PROGNOSTIC DISCUSSION FOR LONG-LEAD OUTLOOKS
CLIMATE PREDICTION CENTER NCEP
NATIONAL WEATHER SERVICE WASHINGTON DC
3 PM EDT THURSDAY MAY 18 2000
PROGNOSTIC DISCUSSION OF SST FORECASTS
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NEGATIVE SST ANOMALIES OF AT LEAST -1 C CONTINUE TO STRADDLE THE EQUATOR FROM
ABOUT 130W WESTWARD TO 155E. HOWEVER - WEAK POSITIVE SST ANOMALIES THAT
DEVELOPED 1 TO 2 MONTHS AGO CONTINUE EAST OF ABOUT 130W. THE SST ANOMALY
IN THE NINO 3.4 REGION (5N-5S / 120-170W) DURING THE LAST WEEK (CENTERED ON 7
MAY) IS ABOUT -0.5 C - AND IN THE NINO 3 REGION (90-150W) IS ABOUT +0.3 C. THIS
WARMING IS CONFINED TO A THIN LAYER OF WATER NEAR THE SURFACE - WITH THE OVERALL
SUBSURFACE THERMAL STRUCTURE (NEGATIVE SUBSURFACE TEMPERATURE ANOMALIES IN THE
EAST AND STRONG POSITIVE SUBSURFACE TEMPERATURE ANOMALIES IN THE WEST)
CONTINUING TO INDICATE MATURE LA NINA CONDITIONS. THE SHALLOW WARM WATER IN THE
EAST IS EXPECTED TO BE REPLACED BY COOL SUBSURFACE WATER - WHICH SHOULD
RESURFACE IF CROSS-EQUATORIAL FLOW (FROM THE SOUTH) OCCURS. SUCH FLOW IS AL-
READY BEGINNING TO BE OBSERVED - AND IS CLIMATOLOGICALLY FAVORED AT THIS TIME
OF YEAR. THE NOW VERY WARM WESTERN PACIFIC SUBSURFACE WATER - THE NEXT EL NINO
IN THE MAKING - HAS NOT SHOWN SIGNS OF MIGRATING EASTWARD DURING THE SPRING
TIME-WINDOW. SIGNIFICANT MIGRATION WOULD NEED TO OCCUR BY EARLY JUNE IN ORDER
TO INITIATE EL NINO CONDITIONS FOR THE SECOND HALF OF YEAR 2000.
FEATURES OF THE ATMOSPHERIC CIRCULATION ALSO CONTINUE TO INDICATE CONTINUING
MATURE LA NINA CONDITIONS. THE SOI FOR APRIL WAS 1.2 (AFTER 1.0 IN MARCH)
- AS SEA LEVEL PRESSURE (SLP) REMAINED ABOVE NORMAL OVER THE EASTERN HALF
OF THE TROPICAL PACIFIC AND BELOW NORMAL THROUGHOUT THE EASTERN INDIAN OCEAN
AND INDONESIA. CONSISTENT WITH THIS PATTERN OF SLP ANOMALIES - THE LOW-LEVEL
EASTERLIES REMAIN STRONGER THAN NORMAL OVER MOST OF THE WESTERN AND WEST-
CENTRAL EQUATORIAL PACIFIC - AND CONVECTION CONTINUES TO BE ENHANCED OVER
INDONESIA AND SUPPRESSED OVER THE CENTRAL EQUATORIAL PACIFIC. ALSO - THE ABOVE
NORMAL SST OVER THE EASTERN PACIFIC IS NOT HIGH ENOUGH TO INITIATE CONVECTION.
ALL MODEL FORECASTS WERE AVAILABLE - INCLUDING A DYNAMICAL FORECAST FROM THE
ATMOSPHERIC COMPONENT OF THE NCEP COUPLED MODEL (BOTH A CONTROL RUN AND A RUN
WITH INTERACTIVE SOIL MOISTURE) AS WELL AS THE ECHAM-3 AND CCM3 GENERAL CIRCU-
LATION MODELS. THE OCEAN FORECAST FROM THE NCEP COUPLED MODEL PREDICTS THE
SSTS IN THE NINO 3.4 REGION TO REMAIN BELOW NORMAL THROUGH JUNE 2000 -- WEAKEN-
ING TO NEAR NEUTRAL DURING JULY-SEPTEMBER AND SLIGHTLY ABOVE NORMAL OCTOBER
THROUGH THE END OF 2000. CCA PREDICTS JUST A BIT COOLER WITH A RISE TO MORE
THAN 0.2 C ABOVE NORMAL SST NOW DELAYED UNTIL SPRING 2001. THE CONSTRUCTED
ANALOG (CA) SST FORECAST PREDICTS CONTINUED LA NINA CONDTIONS THROUGH DECEMBER
2000 WITH A SLOW RETURN TO A ZERO ANOMALY BY MARCH 2001 - AND WEAK POSITIVE
ANOMLIES DEVELOPING DURING SPRING 2001. A CONSOLIDATION OF THE THREE SST FORE-
CASTS PREDICTS A WEAKENING OF SST ANOMALIES TO SLIGHTLY BELOW ZERO FROM JAS
2000 TO JFM 2000 - FOLLOWED BY SUBSTANTIAL WARMING BUT WITH HIGH UNCERTAINTY
IN SPRING AND SUMMER 2001. GIVEN THE CONTINUED LACK OF ANY CURRENT SUBSTANTIAL
EVOLUTION IN THE SUBSURFACE THERMAL STRUCTURE AND THE PERSISTENT LA NINA-
RELATED ATMOSPHERIC CIRCULATION ANOMALIES EVEN INTO MID-MAY (THE LAST PORTION
OF THE TIME WINDOW FOR MAJOR ENSO PHASE SHIFTS) - IT NOW SEEMS LIKELY THAT AT
LEAST WEAK LA NINA CONDITIONS WILL CONTINUE TO BE REFLECTED IN THE GLOBAL ATMOS-
PHERIC CIRCULATION THROUGH THE REMAINDER OF 2000. THIS LIKELIHOOD WILL BECOME A
NEAR-CERTAINTY BY MID-JUNE IF NO SIGNIFICANT CHANGES OCCUR IN THE PACIFIC.
PROGNOSTIC DISCUSSION OF OUTLOOKS - JJA 2000 TO JJA 2001
THE FORECASTS FOR JJA THROUGH SON 2000 ARE BASED HEAVILY ON DYNAMICAL MODEL
GUIDANCE. THE THREE DYNAMICAL MODELS MENTIONED ABOVE WERE AVAILABLE TO
FORECASTERS. THE CCA - OCN - CONSTRUCTED ANALOGUE SOIL MOISTURE TOOL (CAS)
- AND SCREENING MULTIPLE LINEAR REGRESSION TOOL (WHICH NOW INCLUDES SOIL
MOISTURE AS ONE OF THE PREDICTORS) WERE USED ONLY FOR AUXILIARY GUIDANCE
FOR THESE FIRST FOUR LEAD TIMES BUT WERE RELIED UPON MORE HEAVILY AT LONGER
LEADS. THE ENSO COMPOSITES WERE USED ONLY SLIGHTLY DUE TO THE WEAKNESS OF THE
LATE SPRING AND SUMMER IMPACTS FOR THE FIRST LEADS AND THE WEAKNESS OF THE ENSO
OUTLOOK ITSELF FOR THE LONGER LEADS. THE ENSO STATE IS EXPECTED TO BE IN THE
LA NINA DIRECTION ONLY WEAKLY THROUGH THE REMAINDER OF 2000. THE DYNAMICAL
MODELS INDICATED ABOVE NORMAL TEMPERATURES OVER MUCH OF THE U.S. DURING THE
INITIAL FOUR SEASONS - PARTICULARLY THE ECHAM MODEL AND THE NCEP MODEL WITH
INTERACTIVE SOIL MOISTURE. ON AVERAGE - THE REGION WITH THE WARMEST FORECAST
WAS THE CENTRAL AND EAST-CENTRAL U.S. THE CAS TOOL STRONGLY CONCURRED AS WELL.
THE LATEST MODEL FORECASTS HAVE BACKED OFF ON THE COLDNESS PREVIOUSLY
INDICATED FOR ALASKA FOR SUMMER 2000. AT LONGER LEADS - NEGATIVE TEMPERATURE
ANOMALIES ARE FORECAST BY THE STATISTICAL TOOLS IN NORTHWESTERN ALASKA. ONE
OF THE GOVERNING FACTORS FOR ALASKAN TEMPERATURE - THE SOUTHERN COASTAL SST -
HAS BEEN NEAR NORMAL ALONG THE IMMEDIATE COAST AND BELOW NORMAL FARTHER AWAY
FROM THE COAST. THE LATTER FEATURE IS ASSOCIATED WITH THE 2 YEAR OLD LA NINA.
AT THE LONGEST LEAD TIMES MANY OF THE FORECASTS SHOW WARMTH. THIS IS FROM TRENDS
THAT HAVE BEEN OCCURRING MORE RELIABLY OVER THE LAST SEVERAL YEARS. THIS WARMTH
IS INDICATED BY MANY OF THE STATISTICAL TOOLS AT INCREASINGLY USABLE LEVELS OF
ESTIMATED SKILL. THE DESERT SOUTHWEST IN PARTICULAR HAS HAD INCREASES IN THE
MEAN TEMPERATURE THAT ARE LARGE COMPARED TO THE YEAR-TO-YEAR VARIABILITY -
RESULTING IN ABOVE NORMAL TEMPERATURE FORECASTS NEARLY EVERY FORECAST. WHEN
THE BASE PERIOD USED TO DEFINE THE NORMAL CLIMATE ARE CHANGED FROM 1961-90 TO
1971-2000 NEXT YEAR - THIS COMPONENT OF THE FORECAST WILL PLAY A LESSER ROLE.
THE DRYNESS THAT HAS BEEN OCCURRING IN THE MIDWEST - THE SOUTHERN MISSISSIPPI
VALLEY AND PARTS OF THE SOUTHEAST APPEARS LIKELY TO CONTINUE AT LEAST INTO
EARLY SUMMER. WHILE THE LA NINA COMPOSITES IMPLY WETNESS IN PARTS OF THESE
REGIONS - AND SOME CURRENTLY DRY LOCALITIES MAY RECEIVE GOOD RAINFALL DUE TO
SYNOPTIC-SCALE ACTIVITY AND REGIONALLY FOCUSED MCS ACTIVITY - THERE IS NO
INDICATION THAT THE LARGELY DRY CURRENT REGIME WILL COME TO AN END IN A BROAD
SENSE IN THE NEXT 1 TO 2 MONTHS. THE PRESENTLY DEFICIENT SOIL MOISTURE CONDI-
TIONS IN THESE REGIONS TEND TO FAVOR CONTINUATION OF WARMTH AND DRYNESS - AT
LEAST IN THE NEAR TERM. BY MID-SUMMER THE CONTINUATION OF DRYNESS IN THE
SOUTHERN TIER OF STATES WILL BECOME INCREASINGLY UNLIKELY WITH AN ALREADY
ABOVE NORMAL GULF OF MEXICO SST AND THE EXPECTATION OF AN ACTIVE ATLANTIC
HURRICANE SEASON FROM AUGUST TO NOVEMBER. THE DRYNESS IN THE MIDWEST APPEARS
TO BE THE MOST LIKELY TO CONTINUE - AS SHOWN IN THE FORECASTS THROUGH OND.
IN JULY AND AUGUST ENHANCED SOUTHWEST MONSOON RAINFALL IS ANTICIPATED AS A
RESULT OF THE LA NINA - WITH ABOVE MEDIAN RAINFALL IN ARIZONA - NEW MEXICO AND
SOUTHERN COLORADO AND UTAH. ENHANCED SOUTHWEST MONSOON RAINFALL IS USUALLY
ASSOCIATED WITH A CONTINENTAL SCALE PATTERN OF RAINFALL ANOMALIES FEATURING
DRIER THAN NORMAL IN THE MIDWEST AND GREAT PLAINS - AND WETTER THAN NORMAL
CONDITIONS ALONG THE COASTAL PLAINS OF THE SOUTHEAST AND MID-ATLANTIC.
FOR A DESCRIPTION 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/
MULTI-SEASON/13_SEASONAL_OUTLOOKS/TOOLS
NOTE - THESE CLIMATE OUTLOOKS ARE INTENDED FOR USE PRIOR TO THE START OF THEIR
VALID PERIODS. WITHIN ANY GIVEN VALID PERIOD OBSERVATIONS AND SHORT AND MEDIUM
RANGE FORECASTS SHOULD BE CONSULTED. ALSO - THIS SET OF OUTLOOKS WILL BE
SUPERSEDED BY THE ISSUANCE OF THE NEW SET NEXT MONTH ON THURSDAY JUNE 15 2000.
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