PROGNOSTIC DISCUSSION FOR LONG-LEAD OUTLOOKS
CLIMATE PREDICTION CENTER NCEP
NATIONAL WEATHER SERVICE WASHINGTON DC
3 PM EST THURSDAY FEBRUARY 17 2000
PROGNOSTIC DISCUSSION OF SST FORECASTS
. . . . . . . .
CURRENT CONDITIONS IN THE EAST-CENTRAL EQUATORIAL PACIFIC (120-170W LONGITUDE
AND 5S TO 5N - ALSO CALLED NINO 3.4) INDICATE THAT A MODERATE TO STRONG LA NINA
CONTINUES. SST ANOMALIES IN THE NINO 3.4 AREA CURRENTLY AVERAGE AROUND -1.8
CENTIGRADE DEGREES AND HAVE STRENGTHENED SLIGHTLY FURTHER IN THE PAST MONTH.
ANOMALIES OF AT LEAST -2 C STRADDLE THE EQUATOR FROM ABOUT 125W TO THE DATE
LINE. EAST OF 125W A RELATIVE WARM POOL LIES ALONG THE EQUATOR FLANKED BY
COLD WATER MAINLY TO THE NORTH. ANOMALIES OF -1 C EXTEND TO ABOUT 160E. THE
TRADE WINDS CONTINUE AT ABOVE AVERAGE STRENGTH OVER MOST OF THE WESTERN AND
CENTRAL PACIFIC - AND AT NEAR NORMAL STRENGTH OVER THE EASTERN PACIFIC OCEAN.
THERE IS STILL A SUBSTANTIAL AMOUNT OF COLDER THAN NORMAL SUB-SURFACE WATER
BETWEEN THE DATE LINE AND 90W TO MAINTAIN LA NINA CONDITIONS THROUGH SPRING OF
2000. WARMER THAN NORMAL SUB-SURFACE WATER RESIDING IN THE WESTERN PACIFIC IS
GRADUALLY STRENGTHENING AND EXPANDING EASTWARD - THOUGH THE POOL ITSELF
REMAINS CENTERED IN THE WEST. WITH THE EXCEPTION OF THIS SOMEWHAT STRONGER WARM
SUB- SURFACE POOL - CURRENT CONDITIONS IN THE TROPICAL PACIFIC ARE COMPARABLE
TO THOSE OBSERVED ONE YEAR AGO.
ALL MODEL FORECASTS WERE AVAILABLE - INCLUDING A DYNAMICAL FORECAST FROM THE
ATMOSPHERIC COMPONENT OF THE NCEP COUPLED MODEL AS WELL AS THE CCM3 AND THE
ECHAM-3 GENERAL CIRCULATION MODELS. THE OCEAN FORECAST FROM THE NCEP COUPLED
MODEL AND THE CCA PREDICT THE SSTS IN THE NINO 3.4 REGION TO REMAIN BELOW
NORMAL THROUGH MAY OR JUNE 2000 -- STRONGLY BELOW THROUGH MARCH AND THEN
WEAKENING TO NEAR NEUTRAL BY ABOUT JUNE. CCA THEN PREDICTS A CLIMB TO NEARLY 1C
BY SEPTEMBER AND INTO WINTER 2000-01. THE CONSTRUCTED ANALOG (CA) SST FORECAST
PREDICTS A RETURN TO ZERO ANOMALY BY AUGUST WITH TEMPERATURES BECOMING SLIGHTLY
ABOVE ZERO BY NEXT FALL AND WINTER. A CONSOLIDATION OF THE THREE METHODS
PREDICTS ANOMALIES OF AROUND -1.5 C FOR FMA 2000 - REACHING ZERO BY JULY OR
AUGUST AND THEN MOVING TO POSITIVE VALUES BUT WITH HIGH UNCERTAINTY. THIS
UNCERTAINTY WILL RAPIDLY DIMINISH IN JUNE.
PROGNOSTIC DISCUSSION OF OUTLOOKS - MAM 2000 TO MAM 2001
THE FORECASTS FOR MAM THROUGH JJA 2000 ARE BASED MORE HEAVILY ON DYNAMICAL
MODEL GUIDANCE THAN EVER BEFORE. AVAILABLE TO FORECASTERS WERE THE ATMOSPHERIC
GCM RUN OF THE NCEP COUPLED MODEL - THE ECHAM-3 GCM - AND THE CCM3 GCM. THE
CCA - OCN - SOIL MOISTURE TOOL - ENSO COMPOSITES AND SCREENING MULTIPLE LINEAR
REGRESSION TOOL WERE USED ONLY FOR AUXILIARY GUIDANCE FOR THESE FIRST FOUR
LEAD TIMES BUT WERE RELIED UPON MORE HEAVILY AT LONGER LEADS. THE DYNAMICAL
MODELS STRONGLY AND UNANIMOUSLY INDICATED ABOVE NORMAL TEMPERATURES OVER MUCH
OF THE U.S. DURING THE INITIAL FOUR SEASONS - AND THE FORECASTS REFLECT THIS.
THE CANONICAL LA NINA IMPACTS - AS INDICATED BY THE CCA AND THE COMPOSITES
(PAST HISTORICAL DISTRIBUTIONS OF U.S. T AND P ASSOCIATED WITH MODERATE TO
STRONG LA NINA CONDITIONS) - SUPPORT THE MODELS IN THE SOUTHERN U.S. IN THE
NORTH SOME HEDGING TO NEAR NORMAL IS SHOWN IN THE MAM FORECAST IN NORTHERN
MAINE AND NORTHWEST WASHINGTON STATE. IN AMJ ALL OF THE U.S. IS FORECAST TO BE
SOMEWHAT WARMER THAN NORMAL. MUCH OF ALASKA IS PREDICTED TO SEE A CONTINUATION
OF COLD AND DRY CONDITIONS UNTIL SUMMER - AFTER WHICH TIME A MORE NORMAL
CLIMATE SHOULD RESUME. SOME OF THE DRYNESS FORECAST IN ALASKA IN LATE FALL AND
EARLY WINTER 2000 MAY BE OVERESTIMATED DUE TO A REPORTED LOW BIAS IN RECENT
ALASKAN OBSERVATIONAL PRECIPITATION DATA. AT LONGER LEADS MANY OF THE U.S.
FORECASTS SHOW GREATER WARMTH THAN THEY DID IN LAST MONTH'S FORECAST. THIS IS
LARGELY A RESULT OF MORE RECOGNITION OF 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. 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 DYNAMICAL MODELS INDICATE DRYNESS FARTHER TO THE NORTH THAN IN THE CLASSIC
LA NINA. OUR FORECASTS REFLECT THIS MARKED PRECIPITATION DEFICIENCY ACROSS
LARGE PORTIONS OF THE COUNTRY IN MAM - AMJ - MJJ - AND JJA. THIS FORECAST
IMPLIES THAT THE CURRENT DRYNESS IN THE SOUTHERN U.S. - PARTS OF THE WEST -
THE GREAT PLAINS AND THE MIDWEST - IS LIKELY TO CONTINUE AND POSSIBLY WORSEN.
THE SOUTHEAST AND MID-ATLANTIC AND SOUTH-CENTRAL REGIONS COULD EXPERIENCE
SCANT RAINFALL UNTIL SUMMER. GRADUAL CLIMATE CHANGE IS IN THE DIRECTION OF
HEAVIER BUT LESS FREQUENT RAINFALL - MEANING THAT EVEN AN END TO THE DRY
EPISODE IN TERMS OF RAINFALL TOTALS MAY NOT NECESSARILY MEAN THE END TO THE
DRY SOIL CONDITIONS AS A SIGNIFICANT PORTION OF THE WATER COULD RUN OFF.
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 - AND DRYNESS LIKELY IN A SURROUNDING CRESCENT FROM
TEXAS THROUGH THE DAKOTAS TO THE PACIFIC NORTHWEST. THE HEAVIER THAN NORMAL
RAIN WILL INHIBIT THE NORMALLY EXPECTED ABOVE-NORMAL TEMPERATURES IN THE
DESERT SOUTHWEST ASSOCIATED WITH THE LONG-TERM TREND THERE. NOTE THAT EVEN IF
THE LA NINA HAS DISSIPATED BY SUMMER - A SUBSTANTIAL PART OF THE IMPACT
ON THE MONSOON IS BUILT INTO LAND-ATMOSPHERE PROCESSES THAT ARE A RESULT OF
THE LA NINA DURING THE PREVIOUS WINTER AND SPRING -- I.E. NOW.
SIGNIFICANT CHANGES IN THE FORECASTS FROM LAST MONTHS SET ARE NUMEROUS BECAUSE
OF THE HEAVIER WEIGHTING GIVEN TO THE DYNAMICAL MODELS.
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 MAR 16 2000.
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