MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1251 PM EDT SAT SEP 20 2014 VALID SEP 20/1200 UTC THRU SEP 24/0000 UTC ...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR INGEST... ================================================== 12Z NAM/GFS EVALUATION AND PRELIMINARY PREFERENCES ================================================== NAM/GFS INITIALIZATION ERRORS DO NOT APPEAR TO SIGNIFICANTLY AFFECT THEIR LARGE SCALE OUTPUT. ================================================== ...EASTERN U.S...INCLUDING A COASTAL LOW... PREFERENCE: BLEND OF 12Z NAM/GFS WITH 00Z ECMWF MODELS ARE NOW SIMILAR IN HANDLING THE TRACK AND TIMING OF COASTAL LOW PRESSURE AS IT PARALLELS THE ATALANTIC COAST BEFORE BEING ABSORBED AHEAD OF A SYNOPTIC LOW/FRONT NEAR NEW ENGLAND. DAY 3 PRESENTS SOME INTERESTING QUESTIONS...THOUGH. THE 00Z CANADIAN STOOD OUT AS A STRONG OUTLIER AMONG THE OPERATIONAL MODELS...DEEPENING A BACKSIDE SHORTWAVE OVER THE MID-ATLANTIC...AND PULLING THE OFFSHORE FRONTAL ZONE BACK TOWARD THE SHORE. BUT THERE ACTUALLY WAS SOME SUPPORT FROM THE 00Z ECMWF ENSEMBLE...IN WHICH 10 OF THE 50 MEMBERS PRODUCED A SIMILAR SCENARIO...PREDICING A PRONOUNCED FRONTAL WAVE AND ASSOCIATED PRECIPITATION AFFECTING THE MID ATLANTIC ON DAY 3. THE NUMBER OF ECMWF AND CANADIAN ENSEMBLE MEMBERS DEPICTING THIS HAD INCREASED OVER THE PAST FEW CYCLES. STILL...THE ENSEMBLE MAJORITY FAVORS SOMETHING MORE PROGRESSIVE AND MUCH LESS PRONOUNCED IN THE LATTER PART OF THE SHORT RANGE PERIOD. IT WOULD BE UNEXPECTED FOR A WAVE OF SUCH LARGE SCALE AND ORGANIZATION TO FOLLOW IMMEDIATELY IN THE WAKE OF THE BROAD CYCLONE TAKING SHAPE ON DAYS 1-2. WITH THE 12Z NAM/GFS HOLDING THEIR COURSE...WE STILL FAVOR A NAM/GFS/ECMWF BLEND...BUT ACKNOWLEDGE THAT CONFIDENCE FOR THE DAY 3 FORECAST HAS BEEN REDUCED SOMEWHAT...AND WE WILL NEED TO WATCH THE TRENDS CLOSELY. ...SHEAR AXIS TRAPPED IN THE UPPER RIDGE BUILDING OVER THE SOUTHERN PLAINS... PREFERENCE: BLEND OF THE OPERATIONAL MODELS CONVECTIVE FLARE-UPS AND CONVECTIVELY INDUCED MID LEVEL CIRCULATIONS HAD DIMINISHED IN BREADTH SINCE FRIDAY MORNING. THE REMNANTS OF FORMER TROPICAL CYCLONE ODILE WERE...THEREFORE...SEEN ONLY AS A WEAK 700 MB SHEAR AXIS...AND ONE THAT BECOMES LESS PRONOUNCED WITH TIME. A RESIDUAL PLUME OF RICH MOISTURE WILL CONTINUE TO SUPPORT PRECIPITATION...ESPECIALLY IN THE HIGH PLAINS OF WEST TEXAS AND EASTERN NEW MEXICO. MODEL PRECIPITATION OUTPUT IS SIMILAR ENOUGH TO RECOMMEND A CONSENSUS BLEND...BUT THE NAM MAY BE TOO WET FARTHER EAST ACROSS TEXAS BY DAY 3...WHEN THE BUILDING UPPER LEVEL RIDGE WILL TEND TO SUPPRESS ORGANIZED CONVECTION. ...SYSTEM TRACKING FROM CALIFORNIA TO THE NORTHERN PLAINS... PREFERENCE: ANY OF THE OPERATIONAL MODELS THE OPERATIONAL MODELS SHOW STRONG AGREEMENT AT THE SURFACE AND ALOFT. ...DEEP TROUGH LEANING INTO THE PACIFIC NORTHWEST... PREFERENCE: ANY OF THE OPERATIONAL MODELS...LESS WEIGHT ON THE 00Z CANADIAN A LEAD SHORTWAVE WILL BRING HEIGHT FALLS AND THE ONSET OF LIGHT PRECIPITATION WEST OF THE WA/OR CASCADES ON MONDAY. THE WAVE WILL PUSH A WEAK BOUNDARY INTO THE NORTHERN ROCKIES MON/TUE. A DEEPER LONGWAVE TROUGH WILL APPROACH THE COAST BY LATE TUESDAY. MODEL AGREEMENT IS STRONG...ASIDE FROM THE 00Z CANADIAN BEING SOMEWHAT DEEPER AND WETTER WITH THE LEAD WAVE. www.wpc.ncep.noaa.gov/html/model2.shtml... www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml... BURKE