599  
FXUS05 KWBC 191231  
PMD90D  
PROGNOSTIC DISCUSSION FOR LONG-LEAD SEASONAL OUTLOOKS  
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD  
830 AM EDT THU SEP 19 2024  
 
SUMMARY OF THE OUTLOOK FOR NON-TECHNICAL USERS  
 
EL NIñO SOUTHERN OSCILLATION (ENSO)-NEUTRAL CONDITIONS ARE PRESENT, AS  
EQUATORIAL SEA SURFACE TEMPERATURES (SSTS) ARE NEAR-TO-BELOW AVERAGE IN THE  
CENTRAL AND EASTERN PACIFIC OCEAN. DURING THE LAST FOUR WEEKS, NEGATIVE SST  
ANOMALY CHANGES PREVAILED ACROSS THE WESTERN AND CENTRAL EQUATORIAL PACIFIC  
OCEAN. AS SUCH, A LA NIñA WATCH IS IN EFFECT, WITH LA NIñA FAVORED TO EMERGE IN  
SEPTEMBER-NOVEMBER (SON) (71% CHANCE) AND IS EXPECTED TO PERSIST THROUGH  
JANUARY-MARCH (JFM) 2025. HOWEVER, CHANCES OF A MODERATE TO STRONG LA NIñA ARE  
CURRENTLY LESS THAN 50% THROUGH THE FALL AND WINTER. ENSO-NEUTRAL CONDITIONS  
ARE FAVORED TO RE-EMERGE BY THE FEBRUARY-APRIL (FMA) 2025 SEASON.  
 
THE OCTOBER-DECEMBER (OND) 2024 TEMPERATURE OUTLOOK FAVORS ABOVE NORMAL  
TEMPERATURES FOR THE SOUTHERN TWO-THIRDS OF THE CONTIGUOUS UNITED STATES  
(CONUS), THE EASTERN THIRD OF THE CONUS, AND NORTHWESTERN ALASKA. THE LARGEST  
PROBABILITIES (GREATER THAN 60 PERCENT) OF ABOVE NORMAL TEMPERATURES ARE  
FORECAST ACROSS MUCH OF THE SOUTHWEST AND PARTS OF THE RIO GRANDE VALLEY AND  
SOUTHERN HIGH PLAINS. CONVERSELY, A WEAK TILT TOWARD BELOW NORMAL TEMPERATURES  
IS INDICATED FOR SOUTHEAST ALASKA, PARTS OF THE SOUTHERN MAINLAND, AND PARTS OF  
THE ALASKA PENINSULA. THE OND 2024 PRECIPITATION OUTLOOK DEPICTS ENHANCED  
PROBABILITIES OF BELOW-NORMAL PRECIPITATION AMOUNTS ACROSS MUCH OF THE  
SOUTH-CENTRAL AND SOUTHWESTERN CONUS AS WELL AS MOST OF SOUTHEAST ALASKA AND  
PARTS OF THE SOUTHERN MAINLAND. THE GREATEST CHANCES (GREATER THAN 50 PERCENT)  
OF BELOW-NORMAL PRECIPITATION ARE FORECAST FOR MUCH OF THE RIO GRANDE VALLEY  
AND MUCH OF THE SOUTHERN HIGH PLAINS, WHERE PROBABILITIES OF BELOW EXCEED 50  
PERCENT. ABOVE-NORMAL PRECIPITATION IS MORE LIKELY FOR THE NORTHWESTERN CONUS,  
THE GREAT LAKES AND NORTHEAST, AND MUCH OF NORTHWESTERN MAINLAND ALASKA. EQUAL  
CHANCES (EC) ARE FORECAST FOR AREAS WHERE PROBABILITIES FOR EACH CATEGORY OF  
SEASONAL MEAN TEMPERATURES AND SEASONAL ACCUMULATED PRECIPITATION AMOUNTS ARE  
EXPECTED TO BE SIMILAR TO CLIMATOLOGICAL PROBABILITIES.  
 
BASIS AND SUMMARY OF THE CURRENT LONG-LEAD OUTLOOKS  
NOTE: FOR GRAPHICAL DISPLAYS OF THE FORECAST TOOLS DISCUSSED BELOW SEE:  
HTTP://WWW.CPC.NCEP.NOAA.GOV/PRODUCTS/PREDICTIONS/90DAY/TOOLS/BRIEFING  
 
CURRENT ATMOSPHERIC AND OCEANIC CONDITIONS  
 
ENSO-NEUTRAL CONTINUED DURING AUGUST 2024, WITH NEAR-AVERAGE SEA SURFACE  
TEMPERATURES (SSTS) OBSERVED ACROSS MOST OF THE EQUATORIAL PACIFIC OCEAN. THE  
WEEKLY NIñO INDICES DID NOT CHANGE SUBSTANTIALLY DURING THE MONTH, WITH THE  
LATEST WEEKLY INDEX VALUES VARYING BETWEEN +0.1°C (NIñO-4) AND -0.2°C  
(NIñO-3.4, 3, AND 1+2). BELOW-AVERAGE SUBSURFACE TEMPERATURES WERE ALSO SIMILAR  
TO THOSE IN EARLY AUGUST. NEGATIVE TEMPERATURE ANOMALIES CONTINUED TO DOMINATE  
ACROSS MOST OF THE SUBSURFACE EQUATORIAL PACIFIC OCEAN. LOW-LEVEL WIND  
ANOMALIES WERE EASTERLY OVER MOST OF THE EQUATORIAL PACIFIC, AND UPPER-LEVEL  
WIND ANOMALIES WERE EASTERLY OVER THE EAST-CENTRAL PACIFIC. CONVECTION WAS  
SLIGHTLY ENHANCED OVER PARTS OF INDONESIA AND WAS NEAR AVERAGE NEAR THE DATE  
LINE. BOTH THE SOUTHERN OSCILLATION INDEX AND THE EQUATORIAL SOUTHERN  
OSCILLATION INDICES WERE POSITIVE. COLLECTIVELY, THE COUPLED OCEAN-ATMOSPHERE  
SYSTEM REFLECTED ENSO-NEUTRAL.  
 
SINCE LATE AUGUST, THE REAL-TIME MULTIVARIATE MADDEN JULIAN OSCILLATION (RMM)  
INDEX OBSERVATIONS SHOW THE MADDEN JULIAN OSCILLATION (MJO) SIGNAL HAS  
STRUGGLED TO FULLY PROPAGATE OUT OF THE MARITIME CONTINENT. THE SIGNAL HAS  
REGAINED AMPLITUDE IN THE PAST WEEK, BUT STILL HAS SHOWN LITTLE SIGN OF  
EASTWARD PROPAGATION. DYNAMICAL MODELS HAVE COME INTO BETTER AGREEMENT FAVORING  
AN EASTWARD PROPAGATING WESTERN PACIFIC MJO EVENT DURING THE NEXT FEW WEEKS,  
WITH SEVERAL ENSEMBLE MEMBERS MAINTAINING THE SIGNAL WITH A MODERATE TO HIGH  
AMPLITUDE AS IT ENTERS THE WESTERN HEMISPHERE. UPPER-LEVEL VELOCITY POTENTIAL  
FORECASTS FROM THE MODELS REMAIN SOMEWHAT MIXED IN REGARDS TO THE EVOLUTION AND  
STRENGTH OF THE MJO, WITH THE EUROPEAN CENTRE FOR MEDIUM-RANGE WEATHER  
FORECASTS (ECMWF) FAVORING MORE ROBUST ACTIVITY HEADING INTO OCTOBER. DESPITE  
SOME UNCERTAINTIES, CONDITIONS ARE EXPECTED TO BECOME MORE CONDUCIVE FOR  
TROPICAL CYCLONE DEVELOPMENT OVER THE TROPICAL AMERICAS.  
 
PROGNOSTIC DISCUSSION OF SST FORECASTS  
 
THE MOST RECENT INTERNATIONAL RESEARCH INSTITUTE (IRI) FOR CLIMATE AND SOCIETY  
PLUME FORECAST OF THE NIñO-3.4 SST INDEX PREDICTS A WEAK AND A SHORT DURATION  
LA NIñA, AS INDICATED BY THE NIñO-3.4 INDEX VALUES LESS THAN -0.5°C. THIS  
MONTH, THE LATEST NORTH AMERICAN MULTI-MODEL ENSEMBLE (NMME) GUIDANCE WAS  
RELIED UPON, WHICH PREDICTS LA NIñA TO EMERGE IN THE NEXT COUPLE OF MONTHS AND  
CONTINUE THROUGH THE NORTHERN HEMISPHERE WINTER. THE CONTINUATION OF NEGATIVE  
SUBSURFACE TEMPERATURES AND ENHANCED LOW-LEVEL EASTERLY WIND ANOMALIES SUPPORTS  
THE FORMATION OF A WEAK LA NINA. IN SUMMARY, LA NIñA IS FAVORED TO EMERGE IN  
SON (71% CHANCE) AND IS EXPECTED TO PERSIST THROUGH JFM 2025.  
 
PROGNOSTIC TOOLS USED FOR U.S. TEMPERATURE AND PRECIPITATION OUTLOOKS  
 
DYNAMICAL MODEL FORECASTS FROM THE NMME, THE COUPLED FORECAST SYSTEM MODEL  
VERSION 2 (CFSV2) , THE COPERNICUS (C3S) MULTI-MODEL ENSEMBLE SYSTEM WERE USED  
EXTENSIVELY FOR THE FIRST SIX LEADS WHEN THEY ARE AVAILABLE, AS WAS THE  
OBJECTIVE, HISTORICAL SKILL WEIGHTED CONSOLIDATION AND CALIBRATION, BRIDGING,  
AND MERGING (CBAM) GUIDANCE, THAT COMBINES BOTH DYNAMICAL AND STATISTICAL  
FORECAST INFORMATION.  
 
ADDITIONALLY, THE OFFICIAL ENSO FORECAST DEPICTS PROBABILITIES OF LA NIñA THAT  
ARE HIGHER THAN CLIMATOLOGICAL PROBABILITIES THROUGH THE UPCOMING WINTER. THIS  
ANTICIPATED LA NIñA SIGNAL PLAYED A ROLE IN THE CONSTRUCTION OF THESE OUTLOOKS.  
HOWEVER, THE ANTICIPATED LA NIñA IS LIKELY TO BE WEAK, WHICH MAY TEMPER SOME OF  
ITS IMPACTS. AT LATER LEADS, DECADAL TRENDS IN TEMPERATURE AND PRECIPITATION  
WERE INCREASINGLY RELIED UPON IN CREATING THE SEASONAL OUTLOOKS.  
 
PROGNOSTIC DISCUSSION OF OUTLOOKS - OND 2024 TO OND 2025  
 
TEMPERATURE  
 
ABOVE-NORMAL TEMPERATURES ARE FAVORED THROUGHOUT A MAJORITY OF THE SOUTHERN AND  
EASTERN CONUS AND NORTHWESTERN ALASKA DURING OND. CONVERSELY, BELOW NORMAL  
TEMPERATURES ARE MORE LIKELY FOR MUCH OF SOUTHERN ALASKA. EC OF BELOW, NEAR, OR  
ABOVE NORMAL TEMPERATURES ARE FORECAST FOR THE NORTHWESTERN AND NORTH-CENTRAL  
CONUS. THIS EC AREA IS DUE TO WEAK OR CONFLICTING SIGNALS AMONG TEMPERATURE  
TOOLS. PROBABILITIES OF ABOVE-NORMAL TEMPERATURES ARE REDUCED SLIGHTLY RELATIVE  
TO LAST MONTH ACROSS PARTS OF THE NORTH-CENTRAL CONUS TO THE SOUTHEAST DUE TO A  
TREND TOWARD A COLDER SOLUTION IN THE TEMPERATURE CONSOLIDATION COUPLED WITH A  
LOW CONFIDENCE EC FORECAST FOR THE MONTH OF OCTOBER ACROSS MANY OF THESE AREAS.  
ABOVE NORMAL TEMPERATURES REMAIN LIKELY (GREATER THAN 60 PERCENT) ACROSS THE  
SOUTHWEST, DUE TO GOOD AGREEMENT AMONG BOTH DYNAMICAL AND STATISTICAL GUIDANCE.  
THE AREA OF FAVORED ABOVE-NORMAL TEMPERATURES ARE EXPANDED TO INCLUDE MUCH OF  
CALIFORNIA DUE TO A STRONGER SIGNAL IN THIS MONTH’S CONSOLIDATION. GUIDANCE IS  
SIMILAR ACROSS MUCH OF ALASKA RELATIVE TO LAST MONTH. INCREASED PROBABILITIES  
OF BELOW-NORMAL TEMPERATURES ARE INDICATED FOR SOUTHERN PARTS OF THE STATE,  
AIDED IN PART DUE TO AN ANTICIPATED COOLER THAN NORMAL OCTOBER AND THE  
POTENTIAL IMPACTS OF LA NIñA, ESPECIALLY LATER IN THE SEASON. ABOVE-NORMAL  
TEMPERATURES REMAIN FAVORED FOR NORTHWESTERN ALASKA DUE TO RECENT TRENDS AND  
OBSERVED BELOW NORMAL SEA ICE EXTENT.  
 
FROM NOVEMBER-DECEMBER-JANUARY (NDJ) THROUGH JFM, IMPACTS FROM THE PREDICTED LA  
NIñA CONTINUE AS ABOVE NORMAL TEMPERATURES ARE FAVORED ACROSS THE SOUTHERN  
CONUS AND THE EASTERN SEABOARD WHILE ENHANCED BELOW NORMAL TEMPERATURE  
PROBABILITIES PERSIST ACROSS SOUTHEASTERN ALASKA AND EXPAND NORTHWESTWARD  
TOWARD THE CENTRAL MAINLAND AND SOUTHEASTWARD TO THE NORTHWESTERN AND  
NORTH-CENTRAL CONUS BY DECEMBER-JANUARY-FEBRUARY (DJF). BY FMA, THE POTENTIAL  
IMPACTS OF LA NIñA BEGIN TO WANE AS ENSO-NEUTRAL CONDITIONS BECOME INCREASINGLY  
LIKELY. BELOW-NORMAL TEMPERATURES FAVORED FOR MUCH OF ALASKA DISAPPEAR BY  
APRIL-MAY-JUNE (AMJ) AND FROM ALL OF THE CONUS BY MAY-JUNE-JULY (MJJ).  
THEREAFTER, THE FORECAST PATTERN INCREASINGLY REFLECTS TRENDS WITH ABOVE-NORMAL  
TEMPERATURES GENERALLY FAVORED ACROSS MOST OF THE SOUTHERN AND EASTERN CONUS  
NEXT SPRING, EXPANDING TO MOST OF THE WEST COAST BY THE SUMMER AND TO THE  
REMAINDER OF THE CONUS NEXT FALL. ACROSS ALASKA, A QUICK TRANSITION TOWARD  
MILDER THAN NORMAL CONDITIONS IS PREDICTED WITH ABOVE-NORMAL TEMPERATURES  
FAVORED FOR ALMOST THE ENTIRE STATE BY LATE SPRING AND THROUGH THE SUMMER AND  
MUCH OF NEXT FALL.  
 
PRECIPITATION  
 
MODEL AND STATISTICAL GUIDANCE REMAINS GENERALLY CONSISTENT FROM PREVIOUS  
MONTHS DEPICTING ELEVATED PROBABILITIES OF BELOW NORMAL PRECIPITATION FOR MUCH  
OF THE SOUTHERN CONUS, AND INCREASED CHANCES OF ABOVE NORMAL PRECIPITATION FOR  
THE PACIFIC NORTHWEST, GREAT LAKES, AND PARTS OF THE NORTHEAST DURING OND.  
HOWEVER, DESPITE DYNAMICAL MODELS FAVORING BELOW-NORMAL PRECIPITATION IN THE  
SOUTHEAST, EC IS INDICATED DUE TO THE POTENTIAL FOR TROPICAL ACTIVITY EARLY IN  
THE SEASON AND INCREASED PROBABILITIES FOR A WET OCTOBER. PRECIPITATION  
CONSOLIDATION GUIDANCE HAS TRENDED WETTER ACROSS THE NORTHERN TIER OF THE  
CONUS, RESULTING IN AN EXPANSION OF ELEVATED ABOVE-NORMAL PRECIPITATION  
PROBABILITIES ACROSS PARTS OF THE NORTHERN HIGH PLAINS AND GREAT LAKES RELATIVE  
TO LAST MONTH. HOWEVER, DUE TO A LACK OF DYNAMICAL MODEL SUPPORT, EC REMAINS  
FORECAST ACROSS MOST OF THE NORTH-CENTRAL CONUS. A SLIGHT TILT TOWARD  
BELOW-NORMAL PRECIPITATION IS INDICATED FOR PARTS OF SOUTHEASTERN ALASKA, BASED  
PRIMARILY ON DYNAMICAL MODEL GUIDANCE FROM THE CFSV2 AND A DRY SIGNAL DERIVED  
FROM NATURAL ANALOGS TO THE CURRENT ENSO STATE. CONVERSELY, ABOVE-NORMAL  
PRECIPITATION IS FAVORED FOR NORTHERN AND WESTERN ALASKA DUE TO TRENDS WITH  
SUPPORT FROM C3S AND CBAM OUTPUT. EC IS INDICATED FOR MOST OF THE REST OF THE  
FORECAST DOMAIN AS SIGNALS FOR THE VARIOUS DYNAMICAL AND STATISTICAL TOOLS ARE  
TOO WEAK OR CONFLICTING TO ISSUE A FORECAST WITH A SUFFICIENT DEGREE OF  
CONFIDENCE.  
 
AS WE PROGRESS FURTHER INTO LATE FALL AND THROUGH THE WINTER, DYNAMICAL AND  
STATISTICAL GUIDANCE PERSIST WITH A DRY SIGNAL ACROSS THE SOUTHERN CONUS AND  
SOUTHEASTERN ALASKA, CONSISTENT WITH A LA NIñA SIGNATURE. AN EXPANSION OF  
ENHANCED ABOVE NORMAL PRECIPITATION PROBABILITIES IS NOTED ACROSS THE NORTHERN  
CONUS, PEAKING IN COVERAGE DURING THE WINTER MONTHS. A WET SIGNAL IS ALSO  
FAVORED TO PERSIST ACROSS NORTHWESTERN ALASKA AND EXPAND SOUTHWARD FROM THE  
GREAT LAKES ACROSS MUCH OF THE EAST-CENTRAL CONUS LATER IN THE WINTER THROUGH  
THE SPRING, CONSISTENT WITH DYNAMICAL MODEL GUIDANCE AND TRENDS. THEREAFTER,  
TRENDS BECOME THE DOMINANT FACTOR WITH INCREASED CHANCES OF ABOVE NORMAL  
PRECIPITATION GENERALLY INDICATED ACROSS PARTS OF THE NORTHEAST AND PARTS OF  
THE MID-ATLANTIC IN THE SUMMER AND SHIFTING SOUTHWARD ALONG THE MID-ATLANTIC  
COAST BY EARLY FALL. CONVERSELY, TRENDS FAVOR A DRY PATTERN FOR MUCH OF THE  
SOUTHWEST DURING EARLY SPRING SHIFTING NORTHWARD TO THE NORTHERN ROCKIES BY  
LATE SPRING, WITH INCREASED CHANCES OF DRYNESS FOR PARTS OF THE NORTHERN AND  
CENTRAL PLAINS NEXT SUMMER AS WELL. AS DRYNESS POTENTIALLY EXPANDS INTO THE  
NORTHERN AND CENTRAL PLAINS NEXT SUMMER, A CORRESPONDING WEAK TILT TOWARD ABOVE  
NORMAL PRECIPITATION MAKES A BRIEF APPEARANCE FOR PARTS OF THE SOUTHWEST  
MONSOON REGION DURING JUNE-JULY-AUGUST (JJA) AND JULY-AUGUST-SEPTEMBER (JAS).  
THEREAFTER, THE AREA OF ENHANCED PROBABILITIES OF BELOW-NORMAL PRECIPITATION  
BEGINS TO RECEDE OVER THE WEST-CENTRAL CONUS, BASED PRIMARILY ON COMBINED  
STATISTICAL GUIDANCE FROM TRENDS AND ENSO. BY NEXT FALL, A SMALL AREA OF  
FAVORED ABOVE-NORMAL PRECIPITATION APPEARS IN THE NORTHWESTERN CONUS DUE TO  
COMBINED STATISTICAL GUIDANCE FROM TRENDS AND ENSO. ABOVE-NORMAL PRECIPITATION  
IS ALSO FAVORED FOR MUCH OF NORTHERN ALASKA LATE SPRING AND EARLY SUMMER AND  
SHIFTING TO SOUTHEASTERN ALASKA BY THE FALL, CONSISTENT WITH TRENDS.  
 
FORECASTER: SCOTT HANDEL  
 
THE CLIMATIC NORMALS ARE BASED ON CONDITIONS BETWEEN 1991 AND 2020, FOLLOWING  
THE WORLD METEROLOGICAL ORGANIZATION CONVENTION OF USING THE MOST RECENT 3  
COMPLETE DECADES AS THE CLIMATIC 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.  
 
FOR A DESCRIPTION OF OF THE STANDARD FORECAST TOOLS - THEIR SKILL- AND THE  
FORECAST FORMAT PLEASE SEE OUR WEB PAGE AT  
HTTP://WWW.CPC.NCEP.NOAA.GOV/PRODUCTS/PREDICTIONS/LONG_RANGE/TOOLS.HTML  
(USE LOWER CASE LETTERS)  
INFORMATION ON THE FORMATION OF SKILL OF THE CAS FORECASTS MAY BE FOUND AT:  
HTTP://WWW.CPC.NCEP.NOAA.GOV/PRODUCTS/SOILMST_MONITORING/US/OUTLOOK/OUTLOOK.SHTM  
L  
(USE LOWERCASE LETTERS)  
NOTES - THESE CLIMATE OUTLOOKS ARE INTENDED FOR USE PRIOR TO THE START OF THEIR  
VALID PERIOD. WITHIN ANY GIVEN VALID PERIOD OBSERVATIONS AND SHORT AND MEDIUM  
RANGE FORECASTS SHOULD BE CONSULTED.  
 
THIS SET OF OUTLOOKS WILL BE SUPERSEDED BY THE ISSUANCE OF THE NEW SET NEXT  
MONTH ON OCT 17 2024  
 
1991-2020 BASE PERIOD MEANS WERE IMPLEMENTED EFFECTIVE WITH THE MAY 20, 2021  
FORECAST RELEASE.  

 
 
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