MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1250 AM EDT MON SEP 12 2016 VALID SEP 12/0000 UTC THRU SEP 15/1200 UTC ...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR INGEST... ...AN EVALUATION OF THE 00Z NAM/GFS ALONG WITH PRELIMINARY PREFERENCES/FORECAST CONFIDENCE... ...BAND OF VORTICITY MOVING FROM THE BAHAMAS TO THE EASTERN FL PENINSULA... PRELIMINARY PREFERENCE: BLEND OF THE 00Z GFS/12Z ECMWF FORECAST CONFIDENCE: SLIGHTLY ABOVE AVERAGE A BROAD CLUSTER OF SHOWERS AND THUNDERSTORMS ACROSS THE BAHAMAS WILL CONTINUE TO MIGRATE WESTWARD DURING THE NEXT DAY OR SO. THE MID-LEVEL CIRCULATION SHOULD REACH THE EAST-CENTRAL COAST OF FL BY EARLY TUESDAY MORNING SPREADING HEAVY RAINFALL ACROSS THE REGION. THE GUIDANCE VARY ON THE INTENSITY OF THIS FEATURE WITH THE 00Z NAM AND 12Z CMC/UKMET REMAINING ON THE AGGRESSIVE SIDE. FURTHER...THE 00Z NAM IS FARTHER UP THE COAST MORE TOWARD THE PANHANDLE REGION. WILL STICK WITH A MORE CONSERVATIVE APPROACH LEANING ON A MIX OF THE 00Z GFS/12Z ECMWF. ...PERSISTENT TUTT CELL MEANDERING ABOUT THE WESTERN/CENTRAL GULF OF MX... PRELIMINARY PREFERENCE: A NON-00Z NAM MODEL COMPROMISE FORECAST CONFIDENCE: AVERAGE WATER VAPOR IMAGERY ACROSS THE WESTERN/CENTRAL GULF COAST INDICATED A SLOW-MOVING TUTT CELL MOVING EASTWARD ACROSS SOUTHWESTERN LA. THE GUIDANCE GRADUALLY EXPAND THE CIRCULATION AS IT DIPS SOUTH INTO THE MIDDLE OF THE GULF OF MX. BY TUESDAY MORNING...THERE IS SOME WEST/EAST SPREAD WITH THE 12Z ECMWF/ECMWF ENSEMBLE MEAN ON THE WESTERN FLANK WHILE THE 12Z UKMET SITS WELL TO THE EAST. EXCEPT FOR THE 00Z NAM...ALL SOLUTIONS GRADUALLY WEAKEN THIS FEATURE MOVING TOWARD 14/0000Z. THE 00Z NAM MAINTAINS A HEALTHY 588-DM LOW SOUTH OF THE CENTRAL GULF COAST AT THIS POINT IN TIME. ALTHOUGH THERE WERE SOME SPATIAL DIFFERENCES EARLY ON...FEEL A NON-00Z NAM MODEL COMPROMISE WILL SUFFICE HERE. ...LONGWAVE TROUGH TRANSLATING ALONG THE NORTHERN TIER OF THE COUNTRY THIS WEEK... ...ATTENDANT SURFACE COLD FRONT/WAVES... PRELIMINARY PREFERENCE: BLEND OF THE 00Z GFS/12Z ECMWF FORECAST CONFIDENCE: AVERAGE A POSITIVELY-TILTED TROUGH INITIALLY OVER WESTERN/CENTRAL CANADA DOWN INTO THE UPPER INTERMOUNTAIN WEST WILL MOVE SWIFTLY TOWARD THE EAST. THIS UPPER TROUGH WILL SPLIT WITH THE WESTERN FRINGE OF THE SHORTWAVES SINKING DOWN TOWARD THE CENTRAL GREAT BASIN/SOUTHWESTERN STATES. MEANWHILE...THE REMAINING HEIGHT FALLS WILL MIGRATE TOWARD THE EAST WHILE BEING REINFORCED BY A HIGHER LATITUDE IMPULSE CROSSING THE NORTH-CENTRAL U.S. THERE HAS DEFINITELY BEEN A SLOWING TREND COMPARING THE ENSEMBLE SPAGHETTI PLOTS FROM THE PAST FEW MODEL CYCLES...PARTICULARLY FROM RUNS 36 HOURS AGO. RIGHT NOW...ON THE EAST SIDE OF THE SOLUTION SPREAD ARE THE 12Z CMC/UKMET WHILE THE 00Z NAM BECOMES SLOW LATER ON. A COMBINATION OF THE 00Z GFS/12Z ECMWF SEEMS REASONABLE AT THIS POINT AS IT SITS IN THE CENTER OF THE SPREAD...PARTICULARLY AS THE 12Z CMC/UKMET BECOME 6 TO 12 HOURS FAST BY DAY 3. ...INITIAL IMPULSE KICKING OUT OF THE DESERT SOUTHWEST EARLY IN THE PERIOD... ...CLOSED LOW BARRELING THROUGH THE SOUTHWESTERN CONUS ON TUESDAY/WEDNESDAY... PRELIMINARY PREFERENCE: BLEND OF THE 12Z ECMWF/UKMET WITH THE 12Z ECMWF ENSEMBLE MEAN FORECAST CONFIDENCE: SLIGHTLY ABOVE AVERAGE INITIALLY...AN OPEN WAVE WILL TRACK FROM INTERIOR CA TOWARD SOUTHERN UT DURING THE NEXT 24 HOURS WITH ONLY MINOR DIFFERENCES NOTED. TO THE NORTH...EVOLVING SPLIT FLOW WILL CARRY A ROBUST POSITIVELY-TILTED TROUGH THROUGH THE CENTRAL GREAT BASIN AND DOWN INTO THE SOUTHWESTERN STATES AS A CLOSED LOW BY 13/1200Z. RELEVANT MODELS HAVE BEEN RATHER STABLE IN THE INITIAL EVOLUTION LOOKING AT THE PAST 36 HOURS OF RUNS. THERE IS SOME SOLUTION SPREAD BY 14/1200Z WITH ENSEMBLE MEMBERS SCATTERED ABOUT. BUT THE DETERMINISTIC/ENSEMBLE RUNS CLUSTER WELL OVER NORTHERN NV ON WEDNESDAY MORNING. MODEL DIFFERENCES DO INCREASE AS THE CLOSED LOW OPENS UP AND HEIGHT FALLS SPREAD EASTWARD TOWARD THE HIGH PLAINS. THE 18Z GEFS MEAN MOVES A BIT AHEAD OF THE SLOWER 21Z SREF/ECMWF ENSEMBLE MEANS BY DAY 3. GIVEN A SLOW-MOVING CLOSED LOW IN THE FORECAST...WILL ERR ON THE WESTERN END HERE WHICH IS MORE ALONG THE LINES OF THE 12Z ECMWF/UKMET AS SUPPORTED BY THE 12Z ECMWF ENSEMBLE MEAN. www.wpc.ncep.noaa.gov/html/model2.shtml... www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml... RUBIN-OSTER