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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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