MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 138 AM EST THU DEC 18 2014 VALID DEC 18/0000 UTC THRU DEC 21/1200 UTC ...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR INGEST... 00Z MODEL EVALUATION AND FINAL PREFERENCES ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ NAM/GFS INITIALIZATION ERRORS DO NOT APPEAR TO SIGNIFICANTLY AFFECT THEIR LARGE SCALE OUTPUT. PACIFIC SHORTWAVES REACHING THE PLAINS TONIGHT WEAK SURFACE LOW TRACKING ALONG THE GULF COAST ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: 00Z ECMWF CONFIDENCE: AVERAGE THE 00Z NAM IS SLOWER THAN THE CONSENSUS WHILE THE 00Z UKMET/CMC ARE SLIGHTLY MORE AMPLIFIED AT 500 MB WHICH CAUSES THE CORRESPONDING SURFACE LOW TO BE A BIT FARTHER NORTH IN THE UKMET/CMC THAN THE LATEST ENSEMBLE SCATTER LOW PLOTS AND MEANS. THE 00Z GFS IS SLIGHTLY WEAKER THAN THE REMAINING DETERMINISTIC/ENSEMBLE MODEL CONSENSUS NEAR 850 MB. THIS LEAVES THE 00Z ECMWF AS A REASONABLE ALTERNATIVE...WHICH IS CLOSEST TO THE LATEST ENSEMBLE GUIDANCE ACROSS THE SOUTH. NOTE THAT THE 12Z CMC COULD BE USED AS A SECOND OPTION ALONG WITH THE 12Z ECMWF...THOUGH THE 12Z ECMWF WAS A TAD NORTH WITH ITS SURFACE LOW COMPARED TO THE ENSEMBLE GUIDANCE. MID-LEVEL TROUGH AXIS REACHING THE PLAINS FOR SUN MORNING ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: BLEND OF 00Z NAM...00Z GFS...00Z ECMWF CONFIDENCE: AVERAGE THE SIMILAR TIMING AND AMPLITUDE OF THE 00Z NAM/GFS/ECMWF CORRESPOND TO A GOOD COMPROMISE CONCERNING A MID-LEVEL TROUGH FORECAST TO REACH THE PLAINS BY SUN MORNING. THE 00Z UKMET TRENDED WEAKER THAN ITS 12Z RUN BUT REMAINS SLIGHTLY MORE AMPLIFIED ACROSS THE PLAINS WHILE THE 00Z CMC JUST HAS A DIFFERENT EVOLUTION OF SHORTWAVES RESULTING IN AN UNSUPPORTED FLOW PATTERN BY SUN MORNING. THE RELATIVELY ZONAL FLOW OBSERVED ACROSS THE MID-LATITUDES IN THE PACIFIC FAVORS LESS AMPLIFICATION THAN SHOWN BY THE LATEST 00Z UKMET...AND FAVORS THE CURRENT ENSEMBLE GUIDANCE WHICH DOES NOT SUPPORT THE UKMET. SHORTWAVES ENTERING THE NWRN U.S. THROUGH SUN MORNING ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: NON 00Z CMC COMPROMISE CONFIDENCE: AVERAGE THE FAST MID FLOW ENTERING THE PACIFIC NORTHWEST AND NORTHERN CA MAKES IDENTIFYING INDIVIDUAL SHORT WAVES IN THE FLOW DIFFICULT BY SAT/SUN. HOWEVER...SOME OF THE LARGER SCALE DIFFERENCES CAN BE RESOLVED...WITH THE 00Z NAM/GFS SHOWING SLIGHTLY FLATTER FLOW INTO THE PAC NW...WITH THE 00Z ECMWF/UKMET MORE AMPLIFIED. A BLEND OF THESE TWO CAMPS SEEMS APPROPRIATE AT THIS TIME...WHILE THE 00Z CMC APPEARS TO BE LEAST SUPPORTED IN A WEAKER...MORE NORTHERN...PLACEMENT OF MID-LEVEL TROUGHING AND VORTICITY ENERGY ENTERING THE PACIFIC NORTHWEST...UNLIKE THE ENSEMBLE MEANS. CLOSED LOW DEPARTING FROM THE NORTHEAST TODAY AND FRI ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: GENERAL MODEL COMPROMISE CONFIDENCE: ABOVE AVERAGE THE MODELS ONLY SHOW MINOR DIFFERENCES WITH THIS SYSTEM. www.wpc.ncep.noaa.gov/html/model2.shtml... www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml... OTTO