MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 247 AM EDT SUN SEP 27 2015 VALID SEP 27/0000 UTC THRU SEP 30/1200 UTC ...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR INGEST... ...AN EVALUATION OF THE 00Z MODEL GUIDANCE ALONG WITH FINAL PREFERENCES/FORECAST CONFIDENCE... THERE DID NOT APPEAR TO BE ANY INITIALIZATION ERRORS WITH THE 00Z NAM/GFS DEEMED SIGNIFICANT ENOUGH TO IMPACT THEIR FORECASTS. ...UPPER LOW ANCHORING THE WESTERN GULF COAST... FINAL PREFERENCE: BLEND OF THE 00Z GFS/ECMWF WITH THE 12Z ECMWF ENSEMBLE MEAN FORECAST CONFIDENCE: SLIGHTLY BELOW AVERAGE WATER VAPOR IMAGERY TONIGHT SHOWED A BROAD REGION OF CYCLONIC FLOW ACROSS THE WESTERN GULF OF MX WITH NUMEROUS THUNDERSTORMS ERUPTING UNDERNEATH THE CIRCULATION. THERE IS A GENERAL CONSENSUS THE SYSTEM WILL REMAIN NEARLY QUASI-STATIONARY THE NEXT 48 HOURS BEFORE LIFTING NORTH AND EAST. HOW QUICKLY THIS OCCURS WILL HELP DICTATE THE AMOUNT OF SHORTWAVE ENERGY ABSORBED BY THE ADVANCING NORTHERN STREAM TROUGH. THE ENSEMBLE MEANS...PARTICULARLY THE 21Z SREF/12Z ECMWF ENSEMBLE MEANS ARE CLUSTERED WELL AND ARE TO THE SOUTH OF A COUPLE DETERMINISTIC RUNS...00Z NAM/CMC. CHOOSE TO FOLLOW THE FORECAST SUGGESTED BY THOSE ENSEMBLE SYSTEMS WHICH IS SUPPORTED BY THE 00Z GFS/ECMWF. THE OFFICIAL PREFERENCE WILL BE A COMBINATION OF THE 00Z GFS/ECMWF WITH THE 12Z ECMWF ENSEMBLE MEAN TO ACCOUNT FOR SOME OF THE UNCERTAINTIES. ...COASTAL BAROCLINIC ZONE OVER THE WESTERN ATLANTIC... ...ASSOCIATED SURFACE LOWS OFFSHORE... FINAL PREFERENCE: GENERAL MODEL COMPROMISE FORECAST CONFIDENCE: SLIGHTLY BELOW AVERAGE THE SYNOPTIC PATTERN CURRENTLY FEATURES A STRONG ANTICYCLONE ACROSS NEW ENGLAND WITH GENERAL LOW PRESSURE ANCHORING THE SOUTHEASTERN U.S. COAST. THIS CONTINUES TO SUPPORT BRISK NORTHEASTERLY FLOW ALONG THE EASTERN SEABOARD WITH PRECIPITATION SPREADING ALONG THE COAST. AFTER DAILY ATTEMPTS AT LOW DEVELOPMENT...THE GFS/CMC SOLUTIONS HAVE GENERALLY BACKED OFF FOR THE MOST PART. THE FORECAST DOES BECOME COMPLICATED BY ADDITIONAL SHORTWAVE ENERGY MIGRATING NORTHWARD FROM THE GULF COAST WHICH INDUCES ADDITIONAL REGIONS OF LOW PRESSURE. WILL LEAVE THAT FOR THE FINAL SECTION IN THE PMDHMD. OVERALL...WILL FAVOR A GENERAL MODEL COMPROMISE FOR THE INITIAL POSITION OF THE FRONT AND SURFACE WAVES. ...PROGRESSIVE NORTHERN STREAM TROUGH/SURFACE COLD FRONT SWEEPING THROUGH THE NORTHERN TIER... FINAL PREFERENCE: BLEND OF THE 00Z GFS/ECMWF WITH THE 12Z ECMWF ENSEMBLE MEAN FORECAST CONFIDENCE: AVERAGE THE NORTHERN BRANCH OF THE JET WILL BE PROGRESSIVE AND ACTIVE THE NEXT FEW DAYS. HEIGHT FALLS SLIDING EASTWARD THROUGH THE PACIFIC NORTHWEST MAY LEAVE A BATCH OF VORTICITY BEHIND ACROSS THE PACIFIC NORTHWEST. COMPARED TO THE PAST DAY OR TWO OF SCENARIOS...A MAJORITY OF THE SOLUTIONS FAVOR CARRYING MOST OF THE SHORTWAVES DOWNSTREAM. THE 00Z UKMET DOES CONTINUE TO HANG SOME FORM OF VORTICITY BACK ACROSS WA/OR BUT IT IS DEFINITELY NOT AS STRONG OF A SIGNAL. FOCUSING ON THE PASSAGE OF THE LONGWAVE TROUGH...THE PAST FEW SPAGHETTI PLOTS HAVE SHOWN DIMINISHED SPREAD AS THE GEFS MEMBERS TRENDED QUICKER WHILE THE ECMWF SOLUTIONS DID THE OPPOSITE. IT LOOKS LIKE THE RECENT 00Z GFS MOVED TO A SLIGHTLY QUICKER FORECAST WHICH JOINS THE 00Z ECMWF/12Z ECMWF ENSEMBLE MEAN. MEANWHILE...THE 00Z NAM IS SLOWER AND MORE AMPLIFIED WHICH HELPS DRAW ABUNDANT VORTICITY AND MOISTURE FROM THE GULF NORTHWARD LEADING TO A PRETTY HEFTY RAINFALL EVENT ACROSS THE OH VALLEY/MID-ATLANTIC. DISMISSING THIS SCENARIO IN FAVOR OF A BLEND OF THE 00Z GFS/ECMWF WITH THE 12Z ECMWF ENSEMBLE MEAN. ...CLOSED UPPER LOW LOOMING OFF THE WEST COAST ON TUESDAY/WEDNESDAY... FINAL PREFERENCE: BLEND OF THE 00Z GFS/ECMWF WITH THE 12Z ECMWF ENSEMBLE MEAN FORECAST CONFIDENCE: AVERAGE A BROAD POSITIVELY-TILTED TROUGH ENTERS THE PICTURE LATE IN THE PERIOD AS THE SYSTEM SLOWLY MIGRATES EASTWARD TOWARD THE WEST COAST. THE PAST 4 MODEL CYCLES HAVE FEATURED A DISTINCT SLOWING TREND WHICH IS GOOD TO NOTE AS THE SYSTEM IS GENERALLY CUT-OFF FROM THE PREVAILING WESTERLIES. THE 00Z CMC/UKMET HAVE JOINED THE QUICKER 00Z NAM/21Z SREF MEAN WHICH IS A LITTLE PUZZLING GIVEN THE TREND HAS BEEN GENERALLY BEEN THE OPPOSITE. GIVEN THE FORECAST ISSUE IS A SLOW-MOVING UPPER LOW...PLAN ON STAYING WITH THE WESTERN SIDE OF THE SOLUTION ENVELOPE WHICH IS MORE ALONG THE LINES OF THE 00Z GFS/ECMWF AND 12Z ECMWF ENSEMBLE MEAN. ...AREA OF DISTURBED WEATHER LIFTING OUT OF THE YUCATAN PENINSULA... FINAL PREFERENCE: BLEND OF THE 00Z GEFS/12Z ECMWF ENSEMBLE MEANS FORECAST CONFIDENCE: BELOW AVERAGE AN EXPANSIVE REGION OF SHOWERS/THUNDERSTORMS ACROSS THE YUCATAN PENINSULA IS FORECAST TO RISE NORTHWARD INTO THE GULF OF MX. THE NHC TROPICAL OUTLOOK GIVES THE SYSTEM A 40 PERCENT CHANCE FOR TROPICAL DEVELOPMENT WITHIN THE NEXT 48 HOURS. RECENT MODELS SHOW A POSSIBILITY OF AT LEAST TWO SURFACE LOWS ACROSS THE CENTRAL/NORTHERN GULF OF MX WITH THE ECMWF ENSEMBLE SYSTEM MOST EMPHATIC ABOUT THE WESTERN CAMP WHILE THE GEFS SOLUTIONS FAVOR THE MORE EASTERN LOW. GIVEN THE HIGH UNCERTAINTY OF THIS FORECAST...WILL SPLIT BETWEEN THE PAIR. REGARDLESS OF THE SOLUTION CHOICE...THERE IS QUITE THE HEAVY RAINFALL SIGNAL TO THE NORTH AS THIS TROPICAL MOISTURE MAY INTERACT WITH A RATHER STRONG BAROCLINIC ZONE WITHIN THE NORTHERN STREAM. www.wpc.ncep.noaa.gov/html/model2.shtml... www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml... RUBIN-OSTER