MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 201 PM EST FRI DEC 18 2015 VALID DEC 18/1200 UTC THRU DEC 22/0000 UTC ...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR INGEST... 12Z MODEL EVALUATION...INCLUDING THE NAM/GFS/UKMET/CMC AND ECMWF ...UPPER TROUGH SWINGING THROUGH THE GREAT LAKES/OH VALLEY/NORTHEAST... PREFERENCE: GENERAL MODEL COMPROMISE CONFIDENCE: ABOVE AVERAGE A FAIRLY DEEP UPPER TROUGH CROSSING THE GREAT LAKES AND OH VALLEY THROUGH TODAY...AND THEN ACROSS NEW ENGLAND ON SAT. THE MODELS DO HAVE A SURFACE LOW DEVELOPING ALONG A COLD FRONT SLOWLY EXITING THE EAST COAST WHICH FORMS OVER EASTERN NC THIS MORNING AND LIFTS RAPIDLY THROUGH THE GULF OF ME THIS EVENING. THE GUIDANCE IS IN GOOD AGREEMENT WITH THE MASS FIELD EVOLUTION AND THUS A GENERAL MODEL COMPROMISE WILL BE PREFERRED. ...SPLIT-FLOW TROUGH MOVING FROM THE WEST COAST THROUGH THE MIDWEST THIS WEEKEND... ...SURFACE LOW PRESSURE DEVELOPING AND CROSSING THE PLAINS/MIDWEST/GREAT LAKES... PREFERENCE: BLEND OF THE 12Z ECMWF AND 00Z ECENS MEAN CONFIDENCE: BELOW AVERAGE ALL MODELS AGREE IN ALLOWING A SPLIT-FLOW TROUGH TO PROGRESSIVELY CROSS THE WEST THROUGH SAT AND THEN EJECT OUT INTO THE PLAINS ON SUN. MEANWHILE...SURFACE LOW PRESSURE WILL DEVELOP IN THE LEE OF THE ROCKIES OVER THE CENTRAL HIGH PLAINS AND EJECT THROUGH THE MIDWEST AND UPPER GREAT LAKES THROUGH EARLY MON. HOWEVER...IT APPEARS AS THOUGH THERE WILL BE SUFFICIENT STREAM SEPARATION FOR DISTINCT LOW CENTERS IN BOTH THE NORTHERN AND SOUTHERN STREAMS. THE 12Z UKMET IS SLOWER WITH ESPECIALLY THE SOUTHERN STREAM PORTION OF THE TROUGH AND THUS IS SLOWER WITH THE SURFACE LOW EVOLUTION FARTHER SOUTH IMPACTING THE MIDWEST. IT IS FASTER WITH THE NORTHERN STREAM ENERGY/LOW CENTER. THE 12Z NAM ON THE OTHER HAND IS MORE PROGRESSIVE WITH THE ENTIRE TROUGH EVOLUTION AND FOCUSES MUCH OF ITS SURFACE LOW PRESSURE WITH THE NORTHERN STREAM ENERGY. THE 12Z ECMWF AND 12Z CMC SHOW MORE EMPHASIS ON THE SOUTHERN STREAM...AND A WEAKER NORTHERN STREAM REFLECTION...BUT THEY ARE NOT SLOW OR AS DEEP AS THE UKMET. THE 12Z GFS ALONG WITH THE 12Z GEFS MEAN AND 00Z ECENS MEAN SHOW A MORE CONSOLIDATED AREA OF LOW PRESSURE THAT EVOLVES OVER THE CENTRAL PLAINS/MIDWEST AND LIFTS THROUGH THE UPPER GREAT LAKES...BUT IS SLOWER AND SOUTH OF THE NAM. GIVEN THAT THERE SHOULD BE AN ELEMENT OF SPLIT FLOW...THE IDEA OF A SURFACE WAVE REPRESENTING EACH RESPECTIVE STREAM SEEMS REASONABLE. WILL FOR NOW SUGGEST A COMPROMISE BETWEEN THE 12Z ECMWF AND 00Z ECENS MEAN...WITH LIMITED CONFIDENCE GIVEN THE OVERALL MODEL SPREAD. ...SHORTWAVE/COLD FRONT ARRIVING IN THE PACIFIC NORTHWEST EARLY SUN... PREFERENCE: NON-UKMET CONSENSUS CONFIDENCE: ABOVE AVERAGE A PACIFIC SHORTWAVE TROUGH DIVING SOUTHEAST FROM THE GULF OF AK WILL CROSS THE COASTAL RANGES OF WA/OR EARLY SUN AND THEN CROSS THE INTERMOUNTAIN WEST THROUGH MON. THE 12Z UKMET IS STILL JUST A TAD FASTER THAN THE MULTI-MODEL CONSENSUS WITH THIS SYSTEM. THE REMAINING GUIDANCE IS REASONABLY WELL CLUSTERED AND THUS A NON-UKMET CONSENSUS IS PREFERRED AT THIS POINT. ...SHORTWAVE/COLD FRONT/LOW CENTER CROSSING THE PACIFIC NORTHWEST ON MON... PREFERENCE: BLEND OF THE 12Z GFS/12Z CMC AND 12Z ECMWF CONFIDENCE: BELOW AVERAGE ANOTHER FAST-MOVING SHORTWAVE AND ASSOCIATED COLD FRONT WILL ARRIVE ACROSS THE PACIFIC NORTHWEST ON MON...WITH A LOW CENTER THAT WILL LIKELY MOVE IN ACROSS WESTERN WA BY MIDDAY. THE 12Z NAM IS A LITTLE NORTH OF THE MULTI-MODEL CONSENSUS WITH THE LOW TRACK AND SUGGESTS IT MOVES INTO VANCOUVER ISLAND...WITH THE 12Z GFS AND ESPECIALLY THE 12Z CMC AND 12Z ECMWF CLUSTERED A BIT FARTHER SOUTH INTO WESTERN WA. THE 12Z UKMET IS A FAST-OUTLIER WITH THE NEXT SHORTWAVE/FRONT AND SUGGESTS A LOW CENTER WHICH TRACKS WELL SOUTH OF THE MODEL CONSENSUS INTO NORTHWEST OR. WILL DISMISS THE NAM AND UKMET AT THIS POINT IN FAVOR OF THE MORE STRONGLY CLUSTERED GFS/CMC AND ECMWF SOLUTIONS AS COMPROMISE. CONFIDENCE IS LIMITED GIVEN THE OVERALL MODEL SPREAD. www.wpc.ncep.noaa.gov/html/model2.shtml... www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml... ORRISON