MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1212 PM EDT THU OCT 20 2016 VALID OCT 20/1200 UTC THRU OCT 24/0000 UTC ...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR INGEST... 12Z MODEL EVALUATION...INCLUDING THE NAM AND GFS ...LOW PRESSURE SYSTEM TRACKING FROM THE OHIO VALLEY TO NEW ENGLAND... PREFERENCE: BLEND OF THE 12Z GFS/00Z UKMET/00Z ECMWF CONFIDENCE: AVERAGE THE MODELS HAVE CONVERGED WITH THE TIMING AND STRENGTH OF AN AMPLIFYING UPPER TROUGH MOVING INTO THE EASTERN U.S. OVER THE NEXT COUPLE OF DAYS. INITIAL SURFACE LOW DEVELOPMENT ALONG A FRONTAL ZONE OVER THE OHIO VALLEY TODAY WILL GIVE WAY TO A DEEPENING LOW CENTER THAT WILL ELONGATE AND CROSS THE CENTRAL APPALACHIANS AND MID-ATLANTIC STATES BY FRIDAY. THE MODEL GUIDANCE THEN GENERALLY AGREES ON SURFACE LOW PRESSURE CONSOLIDATING AND FOCUSING TOWARD SOUTHERN NEW ENGLAND BEFORE THEN LIFTING NORTH ACROSS NORTHERN NEW ENGLAND THIS WEEKEND. MEANWHILE...A COLD FRONT WILL BE PUSHING OFF THE EAST COAST. THE 12Z NAM APPEARS TO BE A TAD WEAKER THAN THE MODEL CONSENSUS WITH THE SURFACE LOW EVOLUTION. THE 00Z CMC OCCASIONALLY APPEARS TO BE FOCUSING TOO MUCH LOW PRESSURE A LITTLE TOO FAR EAST...PERHAPS IN RELATION TO ITS INTERACTION WITH LOW PRESSURE OFF THE EAST COAST. THE 12Z GFS/00Z UKMET AND 00Z ECMWF GRADUALLY CLUSTER TOGETHER REASONABLY WELL DESPITE SOME MODEST TIMING/DEPTH DIFFERENCES. SO WILL PREFER A GFS/UKMET/ECMWF COMPROMISE AT THIS TIME. ...DEEP UPPER LOW OVER THE GULF OF ALASKA WITH SURFACE LOW APPROACHING THE PACIFIC NORTHWEST... PREFERENCE: BLEND OF THE 12Z GFS/00Z UKMET/00Z ECMWF CONFIDENCE: AVERAGE THE MODELS HAVE A STRONG CLOSED LOW OVER THE GULF OF ALASKA WHICH WILL ALLOW ONE SHORTWAVE AND ASSOCIATED FRONT TO CROSS THE NORTHWEST TODAY. THIS IMPULSE WILL GRADUALLY CROSS SOUTHERN CANADA THROUGH FRIDAY WITH LOW PRESSURE CROSSING THE REGION. FARTHER UPSTREAM...ANOTHER SHORTWAVE AND ASSOCIATED SURFACE LOW/FRONT WILL UNDERCUT THE DEEPER LAYER GULF OF ALASKA LOW AND APPROACH THE PACIFIC NORTHWEST BY SATURDAY AFTERNOON. THE 00Z CMC CONTINUES TO BE SLOWER WITH THE PROGRESSION OF THE SURFACE LOW ON SATURDAY AND ALSO INDICATES MORE UPPER LEVEL RIDGING OVER BRITISH COLUMBIA AND THE PACIFIC NORTHWEST AHEAD OF THE TROUGH AXIS. THE 12Z NAM IS SEEN AS BEING TOO WEAK. THE 12Z GFS/00Z UKMET AND 00Z ECMWF EXHIBIT SOME MINOR TIMING/DEPTH DIFFERENCE...AND SO A BLEND OF THESE SOLUTIONS WILL BE PREFERRED TO RESOLVE THESE DIFFERENCES FOR THE TIME BEING. ...WEAK SHORTWAVE/SURFACE LOW CROSSING THE NORTHERN PLAINS/MIDWEST... PREFERENCE: BLEND OF THE 12Z GFS/00Z UKMET/00Z ECMWF CONFIDENCE: SLIGHTLY ABOVE AVERAGE THE MODELS SHOW A WEAK SHORTWAVE TRAVERSING THE INTERMOUNTAIN WEST AND EJECTING OUT ACROSS THE NORTHERN PLAINS ON SATURDAY BEFORE THEN CROSSING THE MIDWEST. THE 00Z CMC IS THE SLOWEST WITH THIS SYSTEM. MEANWHILE...THE 12Z NAM LOOKS AS IF IT IS TOO FAR NORTH WITH ITS WAVE TRACK. BETTER MODEL CLUSTERING RESIDES WITH THE 12Z GFS/00Z UKMET AND 00Z ECMWF...AND SO A BLEND OF THESE SOLUTIONS WILL BE PREFERRED. ...SURFACE LOW DEVELOPMENT OFF THE EAST COAST... PREFERENCE: BLEND OF THE 12Z GFS/00Z ECMWF CONFIDENCE: BELOW AVERAGE A BROAD AREA OF LOW PRESSURE NORTHEAST OF THE BAHAMAS IS LIKELY TO SHOW ADDITIONAL DEVELOPMENT OVER THE NEXT COUPLE OF DAYS AS IT LIFTS NORTHWARD BETWEEN BERMUDA AND THE EAST COAST. THIS LOW WILL LIKELY GET PULLED NORTHWARD RAPIDLY BY SATURDAY WITH THE DEEP TROUGH OVER NEW ENGLAND AFFECTING IT. THE 00Z CMC CONTINUES TO BE A FASTER OUTLIER SOLUTION WITH THE LOW CENTER REACHING NOVA SCOTIA BY SATURDAY AFTERNOON. THE 00Z UKMET BECOMES A DEEP OUTLIER SOLUTION WHEN COMPARED TO THE OTHER GUIDANCE. THE 12Z NAM IS SLOWER THAN THE MODEL CONSENSUS. CONFIDENCE REMAINS LIMITED IN THE FUTURE EVOLUTION OF THIS LOW...BUT WE WILL CONTINUE TO FAVOR A SOLUTION CLOSE TO A CONSENSUS OF THE BETTER CLUSTERED 12Z GFS AND 00Z ECMWF WHICH IS CLOSE TO THE MODEL CONSENSUS. PLEASE CONSULT THE LATEST NHC TROPICAL WEATHER OUTLOOK FOR MORE INFORMATION ON THIS SYSTEM. www.wpc.ncep.noaa.gov/html/model2.shtml... www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml... ORRISON