PROGNOSTIC DISCUSSION FOR MONTHLY OUTLOOK NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD 300 PM EDT SUN SEP 30 2018 30-DAY OUTLOOK DISCUSSION FOR OCTOBER 2018 THE UPDATE TO THE MID-MONTH OCTOBER TEMPERATURE AND PRECIPITATION OUTLOOK PRIMARILY UTILIZES INFORMATION FROM SHORT-RANGE, MEDIUM-RANGE, EXTENDED-RANGE MODEL GUIDANCE AND CHANGES IN INTEGRATED MONTHLY DYNAMICAL MODEL GUIDANCE INFORMATION FROM MID-MONTH. THE UPDATED OUTLOOKS DEPICTS SOME SUBSTANTIAL CHANGES IN SOME AREAS AS MODEL GUIDANCE HAS BEEN QUITE VARIABLE AT TIMES OVER THE PAST SEVERAL DAYS. THE TEMPERATURE OUTLOOK UPDATE MAINTAINS HIGH PROBABILITIES FOR ABOVE-NORMAL TEMPERATURES FOR MOST OF ALASKA AND FACTORS LISTED AT MID-MONTH HAVE NOT CHANGED AND MODEL GUIDANCE HAS REMAINED CONSISTENT. ANTICIPATION OF ROBUST TROUGHING IMPACTING NORTH AMERICA DESCRIBED EARLIER THIS MONTH CONTINUES TO BE A MAJOR FACTOR IN THE OUTLOOK AS THE ENTRY OF COLDER AIR INTO THE INTERIOR PORTION OF THE CONUS REMAINS FAVORED. THE LATEST GUIDANCE, HOWEVER, NOW INDICATES MORE AMPLIFIED, FULL LATITUDE TROUGHING SHIFTED TO THE WEST-CENTRAL CONUS FOR MUCH OF THE FIRST HALF OF THE MONTH. AREAS NOW MOST FAVORED FOR BELOW-NORMAL TEMPERATURES HAVE INCREASED IN COVERAGE AND SHIFTED WESTWARD TO FOCUS ON THE NORTHERN ROCKIES AND NORTHERN GREAT PLAINS AND TO LESSER CONFIDENCE FOR AN AREA CENTERED ACROSS CALIFORNIA. THIS IS TWO CATEGORY CHANGE IN SOME AREAS OF THE WESTERN CONUS FROM THE PREVIOUS OUTLOOK WHERE RIDGING WAS INITIALLY FAVORED. MIXED SIGNALS OVER THE COURSE OF THE MONTH FROM DYNAMICAL MODEL GUIDANCE MAKES A FORECAST OF EQUAL CHANCES (EC) THE MOST PRUDENT OUTLOOK AT THIS TIME IN THE REMAINING AREAS OF THE WESTERN CONUS. ABOVE-NORMAL TEMPERATURES REMAIN FAVORED FROM THE PREVIOUS OUTLOOK IN THE SOUTHEAST, BUT HAVE BEEN EXPANDED IN COVERAGE AND PROBABILITIES INCREASED SUBSTANTIALLY AS THE ANTICIPATED 500-HPA FLOW PATTERN DURING THE FIRST HALF OF OCTOBER FAVORS STRONGLY AMPLIFIED RIDGING FOR THE EASTERN CONUS. A PARTICULAR CHALLENGE IN THE TEMPERATURE OUTLOOK IS ACROSS THE EASTERN GREAT LAKES REGION AND NORTHEAST WHERE THERE REMAINS UNCERTAINTY FOR HOW MUCH COLDER AIR IS ABLE TO ENTER THIS REGION AFTER MID-MONTH AS INDICATED BY SOME WEEK 3-4 FORECAST MODEL GUIDANCE AND TO THE EXTENT THAT THE EXTENDED-RANGE STRONGLY FAVORED ABOVE-NORMAL TEMPERATURE FORECAST IS EVENTUALLY REALIZED. THE UPDATED OCTOBER PRECIPITATION OUTLOOK CONTINUES TO DEPICT AN ENHANCED LIKELIHOOD FOR ABOVE-NORMAL PRECIPITATION FOR SOUTHWEST ALASKA AND MUCH OF THE SOUTHERN TIER OF THE CONUS, AS WELL AS ENHANCED ODDS FOR BELOW-NORMAL PRECIPITATION FOR THE PACIFIC NORTHWEST, CONSISTENT IN MOST PLACES WITH INFORMATION LISTED AT MID-MONTH. ONE ADDITION IS AN ENHANCED LIKELIHOOD FOR BELOW-NORMAL PRECIPITATION FOR NORTHEAST ALASKA AS A RESULT OF SOME RIDGING IN THIS AREA. HIGH ODDS FOR TROPICAL CYCLONE ROSA AND/OR ITS REMAINING MOISTURE TO IMPACT MUCH OF THE SOUTHWEST CONUS HAVE DRAMATICALLY INCREASED PROBABILITIES FOR ABOVE-NORMAL PRECIPITATION FOR OCTOBER IN THIS AREA AS WELL AS WARRANTING AN EXPANSION OF COVERAGE FOR PARTS OF CALIFORNIA, NEVADA AND THE CENTRAL ROCKIES AS IT INTERACTS WITH A SHORTWAVE TROUGH ENTERING THE PACIFIC COAST EARLY IN THE MONTH. THE ANTICIPATED HIGHLY AMPLIFIED 500-HPA FLOW PATTERN OF A TROUGH - RIDGE PATTERN FROM THE WESTERN CONUS TO THE EASTERN CONUS FAVORS A VERY ACTIVE PATTERN DURING THE FIRST HALF OF OCTOBER WITH A CONSIDERABLY ENHANCED LIKELIHOOD OF STRONG CYCLONES, PRECIPITATION AND ASSOCIATED IMPACTS FOR A RATHER LARGE REGION STRETCHING FROM THE CENTRAL CONUS TO THE GREAT LAKES AND POTENTIALLY NEW ENGLAND AND THE OHIO VALLEY. THIS IS A CONCERN AS THE HIGHEST PROBABILITIES FOR ABOVE-NORMAL PRECIPITATION ARE ANTICIPATED TO BE OVER SOME REGIONS IN THE MIDWEST THAT HAVE SEEN RECENT FLASH FLOODING AND CONSIDERABLY ABOVE-NORMAL STREAMFLOWS. A GENERALLY DRY START TO THE MONTH IN PARTS OF THE MID-ATLANTIC AND SOUTHEAST CONUS ARE OFFSET BY A SHIFT TO POTENTIALLY ELEVATED ODDS OF ENHANCED WETNESS AFTER MID-MONTH SO EC IS FORECAST. ****************************************************************************** ********** PREVIOUS MID-MONTH DISCUSSION IS BELOW ********** ****************************************************************************** ON MOST OCCASIONS, PREDICTABILITY REACHES A GENERAL MINIMUM OVERALL DURING THE MONTH OF OCTOBER FOR THE U.S. FORECAST DOMAIN AS IT IS IN THE MIDDLE OF THE SHOULDER SEASON (SEP-OCT-NOV) THAT RESIDES BETWEEN THE TRANSITION FROM THE SUMMER MONTHS TO THE COLD SEASON. THE HALF-MONTH LEAD MONTHLY FORECAST SUCH AS THE OCTOBER TEMPERATURE AND PRECIPITATION OUTLOOK PRESENTED HERE INCLUDES INFORMATION ACROSS MULTIPLE TIME SCALES (THE EXTENDED RANGE, SUBSEASONAL, AND THE SEASONAL BACKGROUND STATE). THESE FACTORS INFLUENCE THE EVENTUAL OBSERVED MONTHLY MEAN TEMPERATURE AND TOTAL PRECIPITATION AMOUNTS. THE TYPICAL GREATER UNCERTAINTY RESULTS IN A RATHER SIZABLE FORECAST AREA DEPICTING EQUAL CHANCES (EC) FOR THE INITIAL OCTOBER OUTLOOK. A REVIEW OF THE MJO CURRENTLY INDICATES THAT IT REMAINS GENERALLY POORLY DEFINED AND THE ANOMALOUS TROPICAL CONVECTION IN RECENT WEEKS HAS BEEN DOMINATED BY HIGHER FREQUENCY TROPICAL SUBSEASONAL VARIABILITY SUCH AS ATMOSPHERIC KELVIN WAVES AND BURSTS IN TROPICAL CYCLONE ACTIVITY. SOME MODELS DO INDICATE POTENTIAL IMPROVED MJO ORGANIZATION ACROSS THE WESTERN HEMISPHERE AS WE ENTER EARLY OCTOBER, BUT UNTIL THAT PROVES TO BE THE CASE, POTENTIAL MJO INFLUENCE WAS NOT LARGELY UTILIZED IN THE CURRENT OUTLOOK. THE OCTOBER TEMPERATURE OUTLOOK FAVORS ABOVE NORMAL TEMPERATURES FOR ALL OF ALASKA AND MOST OF THE WESTERN CONUS WITH HIGH PROBABILITIES FOR MUCH OF THE ALASKA REGION INCLUDING GREATER THAN 80% PROBABILITY FOR PORTIONS OF NORTHWEST MAINLAND ALASKA. THE TRENDS ASSOCIATED WITH LATE ARRIVAL OF SEA ICE, MUCH ABOVE NORMAL SEA SURFACE TEMPERATURES (SSTS) ALONG MUCH OF THE COASTAL AREAS OF THE STATE, AND STRONG RIDGING AND SOUTHERLY FLOW ANTICIPATED EARLY IN THE MONTH ARE THE BASIS FOR THIS FORECAST. FOR THE WESTERN CONUS, NEARLY UNANIMOUS DYNAMICAL MODEL FORECAST GUIDANCE, LARGE AREAS OF BELOW NORMAL SOIL MOISTURE CONDITIONS AND STRONG LONG TERM TEMPERATURE TRENDS SUPPORT THE FORECAST IN THIS REGION. ABOVE NORMAL TEMPERATURES ARE ALSO MOST LIKELY FOR AREAS OF THE SOUTHEAST, TENNESSEE VALLEY, AND MID-ATLANTIC AS SUBTROPICAL RIDGING, ABOVE NORMAL 500-HPA HEIGHT DEPARTURES, AND SURFACE MEAN SOUTHERLY FLOW ARE STILL ANTICIPATED TO PLAY A ROLE DURING THE MONTH. THE PROBABILITIES ARE MORE MODEST, HOWEVER, AS DYNAMICAL MODEL FORECAST GUIDANCE IS LESS DEFINITIVE IN THIS REGION AND SOME AREAS HAVE SUBSTANTIAL POSITIVE SOIL MOISTURE DEPARTURES AND PERHAPS REMAINING STANDING WATER FROM AN OBSERVED VERY WET SUMMER AND, MOST RECENTLY, IMPACTS FROM HURRICANE FLORENCE. THE MOST CHALLENGING ASPECT OF THE TEMPERATURE OUTLOOK RESIDES ACROSS THE NORTHERN AND CENTRAL PLAINS, GREAT LAKES AND OHIO VALLEY REGIONS. INDICATIONS IN LATE SEPTEMBER ARE THAT A ROBUST RIDGE WILL DEVELOP ACROSS ALASKA WHICH MAY FAVOR DOWNSTREAM TROUGHING ON AVERAGE DURING EARLY PORTIONS OF OCTOBER. THE LATEST WEEK 3-4 FORECAST GUIDANCE FROM THE CFS AND ECMWF FAVOR THIS TO PERSIST SIGNIFICANTLY INTO OCTOBER WITH ELEVATED ODDS FOR BELOW NORMAL TEMPERATURES. IN ADDITION TO THE GENERAL UNCERTAINTY FOR THE FINAL TWO WEEKS IN OCTOBER, THIS IS ALSO AT ODDS WITH NMME DYNAMICAL MODEL GUIDANCE AND LONG TERM TRENDS SO EC IS CURRENTLY FORECAST FOR MUCH OF THIS AREA, HOWEVER, A REGION IS HIGHLIGHTED FAVORING BELOW NORMAL TEMPERATURES IN THE CENTRAL PLAINS AT THIS TIME. AREAS SHOWN IN WHITE LABELED EC DEPICT A FORECAST WHERE THE PROBABILITIES FOR EITHER OF THREE CATEGORIES, BELOW-, NEAR- OR ABOVE-NORMAL MONTHLY MEAN TEMPERATURE ARE NOT SIGNIFICANTLY DIFFERENT FROM CLIMATOLOGICAL PROBABILITIES. THE OCTOBER PRECIPITATION OUTLOOK WAS PARTICULARLY DIFFICULT THIS MONTH AS SIGNALS IN FORECAST GUIDANCE PRODUCTS ARE WEAK AND OFTEN CONFLICTING IN NATURE. THE OUTLOOK DEPICTS AN ENHANCED LIKELIHOOD FOR ABOVE NORMAL PRECIPITATION FOR MOST OF WESTERN AND CENTRAL ALASKA. MORE OPEN WATERS AND ABOVE NORMAL SSTS AS WELL AS GENERALLY CONSISTENT DYNAMICAL MODEL GUIDANCE ARE THE RATIONALE FOR THE OUTLOOK IN THIS REGION. POTENTIAL TROUGHING AS NOTED ABOVE ACROSS THE INTERIOR OF NORTH AMERICA WITH SOME EXTENSION INTO THE CONUS FAVORS ABOVE NORMAL PRECIPITATION FOR SIGNIFICANT PORTIONS OF THE SOUTHERN TIER OF THE CONUS AND THE EASTERN SEABOARD. THIS IS CONSISTENT TO FIRST ORDER WITH THE LATEST WEEK 3-4 DYNAMICAL MODEL GUIDANCE FROM THE ECMWF AND CFS. LONG TERM POSITIVE PRECIPITATION TRENDS ALSO CONTRIBUTE TO THE OUTLOOK ALONG PARTS OF THE EASTERN SEABOARD. ANTICIPATED RIDGING ON AVERAGE DURING THE FIRST HALF OF THE MONTH FAVORS BELOW NORMAL PRECIPITATION IN AREAS OF THE PACIFIC NORTHWEST DOING THE MONTH OF OCTOBER. AREAS SHOWN IN WHITE LABELED EC DEPICT A FORECAST WHERE THE PROBABILITIES FOR EITHER OF THREE CATEGORIES, BELOW-, NEAR- OR ABOVE-NORMAL MONTHLY TOTAL PRECIPITATION ARE NOT SIGNIFICANTLY DIFFERENT FROM CLIMATOLOGICAL PROBABILITIES. FORECASTER: JON GOTTSCHALCK 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. THE NEXT MONTHLY OUTLOOK...FOR NOV ... WILL BE ISSUED ON THU OCT 18 2018 THESE OUTLOOKS ARE BASED ON DEPARTURES FROM THE 1981-2010 BASE PERIOD. $$