PROGNOSTIC DISCUSSION FOR MONTHLY OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
830 AM EST THU NOV 15 2018


30-DAY OUTLOOK DISCUSSION FOR DECEMBER 2018

THE DECEMBER 2018 TEMPERATURE AND PRECIPITATION OUTLOOKS ARE BASED PRIMARILY ON 
RECENT DAILY CFS MODEL OUTPUT, WEEKS 3 TO 4 OUTLOOKS FROM THE CFS AND ECMWF 
MODELS, CONSIDERATIONS OF THE ARCTIC OSCILLATION AND MADDEN-JULIAN OSCILLATION 
(MJO), AND LONG-TERM TRENDS. ALTHOUGH ABOVE-AVERAGE SEA SURFACE TEMPERATURES 
WERE OBSERVED ACROSS THE EQUATORIAL PACIFIC OCEAN DURING THE PAST MONTH, 
ENSO-NEUTRAL CONDITIONS CONTINUE AS CONVECTION REMAINED NEAR AVERAGE OR 
SLIGHTLY SUPPRESSED NEAR THE DATE LINE. CLIMATE SIGNALS TYPICALLY ASSOCIATED 
WITH EL NINO PLAYED A MINOR ROLE IN THE DECEMBER 2018 OUTLOOK, GIVEN THE 
CURRENT ATMOSPHERIC STATE.

THE MADDEN-JULIAN OSCILLATION (MJO) STRENGTHENED AT THE END OF OCTOBER, 
FOLLOWED BY ITS ENHANCED PHASE PROPAGATING EAST FROM THE INDIAN OCEAN TO THE 
MARITIME CONTINENT DURING EARLY TO MID-NOVEMBER. DUE TO THE ONGOING AMPLITUDE 
AND COHERENT WAVE-1 PATTERN, THE MJO IS ANTICIPATED TO CONTINUE SHIFTING EAST 
TO THE WEST PACIFIC.  ALTHOUGH IT REMAINS UNCLEAR HOW MUCH FORCING THE MJO WILL 
HAVE ON THE EXTRATROPICS DURING DECEMBER, ITS EXPECTED EVOLUTION WOULD FAVOR A 
COLD START TO THE MONTH ACROSS MUCH OF THE CENTRAL AND EASTERN CONUS. 

SINCE LATE OCTOBER, THE ARCTIC OSCILLATION (AO) INDEX HAS BEEN NEUTRAL TO 
SLIGHTLY NEGATIVE. MANY OF THE GFS ENSEMBLE MEMBERS INDICATE THAT THE AO 
BECOMES LARGELY NEGATIVE DURING THE REMAINDER OF NOVEMBER. BELOW-NORMAL 
TEMPERATURES ACROSS THE NORTH-CENTRAL AND EASTERN U.S. ARE TYPICALLY ASSOCIATED 
WITH THE NEGATIVE PHASE OF THE AO. THE CFS MODEL AT WEEK-3 INDICATES LARGE 
POSITIVE (NEGATIVE) 500-HPA HEIGHT ANOMALIES AT THE HIGH (MIDDLE) LATITUDES OF 
THE NORTHERN HEMISPHERE, INDICATIVE OF A NEGATIVE AO INDEX PERSISTING INTO THE 
FIRST WEEK OF DECEMBER.  THE ECMWF MODEL AT WEEK-3 FEATURES A HIGHLY AMPLIFIED 
RIDGE ACROSS THE ALEUTIANS AND BERING SEA WITH CORRESPONDING NEGATIVE 500-HPA 
HEIGHT ANOMALIES OVER NORTH AMERICA. THE WEEKS 3-4 CFS AND ECMWF MODELS (ISSUED 
ON NOV 12) WHICH COVERS EARLY DECEMBER FEATURE INCREASED CHANCES FOR BELOW 
NORMAL TEMPERATURES ACROSS PARTS OF THE NORTHERN GREAT PLAINS AND UPPER 
MISSISSIPPI VALLEY. DAILY RUNS OF THE CFS MODEL FOR DECEMBER HAVE TRENDED MUCH 
COLDER ACROSS THE EASTERN THIRD OF THE CONUS DURING THE PAST FEW DAYS AND ITS 
LATEST RUN FROM NOV 14 FEATURES INCREASED CHANCES OF BELOW NORMAL TEMPERATURES 
ACROSS THE GREAT LAKES, OHIO VALLEY, AND NORTHEAST. LARGE UNCERTAINTY ON HOW 
ANOMALOUSLY COLD DECEMBER BEGINS AND THE EVOLUTION OF THE LONGWAVE PATTERN 
LATER IN THE MONTH RESULT IN A LARGE COVERAGE OF EC (EQUAL CHANCES FOR BELOW, 
NEAR, OR ABOVE) IN THE TEMPERATURE OUTLOOK ACROSS THE CENTRAL AND EASTERN 
CONUS. BASED ON THE LATEST DAILY RUNS OF THE CFS MODEL, A SLIGHT TILT IN THE 
ODDS FOR BELOW NORMAL TEMPERATURES IS FORECAST FOR PARTS OF THE GREAT LAKES, 
EASTERN CORN BELT, AND CENTRAL APPALACHIANS. DYNAMICAL MODELS AND LONG-TERM 
TRENDS SUPPORT ENHANCED ODDS FOR ABOVE-NORMAL TEMPERATURES ACROSS THE WESTERN 
CONUS. 

THE PRECIPITATION OUTLOOK FOR DECEMBER, AT THIS HALF-MONTH LEAD, IS BASED 
PRIMARILY ON RECENT DAILY RUNS OF THE CFS MODEL AND THE CALIBRATED NORTH 
AMERICAN MULTI-MODEL ENSEMBLE (NMME). THE FAVORED AREAS OF ABOVE NORMAL 
PRECIPITATION ACROSS THE SOUTHWEST AND PARTS OF THE SOUTHEAST ARE CONSISTENT 
WITH THE EMERGING EL NINO AND THE DECEMBER-JANUARY-FEBRUARY OUTLOOK. THE 
EXPECTATION FOR A SUPPRESSED STORM TRACK, EARLY IN THE MONTH, ELEVATES ODDS FOR 
BELOW NORMAL PRECIPITATION FROM THE MIDDLE MISSISSIPPI AND OHIO VALLEYS 
NORTHEAST TO NEW ENGLAND. HOWEVER, COLD AIR ADVECTION WOULD RESULT IN PERIODS 
OF HEAVY, LAKE-EFFECT SNOW DOWNWIND OF THE GREAT LAKES. THE ENHANCED ODDS OF 
ABOVE NORMAL PRECIPITATION ACROSS THE NORTHERN AND CENTRAL GREAT PLAINS ARE 
BASED IN PART ON THE INCREASED CHANCE OF UPSLOPE SNOW WITH SURFACE HIGH 
PRESSURE SHIFTING SOUTH FROM CANADA. 

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 HIGH AS 2.5 DEGREES C ABOVE NORMAL. THE 
FAVORED AREA OF ABOVE NORMAL PRECIPITATION ACROSS ALASKA IS CONSISTENT WITH THE 
TEMPERATURE OUTLOOK AND IS ALSO 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 FRI NOVEMBER 30 2018

THESE OUTLOOKS ARE BASED ON DEPARTURES FROM THE 1981-2010 BASE PERIOD.
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