027  
FXUS07 KWBC 151231  
PMD30D  
PROGNOSTIC DISCUSSION FOR MONTHLY OUTLOOK  
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD  
830 AM EDT THU APR 15 2021  
 
30-DAY OUTLOOK DISCUSSION FOR MAY 2021  
 
THE MAY 2021 OUTLOOKS ARE ISSUED AGAINST THE BACKDROP OF A WEAKENING LA NINA  
AND AN ONGOING STRONG MJO EVENT. THE NINO 3.4 INDEX CURRENTLY STANDS AT -0.5C,  
WHICH REPRESENTS A SIGNIFICANT WEAKENING RELATIVE TO THE VALUE DURING MUCH OF  
THE PAST WINTER WHEN THE AVERAGE INDEX VALUE WAS NEAR -1.0C. THE OCEANIC LA  
NINA SIGNAL IS GENERALLY FORECAST TO WEAKEN FURTHER IN MAY AND ENSO NEUTRAL  
CONDITIONS ARE EXPECTED TO BECOME THE MOST LIKELY SCENARIO AS WE PROGRESS  
DEEPER INTO SPRING. WEAKENING LA NINAS OF THIS MAGNITUDE AND EVOLUTION TEND TO  
RESULT IN BELOW NORMAL HEIGHTS AT HIGH LATITUDES DURING THE MONTH OF MAY,  
INCLUDING MOST OF ALASKA. FARTHER TO THE SOUTH, ABOVE NORMAL HEIGHTS TYPICALLY  
DOMINATE MOST OF THE CONUS UNDER SUCH CONDITIONS. HOWEVER, COMPLICATING MATTERS  
IS AN ONGOING STRONG MJO EVENT IN THE WESTERN PACIFIC WHICH WOULD TEND TO  
COUNTERACT THIS ENSO SIGNAL ACROSS MUCH OF THE FORECAST DOMAIN. SPECIFICALLY,  
COMPOSITES FROM STRONG (DEFINED IN THIS CASE AS A WHEELER-HENDON INDEX  
AMPLITUDE GREATER THAN 1.0) WESTERN PACIFIC MJO EVENTS OCCURRING THIS TIME OF  
YEAR INDICATE INCREASED TROUGHING ACROSS MUCH OF THE WESTERN CONUS AND LESS  
TROUGHING ACROSS ALASKA DURING THE MONTH OF MAY RELATIVE TO THE UNDERLYING ENSO  
SIGNAL. IT SHOULD BE NOTED, HOWEVER, THAT THE SAMPLE SIZE OF THESE MJO  
COMPOSITES IS RELATIVELY SMALL, THUS, REDUCING THE RELIABILITY OF THE SIGNAL.  
ANALYSIS OF THE MOST RECENT MONTHLY AND SUBSEASONAL DYNAMICAL MODEL GUIDANCE,  
RECENT TRENDS, AS WELL AS CONTRIBUTIONS FROM CURRENT ENVIRONMENTAL CONDITIONS,  
INCLUDING SOIL MOISTURE, SNOW COVER AND DEPTH, SSTS, AND SEA ICE EXTENT, WERE  
ALSO UTILIZED IN THE PRODUCTION OF THESE OUTLOOKS. THESE ADDITIONAL INPUTS WERE  
COLLECTIVELY USED TO RESOLVE THE COMPETING BACKGROUND ENSO AND MJO SIGNALS.  
 
ABOVE NORMAL TEMPERATURES ARE FAVORED FOR THE SOUTHERN TWO-THIRDS OF THE CONUS  
AND FOR MUCH OF THE NORTHEAST CONSISTENT WITH ENSO COMPOSITES, TRENDS, AND  
DYNAMICAL MODEL GUIDANCE. HOWEVER, THE ONGOING MJO EVENT WOULD TEMPER  
PROBABILITIES, RESULTING IN GENERALLY WEAKER ABOVE NORMAL TEMPERATURE  
PROBABILITIES FROM THE ROCKIES WESTWARD. PROBABILITIES OF WARMER THAN NORMAL  
CONDITIONS ARE THE GREATEST (EXCEEDING 60 PERCENT) ACROSS THE SOUTHERN PLAINS  
WHERE WARM SIGNALS FROM ENSO COMPOSITES, DYNAMICAL MODEL GUIDANCE, SOIL  
MOISTURE, AND TRENDS CONVERGE WITH THE GREATEST CONFIDENCE. CONFIDENCE IN ABOVE  
NORMAL TEMPERATURES DIMINISH FARTHER TO THE NORTH AS THE CURRENT STRONG MJO  
EVENT SUPPORTS A COLD SIGNAL ACROSS MUCH OF THE NORTHERN TIER, WHICH ALSO  
COINCIDES WITH AREAS WHERE RECENT TRENDS TOWARD WARMER THAN NORMAL CONDITIONS  
ARE AT ITS WEAKEST OR ABSENT ALTOGETHER. THIS COLD MJO SIGNAL DIRECTLY  
CONFLICTS WITH DYNAMICAL MODEL GUIDANCE, WHICH FAVORS ABOVE NORMAL TEMPERATURES  
ACROSS ALMOST THE ENTIRE CONUS. FOR THESE REASONS, EQUAL CHANCES (OR EC) OF  
ABOVE, NEAR, OR BELOW NORMAL TEMPERATURES ARE INDICATED ACROSS MOST OF THE  
NORTHERN TIER OF THE CONUS. ENHANCED PROBABILITIES OF ABOVE NORMAL TEMPERATURES  
ARE ALSO ABSENT FOR MUCH OF ALASKA. DEEP SNOW COVER, LINGERING SEA ICE, AND  
COLD ENSO SIGNALS WILL LIKELY COMBINE TO COUNTERACT RECENT TRENDS TOWARD ABOVE  
NORMAL TEMPERATURES ACROSS MUCH OF THE STATE. ENHANCED PROBABILITIES OF BELOW  
NORMAL TEMPERATURES ARE INDICATED FOR MUCH OF SOUTHERN MAINLAND ALASKA AND THE  
ALASKA PANHANDLE WHERE COLD SIGNALS FROM INCREASED SNOW DEPTH, SEA ICE, ENSO,  
AND DYNAMICAL MODEL GUIDANCE FROM THE NMME, C3S, AND ECMWF CONVERGE. ABOVE  
NORMAL TEMPERATURES ARE FAVORED FOR PARTS OF NORTHERN ALASKA WHERE TRENDS ARE  
THE STRONGEST. ABOVE NORMAL TEMPERATURES ARE ALSO FAVORED FOR THE ALEUTIANS  
WHERE SSTS ARE NEAR TO ABOVE NORMAL AND TRENDS TILT TOWARD WARMER THAN NORMAL  
CONDITIONS.  
 
THE POTENTIAL FOR ENHANCED NORTH TO SOUTH TEMPERATURE GRADIENTS IS LIKELY TO  
PROMOTE AN ACTIVE PATTERN ACROSS PARTS OF THE CENTRAL AND EASTERN CONUS.  
SPECIFICALLY, DYNAMICAL MODEL GUIDANCE FROM THE CFSV2, COMBINED ENSO/MJO  
COMPOSITES, AND TRENDS SUPPORT A WET SIGNAL STRETCHING FROM PARTS OF THE CORN  
BELT EASTWARD ACROSS THE OHIO VALLEY TO THE MID-ATLANTIC. THEREFORE, ABOVE  
NORMAL PRECIPITATION IS FAVORED FOR THESE AREAS. ENHANCED ABOVE NORMAL  
PRECIPITATION PROBABILITIES ALSO EXTEND SOUTHWARD TO INCLUDE MUCH OF THE  
SOUTHEAST DUE TO SUPPORT FROM ENSO/MJO COMPOSITES AND TRENDS. CONVERSELY, BELOW  
NORMAL PRECIPITATION IS FAVORED ACROSS THE SOUTHERN PLAINS WHERE DRY SIGNALS  
FROM THE NMME, C3S, ECMWF, SOIL MOISTURE, AND TRENDS CONVERGE. FARTHER TO THE  
WEST, UNCERTAINTY IS HIGH AS THE DRY PATTERN FORECAST BY DYNAMICAL MODEL  
GUIDANCE DIRECTLY CONFLICTS WITH A STRONG WET SIGNAL PRODUCED BY MJO COMPOSITES  
FROM NORTHERN CALIFORNIA TO PARTS OF THE CENTRAL AND NORTHERN ROCKIES.  
ADDITIONALLY, TRENDS ARE NOT PARTICULARLY STRONG ACROSS THIS REGION. FOR THESE  
REASONS, EQUAL CHANCES OF BELOW, NEAR, AND ABOVE NORMAL PRECIPITATION ARE  
INDICATED ACROSS MOST THE WESTERN CONUS. CONFIDENCE IS SOMEWHAT HIGHER ACROSS  
ALASKA AS DYNAMICAL MODEL GUIDANCE AND ENSO COMPOSITES GENERALLY AGREE ON A DRY  
SIGNAL ACROSS THE SOUTHWESTERN MAINLAND AND THE ALASKA PENINSULA. CONVERSELY,  
ABOVE NORMAL PRECIPITATION IS FAVORED ACROSS PARTS OF THE ALASKA PANHANDLE AND  
EASTERN MAINLAND ALASKA DUE TO A COMBINATION OF WET SIGNALS FROM ENSO  
COMPOSITES, TRENDS, AND DYNAMICAL MODEL GUIDANCE.  
 
FORECASTER: SCOTT HANDEL  
 
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 MAY WILL BE ISSUED ON FRI APRIL 30 2021  
 
THESE OUTLOOKS ARE BASED ON DEPARTURES FROM THE 1981-2010 BASE PERIOD.  

 
 
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