Prognostic Meteorological Discussion
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000 FXUS10 KWNH 201628 PMDHMD MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1128 AM EST SAT DEC 20 2014 VALID DEC 20/1200 UTC THRU DEC 24/0000 UTC ...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR INGEST... 12Z NAM/GFS EVALUATION AND PRELIMINARY MODEL PREFERENCES NAM/GFS INITIALIZATION ERRORS TO NOT APPEAR TO BE SUFFICIENTLY LARGE TO IMPACT THEIR FORECASTS. ...UPPER-LEVEL LOW ACROSS THE NORTHEAST TODAY... ...SHORTWAVE CROSSING THE OHIO VALLEY TODAY AND MOVING OFF THE MID-ATLANTIC/NORTHEAST COAST ON SUNDAY... PRELIMINARY PREFERENCE: GENERAL MODEL COMPROMISE CONFIDENCE: ABOVE AVERAGE THE NAM AND GFS SOLUTIONS HANDLE THESE FEATURES SIMILARLY TO CONSENSUS. THUS A GENERAL MODEL COMPROMISE IS RECOMMENDED. ...LOW-PRESSURE SYSTEM MOVING ONSHORE IN THE NORTHWEST TODAY AND ENTERING THE NORTHERN PLAINS LATE SUNDAY... PRELIMINARY PREFERENCE: NON-NAM COMPROMISE CONFIDENCE: AVERAGE MODELS DEMONSTRATE RELATIVELY SMALL TIMING DIFFERENCES WITH MOVING THE COLD FRONT INTO THE INTERIOR NORTHWEST TONIGHT. BY SUNDAY MORNING AS THE SYSTEM CROSSES THE NORTHERN ROCKIES THE NAM AND THE GFS ARE BOTH CLUSTERED WELL AROUND THE CONSENSUS AND THE 06Z GEFS AND 00Z ECMWF ENSEMBLE MEANS. BY LATE MONDAY THE NAM SOLUTION BECOMES FARTHER NORTH WITH THE SURFACE LOW RELATIVE TO THE GFS...WHICH REMAINS VERY CLOSE TO THE CONSENSUS OF THE 00Z DETERMINISTIC GUIDANCE CONSENSUS AND THE ENSEMBLE MEANS. ...WEAK SURFACE LOWS ACROSS THE GULF OF MEXICO AND THE SOUTHEAST U.S. COASTLINE... PRELIMINARY PREFERENCE: GFS/00Z ECMWF/00Z UKMET BLEND CONFIDENCE: AVERAGE A FEW WEAK WAVES OF LOW PRESSURE WILL MOVE ALONG A STATIONARY FRONTAL BOUNDARY EXTENDING FROM THE NORTHERN GULF OF MEXICO NORTHEASTWARD TO OFF THE SOUTHEAST U.S. COASTLINE. THE MOST SIGNIFICANT OF THESE LOWS WILL DEVELOP OFF THE CAROLINA COAST ON MONDAY. MODELS SHOW LITTLE DIFFERENCES THROUGH SUNDAY...BUT SPREAD BEGINS TO INCREASE BY MONDAY AS THE AFOREMENTIONED LOW DEVELOPS OFF THE CAROLINAS. THE GFS APPEARS TO BE CLUSTERED REASONABLY WELL WITH THE 00Z ECMWF AND UKMET AS WELL AS THE GEFS AND ECMWF ENSEMBLE MEANS. THE NAM APPEARS TO MOVE THE SURFACE LOW FARTHER NORTH ALONG THE COAST TOO QUICKLY. ...UPPER-LEVEL LOW NORTH OF MINNESOTA THIS MORNING... ...LONGWAVE TROUGH/UPPER-LEVEL LOW EVOLVING ACROSS THE CENTRAL U.S... PRELIMINARY PREFERENCE: GFS/00Z ECMWF/00Z UKMET BLEND (00Z ECMWF/00Z UKMET AFTER 00Z TUES) CONFIDENCE: AVERAGE MODEL DIFFERENCES WITH RESPECT TO THESE FEATURES ARE RELATIVELY SMALL THOUGH SUNDAY...BUT BEGIN TO INCREASE AFTER THAT TIME. THE MAIN DIFFERENCES THAT EMERGE BY MONDAY MORNING SEEM TO STEM FROM DIFFERENCES IN AMPLITUDE AND TIMING OF A SHORTWAVE THAT DIVES INTO THE NORTHERN PLAINS SUNDAY NIGHT EARLY MONDAY WHICH EVENTUALLY CLOSES OFF INTO AN UPPER-LEVEL LOW BY MONDAY AFTERNOON. SPREAD AMONG THE 00Z ECMWF ENSEMBLE MEMBERS APPEARS TO BE LESS THAN IN THE 06Z GEFS MEAN. THE NAM APPEARS TO BE THE SOLUTION DIFFERENT THE MOST FROM CONSENSUS...WITH THE UPPER-LEVEL LOW OVER THE MIDWEST FARTHER NORTH AND WEST THAN ANY OTHER GUIDANCE. THE SAME HOLDS TROUGH FOR THE SURFACE LOW MONDAY NIGHT/EARLY TUESDAY. DIFFERENCES INCREASE FURTHER BY EARLY TUESDAY AS ANOTHER SHORT WAVE MOVING SOUTHWARD INTO THE NORTHERN PLAINS BY LATE TUESDAY. THE GFS SHOWS A MORE AMPLIFIED WAVE WHILE THE 00Z ECMWF DEPICTS A WEAKER AND MORE SPREAD OUT AREA OF 500 HPA VORTICITY WHICH QUICKLY APPEARS TO GET ABSORBED INTO THE LARGE-SCALE TROUGH. THEREFORE...WOULD PUT LESS EMPHASIS ON THE GFS SOLUTION BY TUESDAY. ...WAVE OF LOW PRESSURE DEVELOPING ACROSS THE SOUTHERN PLAINS LATE MONDAY... PRELIMINARY PREFERENCE: GFS/00Z ECMWF BLEND CONFIDENCE: AVERAGE THE GFS AND 00Z ECMWF SOLUTIONS APPEAR TO BE THE MOST CLOSELY CLUSTERED WITH THE ENSEMBLE MEANS AS THE SURFACE LOW DEVELOPS ACROSS THE SOUTHERN PLAINS LATE ON MONDAY AND MOVES INTO THE LOWER MISSISSIPPI VALLEY ON TUESDAY. ...LOW-PRESSURE SYSTEM REACHING THE PACIFIC NORTHWEST EARLY TUESDAY... PRELIMINARY PREFERENCE: GFS/00Z ECMWF/00Z UKMET BLEND CONFIDENCE: AVERAGE THERE APPEARS TO BE REASONABLE CLUSTERING OF SOLUTIONS AMONG THE GFS...00Z ECMWF AND 00Z UKMET AS THIS LOW-PRESSURE SYSTEM APPROACHES THE COAST. THESE SOLUTIONS ALL LIE WITHIN THE BOUNDS DEFINED BY THE 00Z ECMWF ENSEMBLE MEAN AND THE 06Z GEFS MEAN. A BLEND OF THESE SOLUTIONS IS RECOMMENDED AT THIS TIME. ...MODEL TREND GRAPHICS AT www.wpc.ncep.noaa.gov/html/model2.shtml... ...500 MB FORECASTS AT www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml... RYAN $$

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