MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 103 AM EDT SAT SEP 26 2015 VALID SEP 26/0000 UTC THRU SEP 29/1200 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... THERE DID NOT APPEAR TO BE ANY INITIALIZATION ERRORS WITH THE 00Z NAM/GFS DEEMED SIGNIFICANT ENOUGH TO IMPACT THEIR FORECASTS. ...SERIES OF CUT-OFF SYSTEMS ACROSS THE CENTRAL PLAINS...TN VALLEY...AND WESTERN GULF COAST... PRELIMINARY PREFERENCE: FIRST TWO SYSTEMS: GENERAL MODEL COMPROMISE...LATTER SYSTEM: COMPROMISE OF THE 00Z NAM/GFS AND 12Z ECMWF FORECAST CONFIDENCE: AVERAGE WATER VAPOR TONIGHT SHOWS A BROAD REGION OF WEAK MID-LATITUDE FLOW ACROSS THE CENTRAL/EASTERN U.S. THREE KEY FEATURES ARE OF NOTE...A GRADUALLY SHEARING UPPER LOW OVER CENTRAL KS...A BROAD CIRCULATION ANCHORING THE MID-SOUTH...AND LARGE-SCALE CYCLONIC FLOW ACROSS THE WESTERN GULF COAST. ADVANCING FORWARD ABOUT 24 HOURS...SUFFICIENT AGREEMENT IS EVIDENT IN THE GUIDANCE WITH THE INITIAL TWO FEATURES SHEARING IN TIME. REGARDING THE LATTER SYSTEM ACROSS THE WESTERN GULF COAST...THERE IS SYNOPTIC-SCALE AGREEMENT WITH PLACEMENT BUT QUITE A BIT OF VARIABILITY REVOLVING AROUND THE MESOSCALE SHORTWAVES. BY DAY 3...THE 12Z CMC BECOMES MORE COMPACT IN NATURE WHILE THE 12Z UKMET CARRIES THE CIRCULATION TOWARD THE NORTH AND EAST. A COMPROMISE OF THE 00Z NAM/GFS AND 12Z ECMWF APPEARS REASONABLE HERE. ...COASTAL LOW/FRONTAL ZONE ALONG THE WESTERN ATLANTIC... PRELIMINARY PREFERENCE: BLEND OF THE 12Z ECMWF/UKMET FORECAST CONFIDENCE: AVERAGE A STALLED BAROCLINIC ZONE ANCHORING THE WESTERN ATLANTIC WILL SERVE AS A FOCUS FOR DEEP CONVECTION ALONG THE OUTER BANKS. MODELS DEPICT SURFACE LOW DEVELOPMENT...PARTICULARLY THE 00Z GFS/18Z GEFS MEAN AND 12Z CMC. THESE SOLUTIONS CONTINUE TO BE NORTH OF THE OTHER GUIDANCE WITH VERY LITTLE SUPPORT SEEN IN THE ENSEMBLE LOW PLOTS. PREVIOUS CYCLES SHOWED MORE ENSEMBLE LOW CENTERS BUT THE 12Z SUITE SUGGESTS A VERY WEAK SIGNAL. THE PLAN IS TO MAINTAIN A COMBINATION OF THE 12Z ECMWF/UKMET. ...CONVECTIVE LOW/POSSIBLE TROPICAL DEVELOPMENT LIFTING TOWARD THE EASTERN GULF COAST... PRELIMINARY PREFERENCE: BLEND OF THE 12Z ECMWF/ECMWF ENSEMBLE MEAN FORECAST CONFIDENCE: SLIGHTLY BELOW AVERAGE MODELS CONTINUE TO SHOW A BROAD AREA OF DISTURBED WEATHER ACROSS THE SOUTHERN GULF OF MX WITH AN ABRUPT RETURN OF HIGH PWAT AIR TOWARD THE GULF COAST. ENSEMBLE LOW PLOTS CONTINUE TO SUPPORT TWO CAMPS...THE WESTERN SOLUTION FAVORS THE ECMWF MEMBERS WHILE THE EASTERN CLUSTERING IS MOSTLY GEFS MEMBERS. BOTH CLUSTERS HAVE BEEN RATHER CONSISTENT THE PAST 2 DAYS OF MODEL RUNS. WILL LEAN MORE IN THE DIRECTION OF THE ECMWF SUITE AS THE 50 MEMBERS EXHIBIT MUCH TIGHTER CLUSTERING THAN THE GEFS. THE PREFERENCE IS A 12Z ECMWF/ECMWF ENSEMBLE MEAN BLEND. PLEASE VIEW THE LATEST NHC OUTLOOKS FOR MORE INFORMATION ON THE PROSPECTS OF TROPICAL DEVELOPMENT OF THIS SYSTEM. ...PROGRESSIVE HEIGHT FALLS TRACKING THROUGH THE PACIFIC NORTHWEST BY 27/0600Z... ...REMNANT VORTICITY OFF THE WA COAST... PRELIMINARY PREFERENCE: BLEND OF THE 12Z ECMWF WITH THE 18Z GEFS/12Z ECMWF ENSEMBLE MEANS FORECAST CONFIDENCE: SLIGHTLY BELOW AVERAGE AN AREA OF HEIGHT FALLS PEELING OFF A DEEP UPPER LOW ACROSS NORTHWESTERN CANADA WILL MOVE THROUGH THE PACIFIC NORTHWEST BY 27/0600Z. THERE IS SOME UNCERTAINTY IN THIS FORECAST REGARDING HOW MUCH VORTICITY MIGRATES DOWNSTREAM ADJOINING AMPLIFIED FLOW ACROSS CENTRAL CANADA VERSUS PEELING OFF TOWARD THE PACIFIC NORTHWEST. THE 00Z GFS ENDS UP MORE PROGRESSIVE AS IT CARRIES MORE OF THE SHORTWAVE ENERGY EASTWARD WHICH LEADS TO A QUICKER FRONTAL PROGRESSION. THE 12Z ECMWF GARNERS THE MOST SUPPORT FOR THE ENSEMBLE MEANS WHICH CLUSTER RATHER WELL. UTILIZING A SOLUTION LIKE THIS DOES DOWNPLAY THE EXTENT OF A CLOSED LOW LINGERING OVER THE PACIFIC NORTHWEST. GIVEN HOW UNCERTAIN THAT ASPECT OF THE FORECAST IS...WILL CONTINUE TO SHOW A WEAKER SOLUTION. ...BROAD CLOSED LOW OVER THE EASTERN PACIFIC EARLY NEXT WEEK... PRELIMINARY PREFERENCE: BLEND OF THE 18Z GEFS/12Z ECMWF ENSEMBLE MEANS FORECAST CONFIDENCE: SLIGHTLY BELOW AVERAGE TOWARD THE END OF THE PERIOD...THE GUIDANCE SHOW A BROAD UPPER CYCLONE MIGRATING EASTWARD ALONG 40N LATITUDE. THE COMPLICATING FACTOR IS WHETHER THERE IS A CLOSED LOW DOWNSTREAM STILL IN PLACE AS THE 00Z NAM/GFS SHOWED. MULTI-DAY EVALUATIONS OF THE 564-DM SPAGHETTI CONTOURS SHOW A PRETTY CONSISTENT SIGNAL IN THE ECMWF MEMBERS HAVING A CLOSED LOW IN PLACE...ALBEIT WITH A GRADUAL SLOWING TREND. THERE APPEARS TO BE ENOUGH AGREEMENT BETWEEN THE 18Z GEFS/12Z ECMWF ENSEMBLE MEAN TO SUPPORT A MIX OF THE TWO. www.wpc.ncep.noaa.gov/html/model2.shtml... www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml... RUBIN-OSTER