MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 116 PM EDT SUN SEP 13 2015 VALID SEP 13/1200 UTC THRU SEP 17/0000 UTC ...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR INGEST... 12Z NAM/GFS EVALUATION...INCLUDING PRELIMINARY PREFERENCES ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ DEEP CLOSED LOW PIVOTING ACROSS THE NORTHEAST THROUGH MONDAY ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: 12Z GFS / 00Z ECMWF CONSENSUS CONFIDENCE: ABOVE AVERAGE THE MODELS ARE IN RELATIVELY GOOD AGREEMENT WITH THE DEEP CLOSED LOW PIVOTING ACROSS NEW ENGLAND TODAY BEFORE REACHING THE CANADIAN MARITIMES BY LATE MON. HOWEVER...THE 12Z NAM IS A BIT MORE EAST WITH THE LOW LEVEL LOW ON MONDAY...WITH THE GFS AND ECMWF OFFERING UP THE BEST COMPROMISE WITH RESPECT TO THE BEST ENSEMBLE CLUSTERING. WHILE A GENERAL MODEL COMPROMISE MAY WORK FOR THIS SYSTEM GIVEN DIFFERENCES ARE RELATIVELY MINOR...WILL STICK WITH A GFS/ECMWF BLEND FOR THE PREFERENCE GIVEN THEIR AGREEMENT WITH THE MAJORITY/ENSEMBLES. SHORTWAVE TROUGH INFLUENCING CALIFORNIA/SOUTHWEST THROUGH MON ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: GENERAL MODEL COMPROMISE CONFIDENCE: ABOVE AVERAGE THE MODELS DEPICT A SIMILAR LARGE SCALE EVOLUTION OF THIS SYSTEM. INITIAL SHORTWAVE CROSSING THE NORTHWEST BY MON NIGHT LOW PRESSURE DEVELOPING IN THE LEE OF THE NORTHERN ROCKIES ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: BLEND OF THE 12Z GFS/00Z UKMET...THROUGH 00Z/16 GENERAL BLEND OF GUIDANCE AFTER 00Z/16 CONFIDENCE: AVERAGE THE 00Z ECMWF APPEARS TO HAVE THE MOST DIFFERENT HANDLING OF A SHORTWAVE COMING ACROSS THE NORTHWEST BY TUE MORNING...IN BEING A BIT SLOWER THAN THE REMAINING...WELL CLUSTERED...DETERMINISTIC CONSENSUS FOLLOWED NEXT BY THE 00Z CMC. THE GFS AND ECMWF BOTH HOLD GOOD CONTINUITY WITH THEIR PREVIOUS RUNS BUT THE 00Z ECMWF IS FLATTER THAN ITS MEAN BY 00Z/16 WITH THE SHORTWAVE AS IT MOVES OVER SRN CANADA. THE 00Z ECMWF HAS ALSO BEEN CONSISTENTLY AT THE EDGE OF THE GEFS/ECMWF/CMC ENSEMBLE MEMBERS AS THE SURFACE LOW MOVE THROUGH SRN CANADA. THE 12Z GFS AND 00Z UKMET APPEAR TO BE A GOOD FIT TO THE ENSEMBLE GUIDANCE THROUGH ABOUT 00Z/16...AFTER WHICH POINT THE GFS/UKMET GET ON THE FASTER SIDE OF THE SPREAD. HOWEVER...AFTER 00Z/16...THE MAJORITY OF THE IMPACTS FROM THIS SYSTEM ARE IN CANADA...WITH ONLY THE LINGERING FRONT REMAINING INTO THE N-CNTRL CONUS. DESPITE THE GFS/UKMET BEING FASTER THERE IS GOOD OVERALL AGREEMENT ACROSS THE CONUS WITH THIS SYSTEM AFTER 00Z/16. SHORTWAVE TROUGH AXIS OVER THE WEST COAST BY 00Z/16 SURFACE LOW REDEVELOPING OVER THE NRN HIGH PLAINS BY WED EVENING ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: 12Z NAM CONFIDENCE: SLIGHTLY BELOW AVERAGE ANOTHER SHORTWAVE TROUGH IS IDENTIFIED IN THE MODEL GUIDANCE...FORECAST TO REACH THE WEST COAST NEAR 00Z/16...EMBEDDED WITHIN A LARGER SCALE UPPER TROUGH SETTING UP OVER THE WESTERN CONUS. AS THIS WAVE CROSSES OVER THE GREAT BASIN...A RELATED SURFACE LOW SHOULD TRACK ACROSS THE WEST WITH REDEVELOPMENT INTO THE NRN HIGH PLAINS BY WED EVENING. THERE ARE SOME TIMING DIFFERENCES NOTED IN THE GUIDANCE...WITH THE 12Z GFS ON THE FASTER SIDE OF THE GUIDANCE...AND 00Z ECMWF A BIT SLOW. THE MIDDLE OF THE LATEST ENSEMBLE SPREAD SUPPORTS THE 12Z NAM AS THE MIDDLE OF THE SPREAD...FITTING BEST WITH THE LATEST ENSEMBLE MEANS. UPPER TROF AND ATTENDANT COLD FRONT/TROF NEARING THE WEST COAST WED ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: BLEND OF THE 12Z NAM...12Z GFS...00Z ECMWF CONFIDENCE: BELOW AVERAGE THE MODELS SHOW AN ADDITIONAL SHORTWAVE TROUGH AMPLIFYING TOWARD THE WEST COAST ON WED. THE 00Z CMC IS THE STRONGER EDGE OF THE DETERMINISTIC MODELS...WITH THE 12Z NAM...12Z GFS AND 00Z ECMWF APPEARING TO FIT THE MIDDLE OF MODEST DETERMINISTIC/ENSEMBLE SPREAD REGARDING TIMING AND STRENGTH. GIVEN THE EXTENDED TIME FRAME...AND NUMEROUS SHORTWAVE DETAILS WITHIN THE LARGER SCALE TROUGH...CONFIDENCE IS LOW...WITH DETAILS TO BE RESOLVED FURTHER WITH SUBSEQUENT MODEL CYCLES. www.wpc.ncep.noaa.gov/html/model2.shtml... www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml... OTTO