MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1124 AM EST SUN DEC 21 2014 VALID DEC 21/1200 UTC THRU DEC 25/0000 UTC ...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR INGEST... 12Z NAM/GFS EVALUATION AND PRELIMINARY MODEL PREFERENCES MODEL INITIALIZATION ERRORS ARE DESCRIBED UNDER THE RELEVANT SYSTEM HEADER. ...UPPER-LEVEL TROUGH INITIALLY OVER THE NORTHEAST... PRELIMINARY PREFERENCE: GENERAL MODEL COMPROMISE CONFIDENCE: ABOVE AVERAGE THE NAM AND GFS ARE COMPARABLE TO THE GENERAL MODEL CONSENSUS WITH RESPECT TO THIS FEATURE. ...LOW-PRESSURE SYSTEM MOVING FROM THE NORTHERN ROCKIES INTO THE NORTHERN PLAINS TODAY... PRELIMINARY PREFERENCE: 00Z ECMWF/UKMET/CMC BLEND CONFIDENCE: BELOW AVERAGE THE NAM AND GFS BECOME MORE AMPLIFIED WITH THE UPPER-LEVEL SHORTWAVE ACROSS THE NORTHERN PLAINS BY EARLY MONDAY RELATIVE TO THE 00Z NON-NCEP GUIDANCE. THE PARALLEL GFS IS LESS AMPLIFIED THAN THE OPERATIONAL VERSION AT THIS TIME...LENDING SOME CREDENCE TO THE NON-NCEP CAMP OF SOLUTIONS. THE DIFFERENCES APPEAR TO STEM FROM THE FAST-MOVING LOW-AMPLITUDE SHORTWAVE MOVING ONSHORE IN THE NORTHWEST THIS MORNING. THE NAM (AND THE GFS TO A LESSER EXTENT) INITIALIZED 500 HPA HEIGHTS A LITTLE TOO LOW IN THE VICINITY OF THE FEATURES THIS MORNING. THIS COULD IMPACT ITS RESULTANT SOLUTION WITH A MORE AMPLIFIED TROUGH BY THE TIME THE SHORTWAVE REACHES THE NORTHERN PLAINS. ON THE OTHER HAND...THE 00Z ECMWF SEEMS TO BE ON TH LEAST AMPLIFIED END OF THE SPECTRUM WITH THIS FEATURE. THE RESULT OF THESE UPPER-LEVEL DIFFERENCES ARE A STRONGER SURFACE LOW FARTHER SOUTH...RATHER THAN THE NORTHERN SURFACE LOW BEING STRONGER IN THE 00Z ECMWF/UKMET/CMC...AND THE NAM SOMEWHERE IN BETWEEN THE TWO CAMPS. ADDITIONALLY...THE PARALLEL GFS SOLUTION IS CLOSER TO THE 00Z ECMWF/UKMET. ...WAVE OF LOW PRESSURE DEVELOPING IN THE NORTHWEST GULF OF MEXICO TUESDAY AFTERNOON... PRELIMINARY PREFERENCE: 00Z ECMWF/UKMET/CMC BLEND CONFIDENCE: AVERAGE THE NAM APPEARS TO BE MORE SLOWER/MORE AMPLIFIED RELATIVE TO THE GFS AND 00Z NON-NCEP GUIDANCE WITH A SHORTWAVE MOVING ONSHORE IN BRITISH COLUMBIA EARLY MONDAY MORNING...WHICH EVENTUALLY AMPLIFIES AS IT RAPIDLY MOVES SOUTHWARD ACROSS THE ROCKIES MONDAY NIGHT AND INTO THE SOUTHERN PLAINS TUESDAY AS IT ROUNDS THE BASE OF THE LARGE-SCALE TROUGH. AS A RESULT...THE NAM IS SHARPER WITH THE LARGE-SCALE TROUGH AND EVEN CLOSES OFF A LOW AT 500 HPA OVER THE SOUTHERN PLAINS EARLY WEDNESDAY. THESE DIFFERENCES TRANSLATE TO THE SURFACE AS WELL...WITH THE NAM LAGGING BEHIND THE GFS AND NON-NCEP MODELS WITH THE PROGRESSION OF THE SURFACE LOW. THE GFS DEMONSTRATES SOME OF THE SAME TENDENCIES OBSERVED IN THE NAM...ALBEIT TO A LESSER EXTENT. THE 12Z PARALLEL GFS SOLUTION BECOMES A SIGNIFICANT OUTLIER BY LATE TUESDAY WITH AN ANOMALOUSLY DEEP LOW AT THE SURFACE AND UPPER-LEVELS. THE 06Z RUN OF THE PARALLEL GFS LOOKED MUCH MORE LIKE THE 00Z ECMWF/UKMET/CMC. THEREFORE...WILL RECOMMEND A BLEND OF THE NON-NCEP GUIDANCE. ...STATIONARY FRONTAL BOUNDARY AND WEAK WAVES OF LOW PRESSURE AFFECTING THE SOUTHEASTERN U.S... PRELIMINARY PREFERENCE: GENERAL MODEL COMPROMISE CONFIDENCE: ABOVE AVERAGE MODEL DIFFERENCES WITH RESPECT TO THE SURFACE STATIONARY FRONT AND THE WEAK WAVES OF LOW PRESSURE MOVE ALONG IT ARE RELATIVELY SMALL AND NO SIGNIFICANT OUTLIERS ARE NOTED. ...LOW-PRESSURE SYSTEM REACHING THE PACIFIC NORTHWEST TUESDAY INTO WEDNESDAY... PRELIMINARY PREFERENCE: GFS/NAM/00Z ECMWF BLEND CONFIDENCE: ON TUESDAY AS A DEEP PACIFIC LOW PRESSURE SYSTEM MOVES INTO BRITISH COLUMBIA...THE GFS AND NAM SOLUTIONS BOTH LIE WITHIN A REASONABLE RANGE OF SPREAD AND NEITHER CAN BE DISCARDED AT THIS TIME. MODEL SPREAD BECOMES LARGER WITH A SECOND SHORTWAVE THAT REACHES THE PACIFIC NORTHWEST ON WEDNESDAY. WITH THIS FEATURE AS WELL THE GFS AND NAM LIE WITHIN THE MIDDLE OF THE SPREAD OF SOLUTIONS...WITH THE 00Z UKMET AS A FAST OUTLIER. www.wpc.ncep.noaa.gov/html/model2.shtml... www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml... RYAN