEC  
1-12)INDICATES INCREASED CHANCES FOR BELOW-NORMAL TEMPERATURES ACROSS MUCH OF  
THE EASTERN CONUS AND GREAT LAKES, EQUAL CHANCES OF BELOW, NEAR, OR ABOVE  
NORMAL TEMPERATURES ARE NECESSARY FOR THESE AREAS AT THIS TIME LEAD. THE  
SPATIAL PATTERN OF THE TEMPERATURE FORECAST ACROSS THE WESTERN AND  
SOUTH-CENTRAL CONUS IS SIMILAR TO THE SEASONAL OUTLOOK SINCE LA NINA WAS A  
MAJOR FACTOR IN CREATING THE DECEMBER OUTLOOK, BUT THE COVERAGE OF AREAS WHERE  
BELOW- AND ABOVE-NORMAL TEMPERATURES ARE FAVORED IS SMALLER DUE TO UNCERTAINTY.  
DYNAMICAL MODELS, INCLUDING WEEKS 3-4, AND LONG-TERM TRENDS SUPPORT THE  
RELATIVELY HIGH ODDS FOR ABOVE-NORMAL TEMPERATURES ACROSS THE SOUTHWESTERN  
CONUS.  
 
THE PRECIPITATION OUTLOOK FOR DECEMBER IS BASED PRIMARILY ON THE CALIBRATED  
NMME AND IS CONSISTENT WITH LA NINA CONDITIONS AND THE DJF OUTLOOK. THE HIGHEST  
PROBABILITIES FOR BELOW-NORMAL PRECIPITATION ARE FORECAST ACROSS PARTS OF THE  
GULF COAST AND SOUTHEAST WHERE A DRY SIGNAL IS STRONG IN LA NINA COMPOSITES FOR  
NDJ AND PROBABILITIES ARE RELATIVELY HIGH IN THE CALIBRATED NMME. THE BROAD  
AREA OF ENHANCED ODDS FOR ABOVE-NORMAL PRECIPITATION FROM THE PACIFIC NORTHWEST  
EAST TO THE GREAT LAKES IS EITHER SUPPORTED BY LA NINA COMPOSITES OR THE  
CALIBRATED NMME. PROBABILITIES FOR BELOW- AND ABOVE-NORMAL PRECIPITATION ARE  
MODEST DUE TO LIMITED SKILL INHERENT IN A MONTHLY OUTLOOK AT THIS TIME LEAD.  
 
THE HIGHEST ODDS FOR ABOVE-NORMAL TEMPERATURES ACROSS ALASKA ARE FORECAST FOR  
AREAS OF THE STATE ADJACENT TO THE BERING AND CHUKCHI SEAS WHERE SEA SURFACE  
TEMPERATURE ANOMALIES ARE AVERAGING AS MUCH AS 1.5 DEGREES C ABOVE NORMAL.  
ENHANCED ODDS FOR BELOW-NORMAL TEMPERATURES FORECAST ACROSS PARTS OF SOUTHEAST  
MAINLAND ALASKA AND THE ALASKA PANHANDLE ARE BASED ON THE MONTHLY CFS  
TEMPERATURE FORECAST. THE FAVORED AREAS OF BELOW- AND ABOVE-NORMAL  
PRECIPITATION ACROSS ALASKA ARE CONSISTENT WITH THE TEMPERATURE OUTLOOK AND  
SUPPORTED BY THE WEEKS 3-4 CFS AND ECMWF PRECIPITATION FORECASTS ALONG WITH THE  
MONTHLY CFS PRECIPITATION FORECAST.  
 
FORECASTER: BRAD PUGH  
 
THE CLIMATIC NORMALS ARE BASED ON CONDITIONS BETWEEN 1981 AND 2010, FOLLOWING  
THE WORLD METEOROLOGICAL ORGANIZATION CONVENTION OF USING THE MOST RECENT 3  
COMPLETE DECADES AS THE CLIMATE REFERENCE PERIOD. THE PROBABILITY ANOMALIES  
FOR TEMPERATURE AND PRECIPITATION BASED ON THESE NEW NORMALS BETTER REPRESENT  
SHORTER TERM CLIMATIC ANOMALIES THAN THE FORECASTS BASED ON OLDER NORMALS.  
 
AN UPDATED MONTHLY OUTLOOK... FOR DEC WILL BE ISSUED ON THU NOVEMBER 30 2017  
 
THESE OUTLOOKS ARE BASED ON DEPARTURES FROM THE 1981-2010 BASE PERIOD.  
 

 
 
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