MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 125 AM EST SAT DEC 19 2015 VALID DEC 19/0000 UTC THRU DEC 22/1200 UTC ...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR INGEST... 00Z NAM/GFS EVALUATION AND PRELIMINARY PREFERENCES SHORTWAVE TROUGH CURRENTLY MOVING ASHORE THE WEST COAST... PREFERENCE: 1/2 EACH GFS/ECMWF CONFIDENCE: AVERAGE THE NAM MOVES PROGRESSIVELY TOWARD THE FAST SIDE OF THE SOLUTION ENVELOPE AFTER REACHING THE PLAINS AND MAY BE TOO FLAT ALONG THE CANADIAN BORDER...WHICH IMPACTS THE STRENGTH AND POSITION OF 2 SEPARATE LOWS ACCOMPANYING THE FRONTAL SYSTEM. GIVEN RELATIVELY GOOD AGREEMENT BETWEEN THE GFS/ECMWF AND SUPPORT FROM THE UKMET/CANADIAN...RECOMMEND BLENDING THESE 2 SOLUTIONS EQUALLY. SHORTWAVE TROUGH MOVING FROM N. CA TO THE PLAINS SUN-MON... LOW PRESSURE OVER THE LOWER MS VALLEY MON... PREFERENCE: 1/2 EACH GFS/ECMWF CONFIDENCE: AVERAGE THE GFS AND ECMWF SHOW RELATIVELY GOOD AGREEMENT WITH EACH OTHER THROUGH THE PERIOD...INCLUDING THE TIMING AND STRENGTH OF THE FRONTAL BOUNDARY AND LOW ACROSS THE LOWER MS VALLEY...AS WELL AS THE RETURNING THETA-E GRADIENT ALONG THE CENTRAL GULF COAST...WHILE THE NAM LIES NEAR THE OUTER EDGES OF THE GUIDANCE WITH THE SPEED OF THE 500 MB SHORTWAVE TROUGH AS WELL AS THE DEPTH OF THE SURFACE LOW. MODEL SPREAD INCLUDING ENSEMBLE MEMBERS REMAINS CONSIDERABLE AND MAY REMAIN SO FOR ANOTHER 24 OR MORE HOURS UNTIL THESE SYSTEMS MOVE ASHORE AND ARE BETTER SAMPLED. REFER TO THE QPFPFD FOR MORE INFORMATION ON RAINFALL ASSOCIATED WITH THIS SYSTEM. LOW PRESSURE MOVING ASHORE THE PACIFIC NORTHWEST MON... PREFERENCE: 1/2 EACH GFS/ECMWF CONFIDENCE: AVERAGE THE GFS HAS TRENDED ABOUT HALFWAY BETWEEN ITS 18Z RUN AND THE 12-00Z ECMWF...WHICH SUPPORTS THE STRONGER AND FASTER SIDE OF THE GUIDANCE COMPARED TO THE NAM WHICH IS WEAKER AND SLOWER. THE ENSEMBLE MEMBER SPREAD IS NEAR OR SLIGHTLY ABOVE AVERAGE FOR SPREAD WITH RUN-TO-RUN CONTINUITY AVERAGE AS BEST SUGGESTING THAT MODELS MAY CONTINUE TO FLUCTUATE SLIGHTLY FROM ONE CYCLE TO THE NEXT. REFER TO THE QPFPFD AND QPFHSD FOR MORE INFORMATION ON RAINFALL AND WINTER WEATHER ASSOCIATED WITH THIS SYSTEM. www.wpc.ncep.noaa.gov/html/model2.shtml... www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml... JAMES