MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1226 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 NAM/GFS EVALUATION WITH PRELIMINARY MODEL 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 SHORTWAVE/FRONT MOVING BY THE NORTHERN PLAINS SUN/MON ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: CANADIAN/ECMWF COMPROMISE CONFIDENCE: SLIGHTLY ABOVE AVERAGE THE 00Z UKMET IS AN OUTLIER WITH A CLOSED LOW MOVING ACROSS WA/OR SUNDAY. SINCE IT IS MOVING OVER THE MEAN RIDGE POSITION OUT WEST AND UNDER THE BASE OF A TROUGH MOVING ACROSS WESTERN CANADA, IT IS LIKELY TOO STRONG. THE 12Z GFS IS AN OUTLIER WITH 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. THE 12Z NAM IS TOO QUICK WITH THE TROUGH MOVEMENT ACROSS CANADA, WHICH LEADS TO TOO QUICK OF A FRONTAL PROGRESSION INTO THE GREAT LAKES. OVERALL, PREFER A COMPROMISE OF THE 00Z CANADIAN/00Z ECMWF 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 00Z 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. ABOUT ONE-THIRD OF 00Z GLOBAL ENSEMBLE MEMBERS HAVE A LOW IN THIS REGION, AND THEIR SOLUTION IS SLOWER THAN THE 12Z GFS. UNLESS OTHER PIECES OF GUIDANCE CAN DEVELOP THIS SYSTEM, WILL CONTINUE TO TREAT THEIR SOLUTIONS AS A LOW PROBABILITY ALTERNATIVE SCENARIO. A COMPROMISE OF THE 00Z ECMWF/00Z UKMET/12Z NAM IS PREFERRED WITH SLIGHTLY ABOVE AVERAGE CONFIDENCE. UPPER LEVEL SYSTEMS MOVING SOUTH THROUGH THE PLAINS AND NORTHWEST INTO THE MISSISSIPPI VALLEY UPPER LOW NEAR SOUTHERN TX SUNDAY/MONDAY ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: GENERAL MODEL COMPROMISE CONFIDENCE: ABOVE AVERAGE THE GUIDANCE SHOWS GOOD AGREEMENT WITH THESE SYSTEMS. POSSIBLE CONVECTIVE LOW MOVING THROUGH THE GULF ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: GFS/ECMWF/CANADIAN COMPROMISE CONFIDENCE: BY DEFAULT/SEE NHC OUTLOOKS WHILE NHC GIVES 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 YESTERDAY WOULD BE A COMPROMISE OF THE 12Z GFS/00Z CANADIAN/00Z ECMWF, 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