MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 251 PM EDT TUE SEP 15 2015 VALID SEP 15/1200 UTC THRU SEP 19/0000 UTC ...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR INGEST... ...AN EVALUATION OF THE 12Z NAM/GFS ALONG WITH FINAL PREFERENCES/FORECAST CONFIDENCE... THERE DID NOT APPEAR TO BE ANY INITIALIZATION ERRORS WITH THE 12Z NAM/GFS DEEMED SIGNIFICANT ENOUGH TO IMPACT THEIR FORECASTS. ...SOUTHWESTERLY FLOW/EMBEDDED SHORTWAVES ACROSS THE ROCKIES AND ADJACENT PLAINS... ...DEEP SURFACE CYCLONE MOVING INTO MANITOBA... FINAL PREFERENCE: GENERAL MODEL COMPROMISE FORECAST CONFIDENCE: ABOVE AVERAGE WATER VAPOR IMAGERY SHOWED A BROAD LONGWAVE TROUGH EXTENDING FROM THE EASTERN PACIFIC TO THE ROCKY MOUNTAIN REGION. ALONG THE EASTERN PERIPHERY OF THIS TROUGH...EMBEDDED SHORTWAVES WITHIN THE SOUTHWESTERLY FLOW REGIME WHICH IS FORECAST TO DRIVE A WAVE OF LOW PRESSURE UP TOWARD SOUTH-CENTRAL CANADA. OVERALL DIFFERENCES ARE QUITE MINOR WITH THIS INITIAL SYSTEM SO WILL FAVOR A GENERAL MODEL COMPROMISE. ...STRONG VORTICITY LOBES EJECTING FROM THE EASTERN PACIFIC THROUGH THE WESTERN U.S... ...AREAS OF LOW PRESSURE/ADVANCING COLD FRONT... FINAL PREFERENCE: COMPROMISE BETWEEN THE 12Z NAM/ECMWF/UKMET FORECAST CONFIDENCE: AVERAGE THE LONGWAVE TROUGH INITIALLY ANCHORING THE EASTERN PACIFIC/WESTERN U.S. AND POINTS EASTWARD WILL SLIDE EASTWARD IN TIME CARRYING A SERIES OF VORTICITY MAXIMA TOWARD THE PLAINS. REGARDING THE INITIAL IMPULSES MOVING THROUGH THE NORTHERN PLAINS/UPPER MIDWEST ON THURSDAY EVENING...THE 12Z GFS TRENDED TOWARD A FASTER SOLUTION WHICH DISAGREES WITH ALL OF THE ENSEMBLE MEANS. LIKEWISE...THE 12Z CMC IS IN THIS FASTER CAMP WHICH LOOKS TO BE A LOWER PROBABILITY SCENARIO AT THIS TIME. LOOKING FARTHER UPSTREAM...THE 12Z CMC FOLLOWS ITS QUICKER SOLUTION BEING FASTER HERE AS WELL...BUT NOT NEARLY AS FAST AS ITS PREVIOUS CYCLE. A COMBINATION OF THE 12Z NAM/ECMWF/UKMET LOOKS REASONABLE HERE SO WILL RECOMMEND A GENERAL COMPROMISE OF THESE SOLUTIONS. ...UPPER LOW LIFTING UP THE EASTERN SEABOARD FROM WEDNESDAY TO FRIDAY... FINAL PREFERENCE: BLEND OF THE 12Z GFS/ECMWF FORECAST CONFIDENCE: AVERAGE A BROAD CIRCULATION ACROSS THE NORTHEASTERN SECTOR OF THE GULF OF MX IS FORECAST TO LIFT NORTHWARD ALONG THE EASTERN SEABOARD EVENTUALLY REACHING THE MID-ATLANTIC OFFSHORE WATERS BY FRIDAY MORNING. WHILE THE EVOLUTION VARIES AMONG THE MODELS...THE RESULT APPEARS TO BE THE SAME. THE ENSEMBLES ARE IN VERY CLOSE AGREEMENT AFTER THE 12Z GEFS MEAN MOVED CLOSER TO THE 00Z ECMWF ENSEMBLE MEAN. GIVEN THE STRONGER SOLUTION CLUSTERING...WILL NOW SELECT A COMBINATION OF DETERMINISTIC MODELS...A BLEND OF THE 12Z GFS/ECMWF. THE CONFIDENCE WAS RAISED ACCORDINGLY. ...POSSIBLE SURFACE LOW DEVELOPMENT IN THE EASTERN GULF OF MX... FINAL PREFERENCE: BLEND OF THE 12Z GFS/GEFS MEAN FORECAST CONFIDENCE: AVERAGE LATER INTO THE FORECAST PERIOD...MODELS HAVE BEGUN TO AGREE ON SURFACE LOW DEVELOPMENT ACROSS THE EASTERN GULF OF MX. THE 12Z NAM/CMC/UKMET ARE THE FIRST PIECES OF GUIDANCE TO SHOW THIS CYCLONE FORMATION...ALBEIT WITH DIFFERENCES IN PLACEMENT. THE 12Z GFS SHOWS A MORE SUBTLE CIRCULATION AT THIS TIME BUT EVENTUALLY SHOWS SLOW DEEPENING OF THE LOW. MULTI-DAY ENSEMBLE LOW PLOTS DO SHOW A TREND TOWARD MORE MEMBERS INDICATING A LOW CENTER IN THE SOUTHEASTERN GULF OF MX. FORTUNATELY THE 12Z ECMWF DID MOVE TOWARD SHOWING A WEAK LOW SOLUTION BUT THIS STILL DOES LOOK TOO DIFFUSE RELATIVE TO OTHER GUIDANCE AND FORECAST TRENDS. STILL FEEL A LOW WILL DEVELOPMENT...BUT PERHAPS NOT AS INTENSE AS THE 12Z NAM/CMC/UKMET. WPC WILL CONTINUE TO FAVOR A COMBINATION OF THE 12Z GFS/GEFS MEAN WHICH IS A GOOD INTERMEDIATE SOLUTION. www.wpc.ncep.noaa.gov/html/model2.shtml... www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml... RUBIN-OSTER