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. $$