PROGNOSTIC DISCUSSION FOR MONTHLY OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EST FRI NOV 30 2018


30-DAY OUTLOOK DISCUSSION FOR DECEMBER 2018

THE UPDATED TEMPERATURE AND PRECIPITATION OUTLOOKS FOR DECEMBER 2018 ARE BASED 
ON THE WPC TEMPERATURE AND PRECIPITATION FORECASTS DURING THE FIRST WEEK OF 
DECEMBER, THE CPC 8-14 DAY AND WEEKS 3-4 TEMPERATURE AND PRECIPITATION 
OUTLOOKS, AND THE LATEST MONTHLY FORECASTS FROM THE CFS MODEL. ALTHOUGH SEA 
SURFACE TEMPERATURES REMAIN ABOVE NORMAL THROUGHOUT THE EQUATORIAL PACIFIC, 
THERE CONTINUES TO BE A LACK OF ENHANCED CONVECTION NEAR THE DATE LINE. 
THEREFORE, ENSO NEUTRAL CONDITIONS ARE PRESENT. A ROBUST MADDEN-JULIAN 
OSCILLATION (MJO) CONTINUES WITH ITS ENHANCED PHASE PROPAGATING EAST OVER THE 
WESTERN HEMISPHERE DURING LATE NOVEMBER. THE MJO IS FORECAST TO SHIFT EAST TO 
THE INDIAN OCEAN DURING EARLY DECEMBER WHICH WOULD FAVOR A TRANSITION TO A 
WARMER PATTERN FOR MUCH OF THE CONTINENTAL U.S. LATER IN THE MONTH.

THE GFS AND ECMWF ENSEMBLE MEANS REMAIN CONSISTENT AND IN GOOD AGREEMENT THAT 
AN AMPLIFIED UPPER-LEVEL TROUGH DEVELOPS OVER THE EAST PACIFIC EARLY IN THE 
MONTH, WHILE A POSITIVE 500-HPA HEIGHT ANOMALY CENTER BECOMES ESTABLISHED 
ACROSS CENTRAL CANADA. THIS 500-HPA PATTERN FAVORS A TRANSITION TO ABOVE-NORMAL 
TEMPERATURES FOR MUCH OF THE CONUS BY MID-DECEMBER. THIS TRANSITION FROM A COLD 
NOVEMBER AND BEGINNING OF DECEMBER TO A WARMER PATTERN IS ALSO CONSISTENT WITH 
THE PREDICTED MJO FORECAST. THE D+11 TEMPERATURE OUTLOOK FAVORS NEAR TO ABOVE 
NORMAL TEMPERATURES FOR MUCH OF THE CONUS WITH INCREASED CHANCES OF BELOW 
NORMAL TEMPERATURES LIMITED TO THE NORTHEAST. THE NORTH ATLANTIC OSCILLATION 
(NAO) INDEX BECAME MORE NEUTRAL AT THE END OF NOVEMBER AND MOST GFS ENSEMBLE 
MEMBERS INDICATE A POSITIVE NAO THROUGH EARLY TO MID-DECEMBER. THE POSITIVE NAO 
SUPPORTS THE TRANSITION TOWARDS A WARMER PATTERN ACROSS THE CENTRAL AND EASTERN 
CONUS. HOWEVER, PROBABILITIES FOR ABOVE NORMAL TEMPERATURES ARE TEMPERED DUE TO 
THE COLD START TO THE MONTH.

BELOW-NORMAL TEMPERATURES ARE FAVORED FOR THE GREAT BASIN AND CENTRAL ROCKIES 
SINCE THESE AREAS ARE FORECAST TO HAVE THE LARGEST NEGATIVE TEMPERATURE 
ANOMALIES (10 TO 20 DEGREES F) DURING THE FIRST WEEK OF DECEMBER. THE WEEKS 2 
TO 4 OUTLOOKS FAVOR NEAR OR BELOW NORMAL TEMPERATURES FOR THESE AREAS. ALSO, AN 
INCREASE IN SNOW COVER IS EXPECTED DURING THE NEXT WEEK TO TEN DAYS ACROSS THE 
INTERIOR WEST. A SLIGHT TILT IN THE ODDS FOR BELOW NORMAL TEMPERATURES IS 
FORECAST ACROSS NORTHERN NEW ENGLAND WHERE BELOW NORMAL TEMPERATURES ARE LIKELY 
TO PERSIST THROUGH THE FIRST HALF OF THE MONTH. 

DURING THE FIRST WEEK OF DECEMBER, THE DETERMINISTIC MODELS CONTINUE TO 
INDICATE A VIGOROUS SHORTWAVE TROUGH IN THE SOUTHERN STREAM WHICH IS LIKELY TO 
LEAD TO SURFACE LOW DEVELOPMENT ACROSS THE SOUTHERN GREAT PLAINS OR WESTERN 
GULF COAST REGION. THIS SHORTWAVE TROUGH IS FORECAST TO BRING LOCALLY HEAVY 
PRECIPITATION TO SOUTHERN CALIFORNIA AND THE DESERT SOUTHWEST, WHILE WIDESPREAD 
HEAVY PRECIPITATION (1 TO 3 INCHES, OR MORE) IS EXPECTED TO ACCOMPANY THE 
SURFACE LOW ACROSS THE LOWER MISSISSIPPI VALLEY AND SOUTHEAST. A WET PATTERN IS 
LIKELY TO CONTINUE THROUGH MID-DECEMBER ACROSS CALIFORNIA AND THE SOUTHERN TIER 
OF THE CONUS DUE TO AN AMPLIFIED UPPER-LEVEL TROUGH OFFSHORE OF THE WEST COAST, 
AN INFLUX OF SUBTROPICAL MOISTURE, AND A WARMING TREND IN TEMPERATURES.  BASED 
ON HIGH CONFIDENCE IN A WET PATTERN THROUGH AT LEAST MID-DECEMBER, ABOVE-NORMAL 
PRECIPITATION IS FAVORED FOR CALIFORNIA, THE SOUTHWEST, SOUTHERN GREAT PLAINS, 
LOWER MISSISSIPPI VALLEY, AND SOUTHEAST. THE HIGHEST PROBABILITIES (ABOVE 60 
PERCENT) FOR ABOVE NORMAL PRECIPITATION ARE FORECAST ACROSS THE FLORIDA 
PANHANDLE WHERE THE DETERMINISTIC 0Z ECMWF MODEL CONTINUES TO FORECAST 3 TO 7 
INCHES OF RAINFALL DURING THE FIRST WEEK. A SECOND AREA OF HIGH PROBABILITIES 
ACROSS THE CENTRAL GREAT PLAINS AND WESTERN CORN BELT IS RELATED TO HEAVY 
PRECIPITATION (MORE THAN 0.5 INCH, LIQUID EQUIVALENT) FORECAST ON DECEMBER 1. 
THE MOST LIKELY AREA FOR BELOW NORMAL PRECIPITATION EXTENDS FROM THE NORTHERN 
GREAT PLAINS EAST TO NORTHERN NEW ENGLAND AS A DRY START TO THE MONTH IS LIKELY 
WITH ANOMALOUS NORTHERLY FLOW AND PRECIPITATION TOOLS THROUGHOUT THE REMAINDER 
OF THE MONTH HAVE A DRY SIGNAL. 

ABOVE NORMAL TEMPERATURES REMAIN FAVORED FOR ALASKA ALTHOUGH PROBABILITIES ARE 
REDUCED ACROSS EASTERN ALASKA AS 500-HPA HEIGHTS ARE FORECAST TO AVERAGE BELOW 
NORMAL DURING EARLY DECEMBER. THE HIGHEST ODDS FOR ABOVE-NORMAL TEMPERATURES 
ACROSS ALASKA ARE FORECAST FOR AREAS OF THE STATE ADJACENT TO THE BERING SEA 
WHERE SEA SURFACE TEMPERATURE ANOMALIES REMAIN ABOVE NORMAL. THE FAVORED AREAS 
OF ABOVE NORMAL PRECIPITATION ACROSS THE ALEUTIANS AND WESTERN ALASKA ARE BASED 
ON MODEL GUIDANCE FEATURING ABOVE NORMAL PRECIPITATION DURING THE FIRST TWO 
WEEKS AND RECENT DAILY RUNS OF THE CFS MONTHLY FORECAST.

----------- PREVIOUS MESSAGE (FROM NOVEMBER 15) IS SHOWN BELOW ------------

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.

THE NEXT MONTHLY OUTLOOK...FOR JAN ... WILL BE ISSUED ON THU DEC 20 2018

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