MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 241 PM EDT FRI SEP 25 2015 VALID SEP 25/1200 UTC THRU SEP 29/0000 UTC ...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR INGEST... 12Z MODEL EVALUATION WITH PREFERENCES ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ INITIALIZATION ERRORS WITH THE NAM/GFS DO NOT SEEM TO DEGRADE THEIR SHORT RANGE FORECASTS. SHORTWAVE MOVING BY THE NORTHWEST SATURDAY/SUNDAY DEEP CYCLONE SLOWLY APPROACHING NORTHERN CA ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: CANADIAN/ECMWF/NAM COMPROMISE CONFIDENCE: SLIGHTLY ABOVE AVERAGE THE 12Z UKMET/12Z GFS ARE ALONE WITH THEIR HANDLING OF AN UPPER LOW STUCK NEAR WA/OR MONDAY AND TUESDAY. THE SYNOPTIC PATTERN DOES NOT FAVOR ITS EXISTENCE, THOUGH IT DOES FAVOR THE DEEP CYCLONE WEST OF CA. THINK ANYTHING THAT MOVES THROUGH THIS AREA WOULD BE IN THE FORM OF A WEAK SHORTWAVE. OVERALL, PREFER A COMPROMISE OF THE 12Z CANADIAN/12Z ECMWF/12Z NAM WITH SLIGHTLY ABOVE AVERAGE CONFIDENCE. POTENTIAL LOW OFFSHORE THE EAST COAST BEYOND SUNDAY BOUNDARY POTENTIALLY RETROGRADING INTO THE SOUTHEAST MON ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: ECMWF/NAM/UKMET COMPROMISE CONFIDENCE: SLIGHTLY ABOVE AVERAGE BOTH THE 12Z CANADIAN/12Z GFS USE CONVECTIVE FEEDBACK TO MOVE A LOW OFFSHORE THE MID-ATLANTIC AND NEW ENGLAND COASTS, MUCH LIKE YESTERDAY BUT A DAY LATER -- THE FIRST SIGN OF TROUBLE FOR THESE PIECES OF GUIDANCE. ANOTHER SIGN IS THE CANADIAN'S RECENT WEAKENING OF THIS FEATURE BETWEEN ITS 00Z AND 12Z RUNS. ABOUT ONE-THIRD OF 00Z GLOBAL ENSEMBLE MEMBERS HAVE A LOW IN THIS REGION, AND THEIR SOLUTION IS SLOWER THAN THE 12Z GFS. WILL CONTINUE TO TREAT THE GFS/CANADIAN SOLUTIONS AS A LOW PROBABILITY ALTERNATIVE SCENARIO. A COMPROMISE OF THE 12Z ECMWF/12Z UKMET/12Z NAM IS PREFERRED WITH SLIGHTLY ABOVE AVERAGE CONFIDENCE. UPPER LEVEL SYSTEMS MOVING SOUTH THROUGH THE PLAINS AND NORTHWEST INTO THE MISSISSIPPI VALLEY SHORTWAVE/FRONT MOVING BY THE NORTHERN PLAINS SUN/MON ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: GENERAL MODEL COMPROMISE CONFIDENCE: ABOVE AVERAGE THE GUIDANCE SHOWS GOOD AGREEMENT WITH THESE SYSTEMS. UPPER LOW DROPPING THROUGH TX SURFACE LOW IN THE NORTHWEST GULF ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: CANADIAN/NAM/ECMWF COMPROMISE CONFIDENCE: BY DEFAULT/SEE NHC OUTLOOKS WHILE THE GUIDANCE IS AGREEABLE ALOFT, THEIR SURFACE LOW HANDLING IN THE GULF IS QUITE DIFFERENT. THE 12Z UKMET MERGES THIS WITH THE LOW APPROACHING FROM THE SOUTH AND ACCELERATES AHEAD OF THE REST OF THE GUIDANCE -- USUAL BIASES FOR THE UKMET. THE 17Z CONFERENCE CALL WITH NHC SUPPORTED A SOLUTION RESEMBLING A COMPROMISE OF THE 12Z CANADIAN/12Z NAM/12Z ECMWF, WHICH IS PREFERRED BY DEFAULT. POSSIBLE CONVECTIVE LOW MOVING THROUGH THE GULF ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: GFS CONFIDENCE: BY DEFAULT/SEE NHC OUTLOOKS WHILE NHC AT 18Z GAVE THE SYSTEM A 20% CHANCE OF TROPICAL/SUBTROPICAL DEVELOPMENT OVER THE NEXT FIVE DAYS, THE SYNOPTIC PATTERN FAVORS A BROAD AREA OF LOW PRESSURE/POSSIBLE MONSOON DEPRESSION TO MOVE UP FROM CENTRAL AMERICA INTO THE GULF OF MEXICO. THIS IS LIKELY TO BE A SHEARED SYSTEM DUE TO THE UPPER LOW IN PROXIMITY TO ITS EAST. THE GUIDANCE THAT BEST FITS THE NHC/WPC POINTS FROM 17Z TODAY WOULD BE THE 12Z GFS, WHICH IS PREFERRED BY DEFAULT. SEE THE LATEST NHC TROPICAL WEATHER OUTLOOKS FOR MORE ON ITS POTENTIAL AS A TROPICAL/ SUBTROPICAL CYCLONE. www.wpc.ncep.noaa.gov/html/model2.shtml... www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml... ROTH