MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1243 AM EDT THU SEP 24 2015 VALID SEP 24/0000 UTC THRU SEP 27/1200 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. ...QUASI-STATIONARY UPPER LOW ACROSS THE MIDDLE MS VALLEY/CENTRAL PLAINS... PRELIMINARY PREFERENCE: GENERAL MODEL COMPROMISE FORECAST CONFIDENCE: AVERAGE A CYCLONIC CIRCULATION OVER NORTHERN NEB IS SLOWLY DRIFTING EASTWARD PER RECENT WATER VAPOR TRENDS. THIS FEATURE IS FORECAST TO GRADUALLY SAG SOUTHWARD IN RESPONSE TO THE BROAD MID-LEVEL ANTICYCLONE SETTLING IN ACROSS THE ROCKIES. THERE IS A DECENT CONSENSUS THAT THE SYSTEM WILL SHEAR IN TIME ACROSS KS/OK. GIVEN REASONABLE AGREEMENT AMONG THE MODELS...A GENERAL MODEL COMPROMISE SHOULD SUFFICE. ...GENERALLY WEAK CYCLONIC FLOW OVER THE EAST COAST... ...POTENTIAL SURFACE LOW DEVELOPMENT OFF THE CAROLINAS BY EARLY SUNDAY... PRELIMINARY PREFERENCE: BLEND OF THE 12Z ECMWF/UKMET FORECAST CONFIDENCE: SLIGHTLY BELOW AVERAGE A STAGNANT UPPER LOW ACROSS THE DEEP SOUTH IS FORECAST TO GRADUALLY MOVE TOWARD THE NORTH AND WEST. THE EXACT TIMING/POSITION OF THIS FEATURE APPEARS UNCERTAIN AT THIS TIME WITH A DECENT AMOUNT OF SPREAD NOTED AT 26/1200Z. TO COMPLICATE MATTERS...A SERIES OF SEPARATE VORTICITY MAXIMA ARE EXPECTED TO TRACK UP THE EASTERN SEABOARD WHERE SURFACE CYCLOGENESIS SHOULD TAKE PLACE BY SUNDAY MORNING. THE POSITION OF THIS DEVELOPMENT ALONG WITH WHERE THE BAROCLINIC ZONE SETS UP WILL BE KEY PLAYERS IN WHERE THE HEAVY RAINFALL WILL FOCUS. BASED ON THE CURRENT GUIDANCE SUITE AND ENSEMBLE LOW PLOTS...THE NORTHERN MOST SOLUTIONS APPEAR TO BE LOWER PROBABILITY SOLUTIONS. THIS INCLUDES THE 00Z GFS/12Z CMC ALONG WITH THE 18Z GEFS MEAN. THERE SEEMS TO BE ENOUGH CERTAINTY IN THE FORECAST TO LEAN ON DETERMINISTIC MODELS WITH SOME MIXTURE OF THE 12Z ECMWF/UKMET BEING THE LEADING CANDIDATES AT THIS TIME. ...BROAD UPPER LOW ACROSS TX/WESTERN GULF COAST REGION... PRELIMINARY PREFERENCE: A NON-00Z NAM MODEL COMPROMISE FORECAST CONFIDENCE: AVERAGE TOWARD THE END OF THE PERIOD...MODELS AGREE ON A BROAD UPPER LOW ANCHORING THE LOWER RIO GRANDE/WESTERN GULF COAST. WITH THE EXCEPTION OF THE 00Z NAM WHICH IS DISPLACED SOUTHWARD...THERE APPEARS TO BE REASONABLE MODEL AGREEMENT...AT LEAST IN A BROADER SENSE. WILL SUGGEST SOME FORM OF A NON-00Z NAM MODEL COMPROMISE. ...SOUTHWESTERLY FLOW/EMBEDDED IMPULSES ACROSS THE PACIFIC NORTHWEST... PRELIMINARY PREFERENCE: BLEND OF THE 18Z GEFS/12Z ECMWF ENSEMBLE MEAN...WEIGHTED MORE TOWARD THE LATTER FORECAST CONFIDENCE: BELOW AVERAGE THROUGHOUT THE NEXT FEW DAYS...PERSISTENT MID-LEVEL SOUTHWESTERLY FLOW SHOULD BE THE NORM ACROSS THE NORTHWESTERN CORRIDOR OF THE CONUS. THE BIGGEST COMPLEXITY IN THE FORECAST INVOLVES AN AXIS OF HEIGHT FALLS DRAGGING THROUGH COASTAL WA ON SATURDAY. THERE ARE INDICATIONS THAT SOME OF THIS VORTICITY WILL CONTINUE MIGRATING DOWNSTREAM WHILE REMNANT ENERGY COULD PINCH OFF TOWARD THE EASTERN PACIFIC. BASED ON THE PAST FOUR 564-DM SPAGHETTI PLOTS...THE ECMWF ENSEMBLE SYSTEM FIRST SAW THIS SCENARIO WITH OTHER ENSEMBLE SUITES JOINING IT. GIVEN THE INHERENT SPREAD IN THE FORECAST...THE RECOMMENDATION WILL BE A BLEND OF THE 18Z GEFS/12Z ECMWF ENSEMBLE MEANS WITH MORE WEIGHTING TOWARD THE LATTER GIVEN ITS RECENT CONSISTENCY. www.wpc.ncep.noaa.gov/html/model2.shtml... www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml... RUBIN-OSTER