306  
FXUS07 KWBC 302000  
PMD30D  
PROGNOSTIC DISCUSSION FOR MONTHLY OUTLOOK  
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD  
300 PM EST THU NOV 30 2017  
 
30-DAY OUTLOOK DISCUSSION FOR DECEMBER 2017  
 
THE UPDATED TEMPERATURE AND PRECIPITATION OUTLOOKS FOR DECEMBER 2017 ARE BASED  
ON THE WPC TEMPERATURE AND PRECIPITATION FORECASTS DURING THE FIRST WEEK OF  
DECEMBER, THE CPC 8-14 DAY TEMPERATURE AND PRECIPITATION OUTLOOKS, THE LATEST  
DYNAMICAL MODEL GUIDANCE, AND THE TYPICAL INFLUENCES ASSOCIATED WITH LA NINA.  
THE MADDEN-JULIAN OSCILLATION (MJO) CONTINUES TO STRENGTHEN WITH ITS ENHANCED  
(SUPPRESSED) PHASE OVER THE EASTERN INDIAN OCEAN AND MARITIME CONTINENT  
(WESTERN HEMISPHERE). THE ENHANCED PHASE OF THE MJO IS FORECAST TO PROPAGATE  
EAST TO THE WEST PACIFIC AND IT MAY INFLUENCE THE CIRCULATION PATTERN AND  
TEMPERATURES ACROSS THE MID-LATITUDES LATER IN DECEMBER.  
 
A MAJOR LONGWAVE PATTERN CHANGE IS LIKELY TO OCCUR DURING THE FIRST WEEK OF  
DECEMBER AS A HIGHLY ANOMALOUS UPPER-LEVEL RIDGE BECOMES ANCHORED ALONG THE  
WEST COAST WITH AN AMPLIFYING UPPER-LEVEL TROUGH DOWNSTREAM OVER EASTERN NORTH  
AMERICA. FOLLOWING A MILD START TO DECEMBER, THIS MAJOR PATTERN CHANGE IS  
LIKELY TO USHER IN OUTBREAKS OF BELOW TO MUCH BELOW-NORMAL TEMPERATURES TO THE  
EASTERN THIRD OF THE CONUS THROUGH AT LEAST MID-DECEMBER. THE D+11 GFS AND  
ECMWF ENSEMBLE MEANS ON NOV 30 ARE IN EXCELLENT AGREEMENT, MAINTAINING THIS  
ANOMALOUS LONGWAVE PATTERN. THE 6Z GFS ENSEMBLE MEAN FEATURES A POSITIVE  
500-HPA HEIGHT ANOMALY CENTER OF 300 METERS OVER THE ALASKA PANHANDLE WHICH  
STRONGLY FAVORS A STABLE PATTERN OVER NORTH AMERICA AND BELOW-NORMAL  
TEMPERATURES ACROSS THE EASTERN U.S. THE GFS ENSEMBLE MEMBERS INDICATE A  
NEGATIVE ARCTIC OSCILLATION (AO) INDEX THROUGH THE END OF WEEK-2. THE PREDICTED  
EVOLUTION OF THE MJO IS EXPECTED TO INCREASES THE CHANCES THAT THE BELOW NORMAL  
TEMPERATURES PERSIST INTO THE LATTER HALF OF THE MONTH. THE CFS MODEL AT WEEKS  
3-4 (ENDING ON DEC 28) INDICATES BELOW NORMAL TEMPERATURES FROM THE MISSISSIPPI  
VALLEY TO THE EAST COAST. BASED ON EXCELLENT MODEL AGREEMENT AND CONTINUITY  
REGARDING THIS HIGHLY ANOMALOUS PATTERN, BELOW-NORMAL TEMPERATURES ARE FAVORED  
FOR THE EASTERN THIRD OF THE CONUS. THE HIGHEST ODDS FOR BELOW NORMAL  
TEMPERATURES ARE FORECAST ACROSS THE UPPER OHIO VALLEY WHERE THE MOST  
ANOMALOUSLY COLD AIR IS EXPECTED DURING EARLY TO MID-DECEMBER.  
 
THE PERSISTENCE OF A HIGHLY ANOMALOUS RIDGE ALOFT AND EXCELLENT AGREEMENT AMONG  
TEMPERATURE TOOLS SUPPORT INCREASED CHANCES FOR ABOVE NORMAL TEMPERATURES  
ACROSS MUCH OF THE WESTERN U.S. PROBABILITIES AND COVERAGE FOR ABOVE NORMAL  
TEMPERATURES ARE REDUCED ACROSS THE NORTHERN ROCKIES AND NORTHERN GREAT PLAINS  
COMPARED TO THE DYNAMICAL MODEL GUIDANCE. THIS MODIFICATION IS RELATED TO AN  
INCREASING CHANCE OF BELOW NORMAL TEMPERATURES AFFECTING THESE AREAS LATER IN  
THE MONTH, IF THE ENHANCED PHASE OF THE MJO PROPAGATES INTO THE WEST PACIFIC  
AND INFLUENCES THE MID-LATITUDE CIRCULATION PATTERN. ALTHOUGH PERIODS OF  
ANOMALOUS COLD ARE EXPECTED TO AFFECT THE GREAT PLAINS DURING DECEMBER, LARGE  
POSITIVE ANOMALIES AT THE BEGINNING OF THE MONTH AND AN EXPECTED VARIABLE  
TEMPERATURE PATTERN RESULT IN EQUAL CHANCES (EC) OF BELOW, NEAR, OR ABOVE  
NORMAL TEMPERATURES FOR THIS REGION.  
 
BELOW-NORMAL PRECIPITATION IS FAVORED ACROSS THE WESTERN U.S. DUE TO THE  
LIKELIHOOD OF A PERSISTENT UPPER-LEVEL RIDGE THROUGH AT LEAST MID-DECEMBER.  
LITTLE TO NO PRECIPITATION IS LIKELY FOR CALIFORNIA DURING THE FIRST TWO WEEKS  
OF THE MONTH. HIGHER PROBABILITIES OF BELOW NORMAL PRECIPITATION WOULD BE  
FORECAST ACROSS CALIFORNIA, BUT THE CFS MODEL INDICATES AN UPPER-LEVEL TROUGH  
APPROACHING THE WEST COAST DURING THE WEEKS 3-4 PERIOD. A COLD FRONT,  
ASSOCIATED WITH THE ONSET OF THE MAJOR PATTERN CHANGE, IS EXPECTED TO RESULT IN  
PRECIPITATION AMOUNTS OF 0.5 TO 2 INCHES ACROSS THE EASTERN U.S. DURING THE  
FIRST WEEK OF DECEMBER. FOLLOWING THIS FRONTAL PASSAGE, A PROLONGED PERIOD OF  
ANOMALOUS NORTHERLY FLOW ENHANCES ODDS FOR BELOW NORMAL PRECIPITATION FROM THE  
CENTRAL AND SOUTHERN GREAT PLAINS NORTHEAST TO THE MIDDLE MISSISSIPPI VALLEY.  
THE FAVORED AREA OF BELOW NORMAL PRECIPITATION ACROSS THE SOUTHERN GREAT PLAINS  
IS CONSISTENT WITH THE ONGOING LA NINA. A LARGER COVERAGE OF INCREASED CHANCES  
OF BELOW NORMAL PRECIPITATION WOULD BE FORECAST FOR THE EASTERN U.S., BUT  
PRECIPITATION AT THE BEGINNING OF DECEMBER AND POTENTIAL FOR WAVES OF SURFACE  
LOW PRESSURE TO DEVELOP WITH THE AMPLIFIED TROUGH LIMIT FORECAST CONFIDENCE ON  
BELOW NORMAL PRECIPITATION VERIFYING.  
 
A STABLE PATTERN WITH AN ANOMALOUS TROUGH (RIDGE) OVER THE ALEUTIANS (EASTERN  
ALASKA) AND PERSISTENT SOUTHERLY FLOW AT THE SURFACE ENHANCES ODDS FOR ABOVE  
NORMAL TEMPERATURES THROUGHOUT ALASKA. PRECIPITATION TOOLS AT ALL TIME LEADS  
DURING DECEMBER AND THE LONGWAVE PATTERN FAVOR ELEVATED ODDS FOR ABOVE NORMAL  
PRECIPITATION ACROSS THE ALEUTIANS, SOUTHWEST ALASKA, AND ALONG THE SOUTHERN  
COAST OF MAINLAND ALASKA. DOWNSLOPING SLOW, ASSOCIATED WITH PERSISTENT  
SOUTHERLY FLOW OFF THE ALASKA RANGE, TILTS THE ODDS TO BELOW NORMAL  
PRECIPITATION FOR PARTS OF EASTERN MAINLAND ALASKA.  
 
----------- PREVIOUS MESSAGE (FROM NOVEMBER 16) IS SHOWN BELOW ------------  
 
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.  
 
THE NEXT MONTHLY OUTLOOK...FOR JAN ... WILL BE ISSUED ON THU DEC 21 2017  
 
THESE OUTLOOKS ARE BASED ON DEPARTURES FROM THE 1981-2010 BASE PERIOD.  
 

 
 
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