MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 141 AM EST MON DEC 22 2014 VALID DEC 22/0000 UTC THRU DEC 25/1200 UTC ...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR INGEST... 00Z MODEL EVALUATION AND FINAL MODEL PREFERENCES ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ANY NAM/GFS INITIALIZATION ERRORS DO NOT APPEAR TO SIGNIFICANTLY IMPACT THEIR SHORT RANGE FORECASTS. NRN STREAM LOW IMPACTING THE NRN PLAINS AND UPR MS VALLEY TODAY/TUE ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: 00Z NAM / 00Z CMC BLEND CONFIDENCE: AVERAGE THE 00Z GFS MADE A SIGNIFICANT SHIFT FROM ITS PREVIOUS CYCLES IN HAVING STRONGER MID-LEVEL VORTICITY AT THE FRONT EDGE OF A NEWLY DEVELOPED CLOSED LOW OVER THE UPPER MIDWEST MON NIGHT AND ITS UPPER PATTERN DOES NOT FIT WELL WITH THE REMAINING MODEL CONSENSUS. THE 00Z ECMWF/UKMET/CMC HAVE ADJUSTED IN THE DIRECTION OF THE PREVIOUS PREFERENCE...AND STILL DO NOT SUPPORT THE 00Z GFS. GIVEN THE LATEST ENSEMBLE SCATTER LOW PLOTS TUESDAY...THE TWO CLOSEST DETERMINISITC MODELS ARE THE 00Z NAM AND CMC. THE 00Z ECMWF/UKMET ARE CLOSE TO THIS IDEA...BUT PERHAPS STILL A BIT TOO FAR EAST. LONGWAVE TROUGH OVER THE CNTRL U.S. AND LOW PRESSURE MOVING ACROSS THE OHIO RIVER VALLEY ON TUE ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: 00Z CMC / 00Z ECMWF BLEND CONFIDENCE: SLIGHTLY BELOW AVERAGE THE 00Z NAM IS QUICK TO STAND OUT AS WEAKER WITH SECONDARY SHORTWAVE ENERGY DIVING SWD INTO THE SRN HIGH PLAINS TUE MORNING AND IS LESS DEVELOPED WITH LOW PRESSURE ACROSS THE CNTRL GULF COAST TUE NIGHT...ALL COMPARED TO THE REMAINING MODEL CONSENSUS. THE 00Z NAM IS NOT LIKELY TO VERIFY. THIS IS A TOUGH FORECAST WITH MANY DETAILS NOT HANDLED WELL IN ANY SINGLE MODEL. CONSIDERING THE NON-NAM GUIDANCE...A 00Z ECWMF AND 00Z CMC BLEND COMPARE REASONABLY WITH THE LATEST ENSEMBLE SPAGHETTI 500 MB HEIGHT TRENDS AT 540 AND 552 DM. THE 00Z GFS LOOKS OKAY WITH THESE MODELS UNTIL AROUND 00Z/25...AT WHICH POINT THE GFS SURFACE LOW ENDS UP ON THE EDGE OF THE ENSEMBLE SCATTER LOW PLOTS. THE NEW 00Z ECMWF IS SIMILAR TO THE 00Z GFS BUT A TAD FARTHER NORTH OR EAST BUT THE ENSEMBLE LOWS FAVOR A BLENDED SOLUTION BETWEEN THE 00Z ECMWF/CMC. WHEN IT COMES TO THE COLD FRONT SWINGING ACROSS THE EAST...THE 00Z ECMWF HAS SLOWED ENOUGH TO INCLUDE IT WITH THE RELATIVELY UNCHANGED 00Z CMC IN A 50/50 BLEND. GIVEN THE AMPLIFIED PATTERN ALOFT AND SLOWING TRENDS IN THE ENSEMBLE GUIDANCE...SLOWER IS BETTER. IT IS WORTH NOTING THAT THE 00Z GFSP SURFACE LOW IS ALSO BETWEEN THE 00Z ECMWF/CMC AT TIMES BUT IS QUITE DEEP AND WRAPPED BACK TO THE WEST...AND UNSUPPORTED AT THIS TIME IN TERMS OF STRENGTH. WEAK FRONTAL LOW MOVING UP THE EAST COAST TODAY/TUE ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: 00Z GFS/CMC COMPROMISE CONFIDENCE: ABOVE AVERAGE DIFFERENCES ARE SMALL...BUT THE 00Z NAM CONTINUES TO STAND OUT AS FARTHER NORTH WITH A WEAK SURFACE LOW ALONG THE EAST COAST LATER TODAY AND TONIGHT. THE GFS/CMC REPRESENT A GOOD COMPROMISE IN THE MIDDLE OF THE LATEST SPREAD WITH CLOSE AGREEMENT TO THE LATEST ENSEMBLE MEANS...WHILE THE 00Z ECMWF/UKMET APPEAR TO BE FARTHER SOUTH AND INLAND WITH THE LOW TRACK FOR BRIEF PERIODS OF TIME TUE MORNING. SHORTWAVE OVER THE WESTERN U.S. WED NIGHT ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: 00Z ECMWF/ ECMWF BLEND CONFIDENCE: BELOW AVERAGE THE 00Z NAM / 00Z ECMWF BLENDED SOLUTION IS IN THE MIDDLE OF THE LATEST SPREAD AND AGREES REASONABLY WELL WITH THE ENSEMBLE MEANS. THE 00Z GFS IS DEEPER AND SLOWER...AND THE 00Z ECMWF TRENDED DEEPER...BUT STILL A LITTLE FASTER THAN THE GFS. THE 00Z UKMET ALSO TRENDED DEEPER WITH SIMILAR TIMING TO ITS 12Z RUN AND SIMILAR TO THE 00Z ECMWF. LASTLY...THE 00Z CMC ALSO TRENDED DEEPER BUT NOT TO THE EXTENT OF THE GFS/ECMWF/UKMET AND IS ON THE FASTER SIDE OF THE GUIDANCE. ENSEMBLE SPREAD IS QUITE LARGE...LED BY THE DEEPER ECMWF MEMBERS...WHILE THE GEFS MEMBERS ARE MUCH WEAKER/FLATTER. THE PAST 3 CYCLES OF ENSEMBLE GUIDANCE SHOW NO CONVERGENCE TOWARD THE DEEPER OR FLATTER TRACKS...BUT THE AMPLIFIED NATURE OF THE UPPER PATTERN FAVORS TOWARD THE DEEPER END OF THE SPECTRUM ACROSS THE WEST ON THU. THIS IS TOWARD THE 12Z/21 EC MEAN OR A 00Z NAM/ECMWF BLEND. www.wpc.ncep.noaa.gov/html/model2.shtml... www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml... OTTO