MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 255 AM EDT SUN SEP 13 2015 VALID SEP 13/0000 UTC THRU SEP 16/1200 UTC ...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR INGEST... 00Z MODEL EVALUATION...INCLUDING THE NAM/GFS/UKMET/CMC AND ECMWF ...DEEP CLOSED LOW PIVOTING THROUGH THE NORTHEAST... PREFERENCE: NON-NAM CONSENSUS CONFIDENCE: ABOVE AVERAGE THE MODELS ARE IN RELATIVELY GOOD AGREEMENT WITH THE DEEP CLOSED LOW PIVOTING ACROSS NEW ENGLAND ON SUN BEFORE REACHING THE CANADIAN MARITIMES BY LATE MON. HOWEVER...THE 00Z NAM BECOMES THE SLOWEST BY 48 HOURS AND BEYOND. THE REMAINING MODELS ARE WELL CLUSTERED AT THIS POINT...AND SO A NON-NAM CONSENSUS WILL BE PREFERRED. ...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. ...SURFACE LOW TRAVERSING THE U.S/CANADIAN BORDER ON SUN... PREFERENCE: GENERAL MODEL COMPROMISE CONFIDENCE: ABOVE AVERAGE THE MODELS SHOW VERY GOOD AGREEMENT WITH THE EVOLUTION OF A SURFACE LOW MOVING EAST IN ALONG THE U.S./CANADIAN BORDER SUN AND SUN NIGHT. THIS LOW CENTER WILL BE MOVING IN CONJUNCTION WITH A COLD FRONT THAT WILL BE SETTLING SOUTHEAST ACROSS THE NORTHERN PLAINS. A GENERAL MODEL COMPROMISE WILL BE PREFERRED WITH THIS SYSTEM. ...INITIAL SHORTWAVE CROSSING THE NORTHWEST BY MON... ...LOW PRESSURE DEVELOPING IN THE LEE OF THE NORTHERN ROCKIES... PREFERENCE: BLEND OF THE 00Z GFS/00Z UKMET...THROUGH 60 HOURS BLEND OF THE 00Z UKMET/12Z ECENS MEAN...AFTER 60 HOURS CONFIDENCE: AVERAGE THE 00Z NAM/00Z CMC AND 00Z ECMWF ARE OVERALL A LITTLE SLOWER THAN THE MULTI-MODEL CONSENSUS WITH THE INITIAL SHORTWAVE THAT ARRIVES ACROSS THE NORTHWEST ON MON...WITH THE 00Z GFS AND 00Z UKMET A TAD FASTER. THIS ENERGY WILL EJECT NORTHEAST AND CROSS THE NORTHERN ROCKIES AND FAR NORTHERN HIGH PLAINS THROUGH TUES...AND IN DOING SO WILL BE FOSTERING LEE-SIDE CYCLOGENESIS. ALL OF THE MODELS DRIVE SURFACE LOW PRESSURE ACROSS THE NORTHERN PLAINS MON AND TUES. THE 00Z CMC IS THE SLOWEST SOLUTION WITH THE EJECTING LOW CENTER...WITH THE REMAINING GUIDANCE ALL RATHER WELL CLUSTERED ON A SLIGHTLY FASTER PROGRESSION. HOWEVER...AFTER ABOUT 60 HOURS...THE 00Z GFS SUGGESTS A MUCH STRONGER INVERTED TROUGH EXTENDING BACK TO THE WEST AND NORTHWEST OF THE LOW CENTER OWING TO A STRONGER UPPER TROUGH EVOLUTION OVER ALBERTA AND SASKATCHEWAN. THE REMAINING GUIDANCE FAVORS A MUCH MORE SHEARED SHORTWAVE TROUGH EVOLUTION BY CONTRAST GIVEN THE STRONGER UPSTREAM TROUGH THAT IS DIGGING TOWARD THE WEST COAST BY WED. THE INCREASINGLY SHEARED NATURE OF THIS SYSTEM WITH TIME SUGGESTS LEANING TOWARD A MORE PROGRESSIVE SOLUTION. THUS WILL PREFER A BLEND OF THE 00Z GFS AND 00Z UKMET THROUGH ABOUT 60 HOURS...AND THEN LEAN TOWARD A BLEND OF THE 00Z UKMET AND THE 12Z ECENS MEAN THEREAFTER WHICH NICELY REPRESENTS THE GLOBAL MODEL CONSENSUS. ...UPPER TROUGH AMPLIFYING ALONG THE WEST COAST TUES/WED... ...LOW PRESSURE APPROACHING THE WEST COAST EARLY WED... PREFERENCE: 00Z GFS CONFIDENCE: SLIGHTLY BELOW AVERAGE THE MODELS SHOW ADDITIONAL SHORTWAVES AMPLIFYING TOWARD THE WEST COAST ON TUES AND WED...SETTING THE STAGE FOR A LONGER WAVE TROUGH TO EVOLVE ALONG WITH A WAVE OF LOW PRESSURE APPROACHING THE WEST COAST BY EARLY WED. ONE SHORTWAVE WILL CROSS THE WEST COAST LATE TUES AND EARLY WED...WITH ANOTHER ONE JUST OFFSHORE DIGGING TOWARD THE COAST. THE 00Z UKMET IS THE MOST PROGRESSIVE WITH THESE SHORTWAVES...WITH THE 00Z NAM/00Z GFS/00Z CMC AND 00Z ECMWF ALL SLOWER BY COMPARISON. HOWEVER...WITH THE SECOND SHORTWAVE IN PARTICULAR...THE 00Z CMC AND 00Z ECMWF ARE THE SLOWEST OVERALL AND ARE ALSO NOTABLY DEEPER/STRONGER WHICH ALSO TRANSLATES INTO A STRONGER SURFACE WAVE. THE 00Z GFS AND 00Z NAM ARE FLATTER BUT THE GFS IS A BIT MORE PROGRESSIVE THAN THE NAM WITH THE SECOND SHORTWAVE IMPULSE. THE 00Z GEFS MEAN AND 12Z ECENS MEAN DO NOT FAVOR THE STRONGER ECMWF/CMC SOLUTIONS...AND GENERALLY GIVE STRONGER SUPPORT TO THE 00Z GFS. GIVEN THE INCREASING MODEL SPREAD WITH THE SHORTWAVE DETAILS OFF THE COAST EARLY WED...CONFIDENCE IS DECREASING A BIT...BUT FOR NOW WILL PREFER A SOLUTION TOWARD THE RELATIVELY FLATTER/WEAKER 00Z GFS. www.wpc.ncep.noaa.gov/html/model2.shtml... www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml... ORRISON