MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 111 AM EDT MON APR 17 2017 VALID APR 17/0000 UTC THRU APR 20/1200 UTC ...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR INGEST... ...AN EVALUATION OF THE 00Z NAM/GFS ALONG WITH PRELIMINARY PREFERENCES/FORECAST CONFIDENCE... ...GENERAL WEAK PERTURBATIONS TRACKING FROM MX TOWARD THE GULF COAST REGION... PRELIMINARY PREFERENCE: A NON-12Z UKMET MODEL COMPROMISE FORECAST CONFIDENCE: SLIGHTLY BELOW AVERAGE LOW-AMPLITUDE FLOW WILL CONTINUE ACROSS THE LOWER LATITUDES OF NORTH AMERICA AS A SERIES OF WEAK TO MODEST PERTURBATIONS IN THE FLOW TRACK EASTWARD FROM MX TOWARD THE GULF COAST REGION. MANY OF THESE VORTICITY CENTERS WILL BE FOCI FOR ACTIVE CONVECTION...PARTICULARLY IN THE PRESENCE OF AMPLE MOISTURE/INSTABILITY IN PLACE. AS THE 00Z NAM IS OF HIGHER RESOLUTION...IT SHOWS A BIT MORE DEFINITION OF EACH INDIVIDUAL DISTURBANCE. THE GENERAL IDEA IS TO DRAG THE REGION OF STRONGER FORCING SLOWLY TOWARD THE EAST WITH A MEAN SYNOPTIC POSITION ACROSS THE FAR WESTERN GULF OF MX BY 19/1200Z. LIKE YESTERDAY...THE 12Z UKMET IS MORE SHEARED IN NATURE WHICH SEEMS TO BE OUTLYING COMPARED TO THE REST OF THE GUIDANCE SUITE. WHILE THERE ARE A SLEW OF DETAIL DIFFERENCES ON THE MESOSCALE...WILL FOCUS ON THE BROADER PATTERN HERE AND SUGGEST A NON-12Z UKMET MODEL COMPROMISE. ...PROGRESSIVE BAND OF HEIGHT FALLS ACCELERATING FROM THE NORTH-CENTRAL U.S. TO NEW ENGLAND... ...ASSOCIATED FRONTAL ZONE EXITING THE MID-ATLANTIC COAST BY MIDDAY MONDAY... PRELIMINARY PREFERENCE: A NON-12Z CMC MODEL CONSENSUS FORECAST CONFIDENCE: ABOVE AVERAGE AN ACTIVE/PROGRESSIVE FLOW REGIME WILL DOMINATE THE MID-LATITUDES AS A SLEW OF SHORTWAVES PUSH EASTWARD FROM THE NORTH-CENTRAL TIER OF THE U.S. TOWARD NEW ENGLAND. COMPARED TO RECENT DAYS...THE MODELS HAVE COME INTO MUCH BETTER AGREEMENT ACROSS THE BOARD. THE TROUGH AXIS IS FORECAST TO EXIT NEW ENGLAND BY 18/1200Z. WHILE RATHER MINIMAL IN NATURE...THE 12Z CMC IS A BIT SLOWER ALOFT WHICH ALLOWS FOR THE FRONTAL BOUNDARY TO HANG NORTH OF THE CONSENSUS SOLUTION. THE DIFFERENCE DOES GROW SOME IN TIME...ESPECIALLY BY LATE TUESDAY. GIVEN THIS OBSERVATION...THE SUGGESTED PREFERENCE WILL BE A NON-12Z CMC MODEL CONSENSUS. ...FAST-MOVING IMPULSE PUSHING FROM THE CA/OR COAST TOWARD THE DAKOTAS THROUGH 18/1200Z... ...ATTENDANT SURFACE LOW/COLD FRONT SWEEPING THROUGH THE GREAT LAKES/MIDDLE MS VALLEY BY 19/0000Z... PRELIMINARY PREFERENCE: BLEND OF THE 00Z GFS/12Z ECMWF FORECAST CONFIDENCE: ABOVE AVERAGE WATER VAPOR IMAGERY EARLY THIS MORNING SHOWED A NEGATIVELY-TILTED UPPER TROUGH CROSSING INTO SOUTHWESTERN OR WITH THE FEATURE EXPECTED TO REACH THE WESTERN DAKOTAS BY 18/0600Z. SOME SLIGHT DIFFERENCES ARE EVIDENT AS THE PROGRESSIVE IMPULSE MOVES INTO SOUTHERN ONTARIO. MOST NOTABLY...THE 00Z NAM/GFS ARE A TAD QUICKER THAN THE COMBINATION OF THE 12Z CMC/UKMET/ECMWF. AT THIS POINT WILL JUST TAKE AN AVERAGE OF THE TWO CAMPS AND FAVOR A BLEND OF THE 00Z GFS/12Z ECMWF. ...AMPLIFICATION TAKING SHAPE ACROSS THE NORTHERN PLAINS/UPPER MIDWEST ON WEDNESDAY EVENING... ...SURFACE CYCLONE RACING FROM THE CENTRAL PLAINS TO THE LOWER GREAT LAKES... PRELIMINARY PREFERENCE: BLEND OF THE 00Z GFS WITH THE 12Z ECMWF/UKMET FORECAST CONFIDENCE: AVERAGE IN ADVANCE OF AN EXPANSIVE LONGWAVE TROUGH OVER THE EASTERN PACIFIC...A PAIR OF SPEED MAXIMA RACE THROUGH THE INTERMOUNTAIN WEST AND EVENTUALLY INTO THE GREAT PLAINS. ONE SUCH FEATURE TRACKS TOWARD SOUTHERN ALBERTA WHILE THE MORE AMPLIFIED WAVE PUNCHES EASTWARD TOWARD THE CENTRAL HIGH PLAINS. THE 00Z NAM/12Z CMC STAND OUT AS QUICKER OUTLIERS WITH THIS SYSTEM. OF NOTE...THE POSITION OF THE SURFACE WAVE ON WEDNESDAY EVENING SITS ON THE FAR EASTERN SIDE OF THE ENSEMBLE SPREAD. EXCEPT FOR THE 21Z SREF MEAN WHICH FAVORS A QUICKER FORWARD PROGRESSION...THE REMAINING ENSEMBLE SOLUTIONS AGREE ON A SLOWER EVOLUTION. THE BEST CLUSTER RIGHT NOW WOULD FAVOR A BLEND OF THE 00Z GFS WITH THE 12Z ECMWF/UKMET. WILL SEE IF THE NEXT CYCLE OF THE CMC MAKES ANY POSITIVE ADJUSTMENTS TO BE INCLUDED IN THE NEXT ISSUANCE. ...HEIGHT FALLS REACHING THE WEST COAST BY THE END OF THE FORECAST... PRELIMINARY PREFERENCE: BLEND OF THE 00Z NAM/GFS WITH THE 12Z ECMWF FORECAST CONFIDENCE: SLIGHTLY ABOVE AVERAGE BY LATE WEDNESDAY/EARLY THURSDAY...MODELS AGREE ON A LONGWAVE TROUGH EXTENDING ALONG THE ENTIRE EXTENT OF THE WEST COAST. FOR AN 84-HOUR FORECAST...THE SPREAD IS ACTUALLY ON THE LOWER SIDE. ENSEMBLE MEANS CLUSTER NEAR ONE ANOTHER WHICH THE 00Z NAM/GFS AND 12Z ECMWF/GFS-PARALLEL ARE SOMEWHAT IN PHASE WITH. ON THE CONTRARY...THE 12Z CMC IS ON THE SLOWER END WHILE THE 12Z UKMET IS DISPLACED NORTHWARD WITH THE STRONGER POSITIVE VORTICITY ADVECTION. WPC RECOMMENDS A BLEND OF THE 00Z NAM/GFS WITH THE 12Z ECMWF GIVEN SUFFICIENT AGREEMENT IN THEIR OUTPUT. www.wpc.ncep.noaa.gov/html/model2.shtml... www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml... RUBIN-OSTER