MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1255 PM EDT SUN SEP 04 2016 VALID SEP 04/1200 UTC THRU SEP 08/0000 UTC ...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR INGEST... ...AN EVALUATION OF THE 12Z NAM/GFS ALONG WITH PRELIMINARY PREFERENCES/FORECAST CONFIDENCE... ...POST-TROPICAL CYCLONE HERMINE... PRELIMINARY PREFERENCE: THROUGH 06/0000Z: MODEL COMPROMISE BETWEEN THE 12Z NAM/GFS AND 00Z ECMWF/ECMWF ENSEMBLE MEAN...THEREAFTER: BLEND OF THE 06Z GFS/00Z ECMWF ENSEMBLE MEAN FORECAST CONFIDENCE: --- WHAT REMAINS A FAIRLY STRONG POST-TROPICAL CYCLONE...HERMINE CONTINUES TO CHURN THE WESTERN ATLANTIC WATERS WITH MAXIMUM SUSTAINED WINDS AT HIGH-END TROPICAL STORM FORCE. WHILE ITS CURRENT MOTION IS CARRYING IT AWAY FROM THE COAST AT APPROXIMATELY 10 MPH...MODELS FORECAST THIS MOVEMENT TO HALT AND WITH A SLOW NORTHWARD DRIFT THE NEXT COUPLE OF DAYS. EVENTUALLY SOME FORWARD SPEED IS EXPECTED BY TUESDAY WITH A MORE ABRUPT NORTHEASTWARD TRACK. THERE IS STILL SOME WEST-EAST SPREAD NOTED IN THE NATIONAL HURRICANE CENTER (NHC) FORECAST TRACK. EVALUATING THE AVAILABLE SOLUTIONS...THERE ARE PLENTY OF WESTERN OUTLIERS WHICH IN PARTICULAR INCLUDE THE 00Z CMC/UKMET. THROUGH 06/0000Z...A COMPROMISE BETWEEN THE 12Z NAM/GFS AND 00Z ECMWF/ECMWF ENSEMBLE MEAN WOULD AFFORD A SOLUTION CLOSER TO THE NHC FORECAST. THEREAFTER...THE 12Z NAM/00Z ECMWF BECOME QUICKER WHILE THE 12Z GFS NEARLY STALLS THE SYSTEM ON DAY 3. A COMBINATION OF THE FORMER 06Z GFS AND 00Z ECMWF ENSEMBLE MEAN WOULD BE THE CLOSEST FIT FROM 06/0000Z ONWARD. ...LEAD SHORTWAVE LIFTING OUT OF THE CENTRAL GREAT BASIN INTO THE NORTHERN HIGH PLAINS... ...ACCOMPANYING SLOW-MOVING FRONTAL ZONE/WAVES OF LOW PRESSURE... PRELIMINARY PREFERENCE: A NON-12Z NAM MODEL COMPROMISE FORECAST CONFIDENCE: SLIGHTLY ABOVE AVERAGE WATER VAPOR IMAGERY SHOWED A WELL ESTABLISHED LONGWAVE TROUGH EXTENDING OVER THE WESTERN THIRD OF THE COUNTRY WITH EMBEDDED SPEED MAXIMA ACCELERATING NORTHEASTWARD TOWARD THE NORTHERN ROCKIES. GIVEN STOUT MID/UPPER-LEVEL RIDGING DOWNSTREAM...THE CORE OF STRONGER HEIGHT FALLS WILL LIFT UP INTO THE CENTRAL CANADA WITH GENERAL SLOW PROGRESSION OF THE ATTENDANT SURFACE FRONT. OVERALL DIFFERENCES ARE NOT THAT LARGE EXCEPT FOR THE 12Z NAM BEING DISPLACED NORTHWARD WITH THE LOW-LEVEL CONVERGENCE MAXIMA ACROSS THE UPPER MIDWEST. WILL SUGGEST A NON-12Z NAM MODEL COMPROMISE HERE. ...TRAILING SHORTWAVE ACCELERATING THROUGH THE NORTHERN ROCKIES BY 07/1200Z... PRELIMINARY PREFERENCE: BLEND OF THE 12Z GFS/00Z ECMWF FORECAST CONFIDENCE: AVERAGE AS THE FIRST SYSTEM EJECTS TOWARD THE NORTH-CENTRAL CONUS...ADDITIONAL SHORTWAVE ENERGY BEGINS TO DIG INTO THE WESTERN STATES BY EARLY TUESDAY. A RATHER PROGRESSIVE WAVE DESCENDING SOUTHEASTWARD ATOP THE PERSISTENT CENTRAL/EASTERN PACIFIC UPPER RIDGE WILL HELP FURTHER ESTABLISH THE MEAN TROUGH ACROSS THE WESTERN U.S. LIKE RECENT DAYS...THE RECENT GEFS MEMBERS CONTINUE TO BE MORE AMPLIFIED RELATIVE TO THE CONSENSUS. THIS EVENTUALLY LEADS TO TIMING ISSUES WITH THE TROUGH PROGRESSION. GIVEN THE ECMWF SUITE IS FLATTER...IT FAVORS A MORE PROGRESSIVE SOLUTION. AS THE GFS/ECMWF HAVE BEEN CONSISTENT WITH THEIR RESPECTIVE FORECASTS...WPC WILL PLAY THE MIDDLE GROUND BETWEEN THE TWO BARRING ANY SIGNIFICANT CHANGES IN FUTURE 12Z GUIDANCE. ...POTENTIAL TROPICAL DISTURBANCE RACING NORTHWARD TOWARD THE GULF OF CA/SEA OF CORTEZ... PRELIMINARY PREFERENCE: BLEND THE 12Z GFS/00Z ECMWF FORECAST CONFIDENCE: BELOW AVERAGE AN ORGANIZED AREA OF SHOWERS/THUNDERSTORMS LOCATED OFF THE COAST OF MEXICAN STATES MICHOACAN AND GUERRERO CAN EXPECT A 80 PERCENT CHANCE FOR TROPICAL CYCLONE DEVELOPMENT DURING THE 48 HOURS BASED ON THE LATEST NHC TROPICAL OUTLOOK. BY TUESDAY MORNING...THERE ARE A VARIETY SOLUTIONS NOTED ACCOMPANIED BY MARKED STRENGTH DIFFERENCES. ON THE EASTERN SIDE OF THE SPREAD ARE THE NCEP MODELS...12Z NAM/GFS WHICH REACH THE SOUTHERN TIP OF BAJA CA BY 06/1800Z. ENSEMBLE LOW PLOTS INDICATE THE GEFS MEMBERS CONTINUE TO BE SLIGHTLY QUICKER/EASTWARD DISPLACED DURING THE PAST COUPLE OF MODEL CYCLES RELATIVE TO THE ECMWF SUITE. OFF TO THE WEST ARE THE 00Z CMC/UKMET WHICH EXHIBIT LARGE PLACEMENT DIFFERENCES. UNTIL THE TROPICAL CYCLONE DEVELOPS...MOST SPREAD SHOULD REMAIN ON THE HIGHER SIDE. WITH THAT SAID...WILL SPLIT THE DIFFERENCE BETWEEN THE 12Z GFS/00Z ECMWF WHILE AWAITING FUTURE GUIDANCE AS WELL AS MEDIUM RANGE POINTS AT 17Z FROM THE NHC COLLABORATION CALL. www.wpc.ncep.noaa.gov/html/model2.shtml... www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml... RUBIN-OSTER