PROGNOSTIC DISCUSSION FOR LONG-LEAD SEASONAL OUTLOOKS NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD 830 AM EDT THU SEP 21 2017 SUMMARY OF THE OUTLOOK FOR NON-TECHNICAL USERS THE OCTOBER-NOVEMBER-DECEMBER (OND) 2017 TEMPERATURE OUTLOOK INDICATES INCREASED CHANCES OF ABOVE-NORMAL TEMPERATURES FOR MOST OF ALASKA AND THE CONTIGUOUS 48 STATES. THE HIGHEST PROBABILITIES FOR ABOVE-NORMAL TEMPERATURES ARE FORECAST FOR PARTS OF THE SOUTHWEST AND THE NORTH SLOPE OF ALASKA (AT OR ABOVE 60-PERCENT). THE OND 2017 PRECIPITATION OUTLOOK INDICATES INCREASED ODDS OF ABOVE-NORMAL PRECIPITATION FOR WESTERN ALASKA, FROM THE NORTH COAST TO THE ALASKA PENINSULA AND THE ALEUTIANS, AS WELL AS FOR THE PACIFIC NORTHWEST AND NORTHERN ROCKIES. ODDS FOR BELOW-NORMAL PRECIPITATION ARE INCREASED, RELATIVE TO CLIMATOLOGY, ACROSS THE MIDDLE AND LOWER MISSISSIPPI VALLEYS AND ACROSS MUCH OF THE SOUTHEAST. THE CURRENT NEUTRAL PHASE OF ENSO IS EXPECTED TO PERSIST THROUGH THE NORTHERN HEMISPHERE AUTUMN, WITH INCREASING CHANCES FOR A TRANSITION TO LA NINA CONDITIONS LATER IN THE AUTUMN AND WINTER 2017-2018. AS THE ENSO OUTLOOK BECOMES MORE CERTAIN DURING THE NEXT COUPLE OF MONTHS, CORRESPONDING ADJUSTMENTS WILL BE MADE TO THE SEASONAL OUTLOOKS. EQUAL CHANCES (EC) ARE FORECAST OVER AREAS WHERE PROBABILITIES OF ABOVE-, BELOW-, OR NEAR-NORMAL SEASONAL MEAN TEMPERATURES AND SEASONAL ACCUMULATED PRECIPITATION AMOUNTS ARE EXPECTED TO BE SIMILAR TO CLIMATOLOGICAL PROBABILITIES. BASIS AND SUMMARY OF THE CURRENT LONG-LEAD OUTLOOKS NOTE: FOR GRAPHICAL DISPLAYS OF THE FORECAST TOOLS DISCUSSED BELOW SEE: HTTP://WWW.CPC.NCEP.NOAA.GOV/PRODUCTS/PREDICTIONS/90DAY/TOOLS/BRIEFING CURRENT ATMOSPHERIC AND OCEANIC CONDITIONS OCEANIC AND ATMOSPHERIC OBSERVATIONS ACROSS THE EQUATORIAL PACIFIC INDICATE ENSO-NEUTRAL CONDITIONS. DURING THE LAST FOUR WEEKS, SEA-SURFACE TEMPERATURES (SST) HAVE GENERALLY DECREASED OVER THE CENTRAL AND EASTERN EQUATORIAL PACIFIC, INTENSIFYING THE NEGATIVE ANOMALIES IN THOSE REGIONS. ACROSS THE WESTERN PACIFIC, POSITIVE ANOMALIES HAVE SLIGHTLY DECREASED IN COVERAGE AND MAGNITUDE. SUBSURFACE TEMPERATURE MEASUREMENTS IN THE CENTRAL AND EASTERN EQUATORIAL PACIFIC REVEAL A LARGE RESERVOIR OF ANOMALOUSLY COLD WATER FROM ABOUT 50-175 METERS BELOW THE SURFACE. THE PRESENCE AND GROWTH OF THIS FEATURE CONTRIBUTE TO THE ASSESSMENT FOR INCREASING ODDS FOR LA NINA DEVELOPMENT IN THE COMING MONTHS. DECREASING UPPER-OCEAN HEAT CONTENT VALUES REFLECT THIS CONTINUED COOLING. OUTGOING LONGWAVE RADIATION (OLR) ANOMALIES DURING LATER AUGUST AND EARLY SEPTEMBER SHOW BELOW-AVERAGE CONVECTION FROM 140E TO 150W, FROM THE EQUATOR TO 10N. DURING THIS SAME PERIOD, THE TRADE WINDS HAVE BEEN ABOVE AVERAGE WEST OF THE DATE LINE WITH REDUCED TRADES OVER THE EASTERN PACIFIC. THE REDUCED TRADES OVER THE EASTERN PACIFIC HAVE BEEN DISPLACED NORTHWARD, SO ARE LIKELY TO HAVE HAD LESS OF AN INFLUENCE ON EQUATORIAL SST VALUES. OVERALL, THE OCEAN AND ATMOSPHERE SYSTEM REMAINS CONSISTENT WITH ENSO-NEUTRAL CONDITIONS. PROGNOSTIC DISCUSSION OF SST FORECASTS THE MOST RECENT PREDICTIONS FROM THE NCEP CLIMATE FORECAST SYSTEM (CFSV2) AND THE NORTH AMERICAN MULTI-MODEL ENSEMBLE (NMME) INDICATE THE FORMATION OF LA NINA AS SOON AS THE NORTHERN HEMISPHERE FALL 2017. THE NMME MEAN INDICATES A NINO 3.4 VALUE (-0.6 DEG C) BELOW THE LA NINA THRESHOLD BY NOVEMBER OF 2017, WITH CONSTITUENT ENSEMBLE MEMBERS VARYING FROM -0.3 TO -1.1 BY DECEMBER OF 2017. THE CONSTRUCTED ANALOG BASED ON SST PATTERNS (SST-CA), CCA, AND MARKOV MODEL INDICATE ENSO NEUTRAL CONDITIONS THROUGH THE NORTHERN HEMISPHERE WINTER. THE CPC CONSOLIDATION PREDICTS ENSO NEUTRAL CONDITIONS THROUGH THE AUTUMN, WITH INCREASING ODDS FOR LA NINA BY WINTER 2017-18. FORECASTERS FAVOR THE NMME PREDICTIONS IN PART BECAUSE OF THE RECENT COOLING OF SURFACE AND SUB-SURFACE TEMPERATURE ANOMALIES, AND ALSO BECAUSE OF THE HIGHER DEGREE OF FORECAST SKILL AT THIS TIME OF YEAR. THE OFFICIAL CPC/IRI OUTLOOK INDICATES A 62 PERCENT CHANCE OF LA NINA DEVELOPMENT DURING NOVEMBER-DECEMBER-JANUARY 2017-18, AN INCREASE OF 36 PERCENTAGE POINTS FROM LAST MONTH. THE OFFICIAL OUTLOOK INDICATES A LIKELY RETURN TO ENSO NEUTRAL CONDITIONS BY NEXT SPRING, MARCH-APRIL-MAY 2018. PROGNOSTIC TOOLS USED FOR U.S. TEMPERATURE AND PRECIPITATION OUTLOOKS THE OUTLOOKS ARE BASED ON DYNAMICAL MODEL GUIDANCE FROM THE NMME SUITE, REGRESSIONS OF TEMPERATURE AND PRECIPITATION ON THE CPC NINO 3.4 CONSOLIDATION, AND TRENDS. THE SST-CA TOOL WAS NOT USED IN THE CREATION OF OUTLOOKS THIS MONTH, AS IT WAS THE ONLY TOOL INDICATING POSITIVE NINO 3.4 VALUES THROUGH WINTER 2017-18. PROGNOSTIC DISCUSSION OF OUTLOOKS - OND 2017 TO OND 2018 TEMPERATURE ABOVE-NORMAL TEMPERATURES ARE FAVORED FOR MUCH OF ALASKA AND FOR MOST OF THE CONTIGUOUS 48 STATES DURING OND 2017. THE ODDS FOR ABOVE-NORMAL TEMPERATURES ACROSS THE NORTHWEST, NORTHERN ROCKIES, AND NORTHERN GREAT PLAINS HAVE BEEN REDUCED FROM LAST MONTH'S OUTLOOK, AS THE INCREASING ODDS FOR LA NINA WOULD FAVOR BELOW-NORMAL TEMPERATURES DURING THE LATTER PORTIONS OF OND. ACROSS SOUTHEAST ALASKA AND THE ALASKA PANHANDLE, AS WELL AS PORTIONS OF THE NORTHERN GREAT PLAINS, ODDS FOR ABOVE-NORMAL TEMPERATURES WERE REDUCED ENOUGH THAT SOME AREAS TRANSITION TO EQUAL CHANCES. THE REDUCTION IN PROBABILITIES FOR ABOVE-NORMAL TEMPERATURES FROM THE NORTHWEST TO THE NORTHERN GREAT PLAINS AND OVER SOUTHEAST ALASKA CONTINUES THROUGH THE MAM 2018 OUTLOOKS, REFLECTING THE POSSIBILITY FOR LA NINA TO INFLUENCE THE TEMPERATURE PATTERN OVER THE FORECAST DOMAIN. THESE SAME SIGNALS, A COLDER FORECAST RELATIVE TO LAST MONTH ACROSS THE NORTHERN CONUS, ARE LARGELY REFLECTED IN THE NMME MODEL SUITE, THOUGH THE DYNAMICAL MODELS ARE SLIGHTLY COLDER FOR THE PACIFIC NORTHWEST AND THE ALASKA PANHANDLE. THE UNCERTAINTY AROUND THE ENSO OUTLOOKS PRECLUDES A COMPLETE SWITCH IN FORECAST CATEGORY DURING THE WINTER OF 2017-18. THROUGH FMA 2018, ODDS FOR ABOVE-NORMAL TEMPERATURES REMAIN INCREASED RELATIVE TO CLIMATOLOGY ACROSS THE SOUTHERN TIER OF THE CONUS, SUPPORTED BY TRENDS AND NMME MODEL OUTPUT. MODEL OUTPUT AND TRENDS DISAGREE OVER THE PACIFIC NORTHWEST (NO TREND BUT BELOW-AVERAGE TEMPERATURES IN THE OUTLOOKS) AND NORTHERN CONUS (COLD TREND BUT EC OR WARM IN MODELS), SO EC IS INDICATED IN THOSE REGIONS, UNTIL WE GET CLOSER IN TIME AND FORECAST CONFIDENCE INCREASES. AS THE LEADS PROGRESS THROUGH 2018, TRENDS ARE LIKELY TO BE THE DOMINANT FACTOR AS ENSO IS LIKELY TO RETURN TO A NEUTRAL STATE. TRENDS FAVOR ABOVE-NORMAL TEMPERATURES FOR ALL OF ALASKA AND MUCH OF THE CONUS, WITH A WEAKNESS NEAR THE GREAT LAKES AND OHIO VALLEY DURING SUMMER, WHICH SHIFTS TOWARD THE SOUTHEAST DURING AUTUMN. PRECIPITATION SIGNIFICANT CHANGES HAVE BEEN MADE TO THE OND 2017 OUTLOOK FOR PRECIPITATION, WHEN COMPARED TO LAST MONTH. THESE CHANGES REFLECT THE RELATIVELY DRAMATIC SWITCH IN MODEL OUTPUTS AS WELL AS THE DECISION TO DISCOUNT THE SST-CA DURING THIS CYCLE. IN GENERAL, PROBABILITIES ARE LOW, DUE TO THE UNCERTAINTY REGARDING THE STATE OF ENSO. THE OND OUTLOOK INDICATES INCREASED ODDS FOR ABOVE-NORMAL PRECIPITATION OVER WESTERN ALASKA, CONSISTENT WITH NMME OUTPUTS AND CONDITIONS ASSOCIATED WITH LA NINA, THOUGH PROBABILITIES ARE LOW. TRENDS, ESPECIALLY AS EXPRESSED DURING THE LATER AUTUMN AND WINTER SEASONS, AND A SLIGHT TILT TOWARD LA NINA, FAVOR BELOW-NORMAL PRECIPITATION FOR THE SOUTHEAST AND PORTIONS OF THE MIDDLE AND LOWER MISSISSIPPI VALLEY. TRENDS FOR DRIER-THAN-AVERAGE CONDITIONS ALONG THE SOUTHERN TIER OF THE CONUS WITH WETTER-THAN-AVERAGE CONDITIONS ACROSS THE NORTHERN TIER, AS WELL AS A TILT TOWARD LA NINA INFLUENCE THE OUTLOOK THROUGH FMA 2018. TRENDS FOR ABOVE-NORMAL PRECIPITATION ARE WEAKEST IN JFM, BUT THAT IS ALSO WHEN ANY LA NINA FOOTPRINT WOULD BE MOST INTENSE. FOR MAM 2018 THROUGH ASO 2018, TRENDS, AS CALCULATED IN THE 15-YEAR OPTIMAL CLIMATE NORMALS (OCN) WERE USED AS THE PRIMARY INPUT. FORECASTER: MATTHEW ROSENCRANS THE CLIMATIC NORMALS ARE BASED ON CONDITIONS BETWEEN 1981 AND 2010, FOLLOWING THE WORLD METEROLOGICAL ORGANIZATION CONVENTION OF USING THE MOST RECENT 3 COMPLETE DECADES AS THE CLIMATIC 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. FOR A DESCRIPTION OF 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/LONG_RANGE/TOOLS.HTML (USE LOWER CASE LETTERS) INFORMATION ON THE FORMATION OF SKILL OF THE CAS FORECASTS MAY BE FOUND AT: HTTP://WWW.CPC.NCEP.NOAA.GOV/PRODUCTS/SOILMST_MONITORING/US/OUTLOOK/OUTLOOK.SHTM L (USE LOWERCASE LETTERS) NOTES - THESE CLIMATE OUTLOOKS ARE INTENDED FOR USE PRIOR TO THE START OF THEIR VALID PERIOD. WITHIN ANY GIVEN VALID PERIOD OBSERVATIONS AND SHORT AND MEDIUM RANGE FORECASTS SHOULD BE CONSULTED. THIS SET OF OUTLOOKS WILL BE SUPERSEDED BY THE ISSUANCE OF THE NEW SET NEXT MONTH ON OCT 19 2017 1981-2010 BASE PERIOD MEANS WERE IMPLEMENTED EFFECTIVE WITH THE MAY 19, 2011 FORECAST RELEASE. $$