MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1227 AM EDT TUE AUG 04 2015 VALID AUG 04/0000 UTC THRU AUG 07/1200 UTC ...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR INGEST... 00Z NAM/GFS EVALUATION WITH PRELIMINARY MODEL PREFERENCES ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ NAM/GFS INITIALIZATION ERRORS DO NOT APPEAR TO DEGRADE THEIR SHORT RANGE FORECASTS. CLOSED LOW APPROACHING CA THU NIGHT ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: NON-UKMET COMPROMISE CONFIDENCE: ABOVE AVERAGE THE 12Z UKMET WAS A SLOW OUTLIER, WITH THE MODEL CONSENSUS IN GOOD AGREEMENT WITH THE 18Z GEFS AND 12Z ECMWF ENSEMBLE MEANS. A NON-UKMET COMPROMISE IS PREFERRED HERE WITH ABOVE AVERAGE CONFIDENCE. POSSIBLE CLOSED LOW MOVING ALONG WESTERN US/CAN BORDER ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: NAM/ECMWF COMPROMISE CONFIDENCE: AVERAGE THE GUIDANCE SHOWS CONSIDERABLE SPREAD WITH THIS SYSTEM, WITH THE 00Z GFS THE QUICKEST AND THE 12Z CANADIAN THE SLOWEST -- BOTH FITTING KNOWN BIASES. WITH THE SYSTEM IS MOVING OVER THE TOP OF A SOUTHERN STREAM RIDGE (ARGUING FOR A FASTER SOLUTION), IT IS MOVING WITHIN THE SOUTHWEST PORTION OF AN OMEGA-TYPE BLOCK ACROSS CANADA (WHICH ARGUES FOR SLOWER). THE INTERMEDIATE GUIDANCE HERE OF THE 12Z ECMWF/00Z NAM SPLIT THE DIFFERENCE SEEN IN THE 12Z ECMWF AND 18Z GEFS ENSEMBLE MEANS, AND A COMPROMISE OF THE TWO IS PREFERRED WITH AVERAGE CONFIDENCE. NORTHERN STREAM SHORTWAVES CROSSING THE DAKOTAS/MN ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: GFS/NAM COMPROMISE CONFIDENCE: AVERAGE THE 12Z ECMWF WAS ANOMALOUSLY STRONG ALOFT AND AT THE SURFACE WITH THE FIRST WAVE, WHICH WAS NOT SUPPORTED BY THE 12Z GLOBAL ENSEMBLE GUIDANCE. BECAUSE OF THIS, THE ECMWF BECAME FLATTER WITH THE SECOND WAVE THE REST OF THE GUIDANCE SHOWS A STRONGER SOLUTION FOR ALOFT. THE 12Z UKMET IS UP TO ITS OLD TRICKS AND ATTEMPTS TO DEVELOP A TROPICAL CYCLONE-LIKE LOW WHICH MOVES OUT OF SD INTO MN...THIS IS NOT PREFERRED. OVERALL, A COMPROMISE OF THE 00Z GFS/00Z NAM FITS IN BEST WITH THE 12Z ECMWF/18Z GEFS MEAN SOLUTIONS. THIS IS PREFERRED WITH AVERAGE CONFIDENCE. SHORTWAVE CROSSING THE MID-SOUTH/SOUTHEAST WED/THU ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: GFS/ECMWF COMPROMISE CONFIDENCE: AVERAGE THE GUIDANCE HAS TRENDED NOTICEABLY WEAKER WITH THIS SYSTEM SINCE YESTERDAY. THE 12Z CANADIAN IS A STRONG OUTLIER -- ITS USUAL BIAS. THE 00Z NAM SURFACE LOW LIES COMPLETELY OUTSIDE THE NORTHEAST FRINGE OF THE 12Z GLOBAL ENSEMBLE SPREAD BY FRIDAY MORNING. A COMPROMISE OF THE 00Z GFS/12Z ECMWF LOOKS REASONABLE HERE, AND IS PREFERRED WITH AVERAGE CONFIDENCE. LOW PRESSURE MOVING FROM GA COAST TO THE CAROLINA COAST ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: GFS/NAM/ECMWF COMPROMISE CONFIDENCE: ABOVE AVERAGE THROUGH TUESDAY NIGHT, THE GUIDANCE HANDLES THIS WEAK SYSTEM SIMILARLY. BY WEDNESDAY MORNING, BOTH THE 12Z CANADIAN AND 12Z UKMET DEVELOP THE SYSTEM AS A TROPICAL CYCLONE-LIKE ENTITY AS IT PULLS AWAY FROM NC, LYING ON THE EASTERN AND SOUTHERN FRINGE OF THE 12Z GLOBAL ENSEMBLE ENVELOPE OF SOLUTIONS. A COMPROMISE OF THE 00Z GFS/00Z NAM/12Z ECMWF LOOKS REASONABLE HERE WITH ABOVE AVERAGE CONFIDENCE. www.wpc.ncep.noaa.gov/html/model2.shtml... www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml... ROTH