THE WEATHER PREDICTION CENTER

COLLEGE PARK, MD


MODEL DIAGNOSTIC DISCUSSION


[Abbreviations and acronyms used in this product]
Geographic boundaries:    Map 1- [Color] [B/W Print Version]      Map 2 - [Color] [B/W Print Version]



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





Last Updated: 1128 AM EST SAT DEC 20 2014