MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1153 PM EST TUE DEC 13 2016 VALID DEC 14/0000 UTC THRU DEC 17/1200 UTC ...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR INGEST... 00Z NAM/GFS/UKMET MODEL EVALUATION ALONG WITH PREFERENCES ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ POLAR VORTEX LOBE OVER NORTHERN ONTARIO AND QUEBEC ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: NON-CMC COMPROMISE CONFIDENCE: ABOVE AVERAGE IMPRESSIVE SUB-480DM HEIGHTS FROM A LOBE OF THE POLAR VORTEX ARE CURRENTLY OVER NORTHERN ONTARIO, AND THIS DEEP UPPER LOW IS FORECAST TO TRACK ACROSS JAMES BAY AND THEN OVER CENTRAL QUEBEC THROUGH THURSDAY. GIVEN THAT THIS FEATURE WILL ESTABLISH A DEEP UPPER LEVEL TROUGH OVER THE EASTERN U.S., THIS WILL USHER IN THE COLDEST AIRMASS SO FAR THIS SEASON ACROSS MUCH OF THE NORTHERN AND EASTERN U.S. TO CLOSE OUT THE WEEK. MODELS FOR THE MOST PART ARE IN STRONG AGREEMENT WITH THE OVERALL STRUCTURE WITH SLIGHT DIFFERENCES EXISTING ON THE SMALLER SCALE DUE TO VARIANCES WITH INDIVIDUAL PERTURBATIONS AROUND THE LOW TOWARD THE END OF THE SHORT RANGE PERIOD. THE 12Z CMC APPEARS TO DEVIATE THE MOST WITH THE CORE OF THE VORTEX TO THE NORTH OF THE MODEL CONSENSUS BEGINNING BY 36 HOURS OUT, SO A NON-CMC COMPROMISE APPEARS PRUDENT. NORTHERN STREAM TROUGH REACHING THE NORTHERN PLAINS FRIDAY NIGHT ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: 00Z NAM/00Z GFS/12Z ECMWF CONFIDENCE: AVERAGE TOWARDS THE END OF THE SHORT RANGE FORECAST PERIOD, A DEEP TROUGH ORIGINATING FROM NORTHERN CANADA IS PROGGED TO REACH THE NORTHERN PLAINS AND UPPER MIDWEST, HERALDING THE ARRIVAL OF A SECOND ARCTIC AIRMASS FOR THE WEEKEND. SOME PHASING OF THIS TROUGH WITH THE SOUTHERN STREAM TROUGH OVER THE DESERT SOUTHWEST IS LIKELY. THE 00Z UKMET APPEARS A LITTLE MORE AMPLIFIED WITH THE LEADING EDGE OF THE TROUGH COMPARED TO THE MODEL CONSENSUS, BUT OTHERWISE GOOD MODEL AGREEMENT EXISTS. UPPER TROUGH INFLUENCING THE WEST COAST AND INTERMOUNTAIN WEST ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: 00Z GFS/18Z GEFS MEAN/12Z ECMWF CONFIDENCE: SLIGHTLY BELOW AVERAGE A WELL-DEFINED UPPER LEVEL TROUGH OVER THE NORTHEAST PACIFIC IS EXPECTED TO SLOWLY PIVOT SOUTHEASTWARD TOWARDS THE DESERT SOUTHWEST BY SATURDAY MORNING, WITH A WELL-DEFINED SHORTWAVE IN THE BASE OF THE TROUGH THAT WILL CROSS CALIFORNIA ON FRIDAY AND FRIDAY NIGHT. AT THE SURFACE, AN INITIAL WAVE OF LOW PRESSURE IS EXPECTED TO REACH THE COAST WEDNESDAY EVENING, FOLLOWED BY A SECOND LOW AND ACCOMPANYING COLD FRONT. A LARGE AREA OF LOW PRESSURE IS EXPECTED TO MATERIALIZE OVER THE INTERMOUNTAIN WEST BY FRIDAY IN RESPONSE TO THIS UPPER LEVEL TROUGH. MODELS ARE BEGINNING TO CONSOLIDATE BETTER WITH THE OVERALL EVOLUTION OF THE TROUGH AND ITS ASSOCIATED SHORTWAVE ENERGY COMPARED TO EARLIER RUNS. HOWEVER, NUMEROUS DETAIL DIFFERENCES STILL EXIST WITH THE STRUCTURE AND TIMING OF THE INDIVIDUAL IMPULSES TRACKING TOWARDS THE COAST. THE 12Z ECMWF IS PERHAPS A TAD FLATTER THAN ITS PREVIOUS RUN BUT IS STILL IN GOOD AGREEMENT WITH THE 00Z GFS, AND ALSO THE GEFS MEAN. www.wpc.ncep.noaa.gov/html/model2.shtml... www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml... HAMRICK