PROGNOSTIC DISCUSSION FOR LONG-LEAD OUTLOOKS
CLIMATE PREDICTION CENTER NCEP
NATIONAL WEATHER SERVICE WASHINGTON DC
3 PM EST THURSDAY NOV 16 2000
. . . . . . . .
. . BASIS AND SUMMARY OF THE CURRENT LONG-LEAD OUTLOOK
THESE OUTLOOKS REFLECT THE EXPECTED U.S. CLIMATE ANOMALIES FOR ENSO-NEUTRAL
TO BORDERLINE COLD-ENSO CONDITIONS FOR DJF - AND ENSO NEUTRAL CONDITIONS FROM
JFM TO EARLY SUMMER. MOREOVER WE ASSUME A NEAR-NEUTRAL SEASONAL PHASE
OF THE NORTH ATLANTIC OSCILLATION (NAO). THE NAO AND MANY OTHER MODES
OF ATMOSPHERIC INTERNAL VARIABILITY HAVE LOW PREDICTABILITY.
ESTIMATES OF DECADAL TRENDS PLUS ENSO-NEUTRAL AND COLD COMPOSITES THUS
PROVIDE THE BASIS FOR THE SEASONAL OUTLOOKS FOR THE UPCOMING WINTER.
ON AVERAGE - THIS WINTER WILL LIKELY BE COOLER THAN THE LAST THREE - WHICH
WERE EXCEPTIONALLY WARM. NONETHELESS - WE STILL EXPECT GENERALLY WARMER THAN
NORMAL TEMPERATURES FOR THE SOUTHERN HALF OF THE U.S. - COMPARED TO THE
1961-1990 CLIMATOLOGY THAT IS - A NORMAL WHICH WE WILL CONTINUE TO USE TO
COMPUTE ANOMALIES FROM UNTIL EARLY NEXT SPRING WHEN THE 1971-2000 NORMALS WILL
BECOME THE OFFICIAL STANDARD. CONFLICTING FORECAST TOOLS INDICATE THAT
CLIMATOLOGICAL ODDS SHOULD BE USED IN MOST OF THE NORTHERN HALF OF THE U.S
FROM DJF TO FMA. TEMPERATURE AND PRECIPITATION VARIABILITY WILL LIKELY BE
ENHANCED - WHICH IN VIEW OF THE CL FORECAST INDICATES AN INCREASED LIKELIHOOD
OF SIGNIFICANT WINTER WEATHER EXTREMES - INCLUDING COLD AIR OUTBREAKS EXTENDING
INTO THE DEEP SOUTH - SNOWY DAYS IN THE GREAT LAKES AND NORTHEAST - AND DAYS
BELOW 0 DEGREES F IN THE NORTHERN PLAINS AND MIDWEST.
STATISTICAL PREDICTIONS OF NINO3.4 INDICATE THE POSSIBILITY OF A WARM EVENT BY
WINTER 2001/2002 WITH ENOUGH CONFIDENCE TO PLAY A ROLE IN THE TWO
LONGEST LEADS.
. . CURRENT ATMOSPHERIC AND OCEANIC CONDITIONS
NEAR-NORMAL TO SLIGHTLY COLD ATMOSPHERIC AND OCEANIC CONDITIONS PREVAILED IN
THE TROPICAL PACIFIC DURING OCTOBER. WEAK NEGATIVE SST ANOMALIES THAT WERE
FOUND ACROSS PORTIONS OF THE CENTRAL AND EASTERN TROPICAL PACIFIC DURING
SEPTEMBER HAVE STRENGTHENED SOMEWHAT. OCTOBER FEATURED SOMEWHAT STRONGER
THAN NORMAL TRADE WINDS AND CONVECTION REMAINED DEPRESSED NEAR THE DATELINE.
THE OCEANIC THERMOCLINE IS DEEPER-THAN-NORMAL IN THE EQUATORIAL WEST-CENTRAL
AND WESTERN PACIFIC - WITH TEMPERATURES AVERAGING UP TO 3 DEGREES CELSIUS
ABOVE NORMAL AT THERMOCLINE DEPTH. THE NEGATIVE TEMPERATURE ANOMALIES THAT
HAVE CHARACTERIZED THE SUBSURFACE THERMAL STRUCTURE IN THE EASTERN PACIFIC
SINCE LATE 1998 - WHILE WEAKER THAN IN OCTOBER 1999 - HAVE REASSERTED
THEMSELVES SOMEWHAT. DURING SUMMER AND FALL THE PATTERN OF TROPICAL
CONVECTION FEATURED INTRASEASONAL ACTIVITY OVER THE EASTERN INDIAN OCEAN
- INDONESIA - AND THE WESTERN TROPICAL PACIFIC - BUT ALSO REMAINED
GENERALLY CONSISTENT WITH WEAK COLD EPISODE CONDITIONS IN THAT THE
SUPPRESSED CONVECTION NEAR THE DATELINE WAS BROKEN THRUOGH ONLY ONCE
- IN AUGUST.
. . PROGNOSTIC DISCUSSION OF SST FORECASTS
WITH THE INITIAL CONDITIONS SOMEWHAT COLDER THAN A MONTH AGO - THE NCEP
STATISTICAL MODELS (CONSTRUCTED ANALOG (CA) METHOD AND CANONICAL
CORRELATION ANALYSIS (CCA)) CONTINUE TO PREDICT BELOW NORMAL SSTS.
THE NCEP COUPLED MODEL HOLDS TEMPERATURES CLOSER TO NEAR-NORMAL THROUGH
THE END OF THE YEAR. THE MAGNITUDE OF THE FORECAST NEGATIVE SST ANOMALIES
IN CCA AND CA IS ON THE ORDER OF AT MOST 0.5-0.7 DEGREE C FOR NINO 3.4
WHICH IS ENOUGH TO SUGGEST LA NINA CONDITIONS MAY PLAY A
SLIGHT ROLE IN DJF. THIS IS A SLIGHT CHANGE IN THINKING FROM A MONTH AGO
WHEN THE COLD EVENT WAS CONSIDERED MORE DEFINITIVELY OVER.
REMARKABLY - ALL MODELS ARE IN PERFECT AGREEMENT IN FORECASTING A
CROSS-OVER TO POSITIVE SST ANOMALIES IN MARCH 2001. IN FACT ALL
TOOLS HAVE BEEN INDICATING THIS CROSS-OVER IN MARCH 2001 FOR THE
LAST 5 MONTHS. THE QUESTION IS HOW RAPID WILL THE WARMING BE THEREAFTER.
FROM A CLOSE INSPECTION OF THE TOOLS OVER THE LAST SEVERAL
MONTHS IT APPEARS THAT THE FORECAST WARMING IN DJF 2001/2002
INCREASES WITH THE MAGNITUDE OF THE COLD ANOMALIES BEFORE THE
CROSS OVER IN MARCH 2001. CURRENT INCREASES IN THE COLD CONDITION THUS SET
UP A SCENARIO FOR AN INCREASED LIKELIHOOD OF A WARM EVENT IN 2001/2002 - A
CONSIDERATION WE FACTORED INTO THE NDJ AND DJF 2001/2002 U.S. FORECASTS.
THE OFFICIAL SST FORECAST - WHICH IS A CONSOLIDATION OF THE 3 MODELS BASED
ON PAST PERFORMANCE AND CO-LINEARITY - INDICATES BELOW NORMAL NINO3.4
AT EARLY LEADS BUT COMES INTO AGREEMENT WITH ALL TOOLS ON FORECASTING
A RELATIVELY QUICK TRANSITION IN MARCH 2001 AND A GOOD LIKELIHOOD FOR
AN - AT LEAST MILD - WARM EVENT FOR WINTER 2001/2002. FORECASTS THIS
FAR OUT ARE OBVIOUSLY UNCERTAIN. USERS ARE ADVISED TO MONITOR THE
MONTHLY UPDATES OF THESE FORECASTS AS WE GET CLOSER TO NEXT WINTER.
PROGNOSTIC TOOLS USED FOR U.S. TEMPERATURE AND PRECIPITATION OUTLOOK
IN GENERAL FEW CHANGES WERE MADE FORM LAST MONTH'S SET. THE CCA AND SMLR
FORECASTS WERE CONSULTED AT ALL LEAD TIMES AND THE CMP FORECASTS WERE
USED FOR DJF THROUGH MAM. THE PRIMARY TOOL FOR THIS SET OF FORECASTS IS
THE TREND - AS RENDERED BY THE OCN TOOL RELATIVE TO THE 1961-1990
CLIMATOLOGY. SEVERAL ADDITIONAL ESTIMATES OF THE TREND WERE CONSTRUCTED
FOR THE FORECASTS MADE ONE MONTH AGO - INCLUDING A VERSION OF OCN FOR
ENSO-NEUTRAL CONDITIONS - AND A RESIDUAL TREND THAT ACCOUNTS FOR THE
CONTRIBUTIONS OF THE LEADING MODES OF NATURAL VARIABILITY. A SET OF
COMPOSITES OF PRECIPITATION AND TEMPERATURE ANOMALIES FOR ENSO-NEUTRAL
YEARS - IN WHICH A 9-YEAR RUNNING MEAN WAS FIRST REMOVED FROM THE COMPLETE
TIMESERIES - WAS ALSO CONSULTED. ENSO COLD COMPOSITES WERE USED FOR
DJF 2000/2001 AND ENSO WARM COMPOSITES WERE USED FOR DJF 2001/2002. SMALLER
MODIFICATIONS ARE MENTIONED BELOW WHERE RELEVANT.
PROGNOSTIC DISCUSSION OF OUTLOOKS - DJF 2000/2001 TO DJF 2001/2002
THE TEMPERATURE FORECAST FOR DJF 2000 CALLS FOR WARMER-THAN-NORMAL
TEMPERATURES IN THE SOUTHERN ONE THIRD OF THE NATION - PARTICULARLY IN THE
DESERT SOUTHWEST AND THE WESTERN GULF STATES. COMPARED TO A MONTH AGO WE
REDUCED WARMTH ALONG THE WEST COAST BECAUSE OF COLD LOCAL SST -
AND WE REDUCED WARMTH IN THE ROCKIES BECAUSE OF EARLY SNOW COVER
WHICH MAY HELP TO KEEP THE BOUNDARY LAYER COLD EVEN IF THE
MID-TROPOSHERIC CIRCULATION TURNS MILDER.
WETTER-THAN-NORMAL CONDITIONS IN THE SOUTHERN PLAINS AND ALONG PORTIONS
OF THE WESTERN GULF COAST ARE LIKEWISE SUPPORTED BY TREND ESTIMATES FOR
ENSO-NEUTRAL CONDITIONS. BECAUSE OF WEAK LA NINA CONDITIONS WE OPTED FOR
BELOW MEDIAN PRECIPITATION IN SOUTHERN ARIZONA AND FLORIDA. ADDING
THESE TWO BELOW MEDIAN PRECIPITATION AREAS IS A CHANGE FROM LAST MONTH.
THE FORECASTS FOR JFM 2001 THROUGH MAM 2001 REFLECT THE LONG TERM TRENDS
TEMPERED BY COMPOSITES FOR ENSO-NEUTRAL CONDITIONS (IN WHICH THE TREND HAS
BEEN REMOVED). ABOVE NORMAL TEMPERATURES ARE EXPECTED OVER THE SOUTHERN
HALF OF THE LOWER 48 STATES. PROBABILITIES FOR ABOVE NORMAL
TEMPERATURES ARE HIGHEST IN THE SOUTH AND SOUTHWEST WHERE YEAR-TO-YEAR
VARIABILITY IS SMALL IN RELATION TO THE TRENDS. THE BATTLE GROUND IS THE
NORTHEAST. BASED ON THE CMF AND NEUTRAL ENSO COMPOSITES BELOW NORMAL TEMPS
ARE LIKELY - BUT THIS TENDENCY IS OFFSET BY STRONG RECENT TRENDS FOR WARM
WINTERS. AFTER CONSIDERABLE DISCUSSION WE BELIEVE THAT ON BALANCE
CLIMATOLOGICAL ODDS APPLY OVER THE NORTHERN HALF OF THE COUNTRY - MOST
NOTABLY IN THE REGION FROM THE NORTHERN PLAINS TO THE NORTHEAST.
CURRENTLY THERE IS NO CAPABILITY TO FORECAST THE SEASONAL PHASE OF THE
NAO - WHICH INTRODUCES LARGE UNCERTAINTY IN THE FORECASTS ALONG THE
EASTERN SEABOARD. THE TREND TOOLS INDICATE ABOVE MEDIAN PRECIPITATION
EXTENDING FROM THE WESTERN GULF COAST STATES TO THE CENTRAL APPALACHIANS
AND DRYNESS LINGERING OVER FLORIDA THRU JFM. OVER SOUTHERN ALASKA
TEMPERATURES ARE EXPECTED TO REMAIN COLD UNTIL FMA 2001 DUE TO CCA
AFTER WHICH THE TREND TOOLS TAKE OVER AND INDICATE THAT TEMPERATURES
WILL BE ABOVE NORMAL. CLIMATOLOGICAL PROBABILITIES FOR PRECIPITATION
IN ALASKA ARE FORECAST FOR ALL SEASONS.
THE FORECASTS FOR AMJ THROUGH JJA 2001 MOSTLY REFLECT TEMPERATURE AND
PRECIPITATION TRENDS. THE WINTER WARMING TREND CONTINUES INTO SPRING IN THE
WESTERN U.S. - BUT FORECAST PROBABILITIES FOR ABOVE NORMAL TEMPERATURES
IN THE SOUTHEAST U.S. DIMINISH RAPIDLY FROM WINTER TO SPRING ONLY TO
REAPPEAR IN SUMMER. PRECIPITATION TRENDS ARE TOWARD WETTER SPRINGTIME
CONDITIONS IN THE PACIFIC NORTHWEST - AND THIS IS SUPPORTED BY CCA.
OCN INDICATES STRONG WARMING TRENDS IN ALASKA IN THE SPRING AND EARLY
SUMMER WITH THE CCA SUPPORTING THIS BY EARLY SUMMER FOR THE INTERIOR AREAS.
THE FORECAST FOR JAS 2001 TROUGH DJF 2001/2002 SHOWS WARMING TRENDS FOR MUCH
OF THE WEST - AND IN PORTIONS OF THE SOUTHERN STATES - DIMINISHING FROM
JAS TO OND. TRENDS INCREASE THE LIKELIHOOD OF SUB-NORMAL TEMPERATURES
IN THE CENTER OF THE NATION DURING JAS AND ASO. CCA INDICATES BELOW
MEDIAN PRECIPITATION IN JAS AND ASO FOR PORTIONS OF THE PACIFIC NORTHWEST
AND THE GREAT BASIN - WITH SOME SUPPORT FROM OCN.
FOR NDJ AND ESPECIALLY DJF2001/2002 WE STRETCHED THE TREND TOOLS TO LOOK
SOMEWHAT LIKE A WARM EVENT COMPOSITE. MOST NOTABLY WE EXTENDED THE ABOVE
MEDIAN PRECIP FROM TEXAS BACK INTO CALIFORNIA AND EASTWARD AS WELL INTO THE
GULF STATES. DURING WARM ENSO THE TRENDS FOR WARM WEATHER IN THE NORTHEAST
WOULD COME OUT UNABATED AND MAY EXTEND WESTWARD INTO THE UPPER MIDWEST
AND NORTHWEST. ON THE OTHER HAND THE SOUTHEAST MAY NOT BE AS WARM AS
INDICATED BY TREND ALONE. PROBABILTY ANOMALIES ARE MODEST IN ALL CASES
AT A LEAD OF 12.5 MONTHS.
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 DECEMBER 14
2000.
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