MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 239 PM EDT SUN AUG 31 2014 VALID AUG 31/1200 UTC THRU SEP 04/0000 UTC ...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR INGEST... ...AN EVALUATION OF THE 12Z CMC/UKMET/ECMWF ALONG WITH FINAL PREFERENCES/FORECAST CONFIDENCE... THERE DID NOT APPEAR TO BE ANY INITIALIZATION ERRORS WITH THE 12Z NAM/GFS DEEMED SIGNIFICANT ENOUGH TO IMPACT THEIR FORECASTS. ...AREA OF HEIGHT FALLS MOVING THROUGH THE LOWER GREAT LAKES/NORTHEASTERN U.S... FINAL PREFERENCE: GENERAL MODEL COMPROMISE FORECAST CONFIDENCE: ABOVE AVERAGE A PAIR OF SHORTWAVES MOVING THROUGH THE LOWER GREAT LAKES AND OH VALLEY...RESPECTIVELY...WILL EVENTUALLY REACH THE NORTHEASTERN U.S. BY LATER TONIGHT. THE FORMER WAVE SHOULD REMAIN MORE INTACT RELATIVE TO THE OH VALLEY IMPULSE WHICH WILL BE A LITTLE MORE SHEARED IN NATURE. GIVEN THE LIMITED TIME THIS SYSTEM AFFECTS THE CONUS...THE MODELS ARE IN CLOSE ENOUGH AGREEMENT TO AFFORD A GENERAL MODEL COMPROMISE HERE. ...LONGWAVE TROF EXTENDING FROM THE ROCKIES THROUGH THE GREAT LAKES... ...PROGRESSIVE COLD FRONT... FINAL PREFERENCE: BLEND OF THE 12Z GEFS/09Z SREF/00Z ECMWF ENSEMBLE MEANS FORECAST CONFIDENCE: SLIGHTLY ABOVE AVERAGE WATER VAPOR IMAGERY SHOWED A LONGWAVE TROF INITIALLY SPANNING MUCH OF THE WESTERN TO CENTRAL U.S. WITH SEVERAL EMBEDDED SHORTWAVES RACING TOWARD THE NORTH AND EAST. THE LEAD ENERGY AND ITS ACCOMPANYING SPEED MAX TRACKING TOWARD THE MIDDLE MS VALLEY WILL CARRY AN INHERENT HEAVY RAINFALL AND SEVERE WEATHER THREAT. EARLY ON...THE 12Z GFS APPEARS A TAD QUICKER ALOFT AND AT THE SURFACE WITH THE BAROCLINIC ZONE. OTHER MODELS APPEAR TOLERABLE WITHIN THE GENERAL NOISE THROUGH MONDAY WITH THIS SHORTWAVE. AS THIS SYSTEM LIFTS ACROSS THE CANADIAN BORDER ADDITIONAL LONGWAVE ENERGY WILL FILL IN ACROSS THE CENTRAL U.S. THE 12Z NCEP GUIDANCE ARE A TAD QUICKER RELATIVE TO THE 12Z ECMWF/CMC/UKMET WITH THIS ASPECT OF THE TROF. AT THE SURFACE...THE ENSEMBLE MEANS ALL ARE FAIRLY CLOSE TO ANOTHER THROUGH 02/0000Z WITH THE POSITION OF THE FRONT AND AREA OF LOW PRESSURE OVER ONTARIO. ALL AND ALL...THE ENSEMBLE MEANS SEEM TO AFFORD THE BEST COMPROMISE AS THE 12Z GUIDANCE HAS SHOWN MORE VARIATION IN THEIR SOLUTIONS. WILL SWITCH OVER TO A BLEND OF THE MEANS WITH CONFIDENCE REDUCED BUT STILL A BIT ABOVE AVERAGE GIVEN THE ENSEMBLE AGREEMENT. ...AMPLIFIED SHORTWAVE REACHING THE PACIFIC NORTHWEST BY TUESDAY AFTERNOON... FINAL PREFERENCE: BLEND OF THE 12Z GFS/CMC/ECMWF FORECAST CONFIDENCE: SLIGHTLY BELOW AVERAGE AS THE LONGWAVE TROF BEGINS TO EJECT EASTWARD...ADDITIONAL SHORTWAVE ENERGY IS FORECAST TO DROP DOWN FROM THE NORTHEAST PACIFIC AND BRING AN AMPLIFIED SYSTEM INTO THE PACIFIC NORTHWEST BY TUESDAY AFTERNOON. BY MID-WEEK THE 12Z NAM LOOKS TOO SLOW AND IS WEST OF THE ENSEMBLE MEANS INCLUDING THE 09Z SREF MEAN. THE 564-DM SPAGHETTI PLOT SHOWS AGREEMENT THAT A POSITIVELY-TILTED TROF WILL BE LOCATED OVER THE NORTHERN GREAT BASIN/ROCKIES WITH SOME SPREAD REGARDING TIMING AND HOW MUCH ENERGY HANGS BACK TOWARD THE SOUTHWEST. THE ENSEMBLE MEANS ACTUALLY DIVERGE FROM ONE ANOTHER A BIT MORE THAN PREVIOUSLY AS THE 12Z GEFS MEAN IS WELL AHEAD OF THE DETERMINISTIC 12Z GFS ALONG WITH ANY OTHER SOLUTION. BELIEVE THE 12Z GEFS MEAN MAY BE TOO FAST. OVERALL...THE 12Z CMC/ECMWF ARE IN A SLIGHTLY SLOWER CAMP WHILE THE 12Z GFS/UKMET ARE OFF TO THE EAST. BY 04/0000Z...THE 12Z UKMET BECOMES FASTER SO WILL LEAN A BIT MORE HEAVILY ON THE CLUSTERING OFF THE 12Z GFS/CMC/ECMWF AS THEY SIT IN THE CENTER OF THE SPREAD. ...PAIR OF EASTERLY WAVES AFFECTING EASTERN MEXICO AND FL...RESPECTIVELY... FINAL PREFERENCE: EASTERN MEXICO WAVE: NON-12Z NAM MODEL COMPROMISE...FL WAVE: NON-12Z NAM/UKMET MODEL COMPROMISE FORECAST CONFIDENCE: AVERAGE TWO EASTERLY WAVES ARE FORECAST TO AFFECT THE BAY OF CAMPECHE AND THE BAHAMAS...RESPECTIVELY. WITH REGARD TO THE SYSTEM APPROACHING EASTERN MEXICO...THE 12Z NAM IS DEFINITELY THE MOST NORTHERN SOLUTION WHICH WOULD BRING HEAVY RAINFALL INTO SOUTH TX ON DAY 3. THE OTHER MODELS ARE CLOSE ENOUGH WITH THEIR MORE SOUTHERN POSITIONS WITH LESS IMPACTS ACROSS THE LOWER 48. MEANWHILE...WITH THE EASTERLY WAVE CROSSING THE BAHAMAS...THE 12Z NAM WAS QUICKER BEING OFF TOWARD THE WEST WHILE THE 12Z UKMET DEPICTS A MORE SUPPRESSED SOLUTION. THE 12Z GFS/ECMWF/CMC LEAN FAVORABLY TOWARD CLUSTERED ENSEMBLE MEANS SO A NON-12Z NAM/UKMET PREFERENCE IS RECOMMENDED. www.wpc.ncep.noaa.gov/html/model2.shtml... www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml... RUBIN-OSTER