Prognostic Meteorological Discussion
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000
FXUS10 KWNH 202112
PMDHMD

MODEL DIAGNOSTIC DISCUSSION...CORRECTED
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
411 PM 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 MODEL EVALUATION AND FINAL 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...

PREFERENCE: GENERAL MODEL COMPROMISE
CONFIDENCE: ABOVE AVERAGE

MODELS HANDLE THESE FEATURES SIMILARLY. THUS A GENERAL MODEL
COMPROMISE IS RECOMMENDED.


...LOW-PRESSURE SYSTEM MOVING ONSHORE IN THE NORTHWEST TODAY AND
ENTERING THE NORTHERN PLAINS LATE SUNDAY...

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 12Z 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
ECMWF/UKMET/CMC AND THE ENSEMBLE MEANS.


...WEAK SURFACE LOWS ACROSS THE GULF OF MEXICO AND THE SOUTHEAST
U.S. COASTLINE...

PREFERENCE: GFS/ECMWF/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 ECMWF AND UKMET AS WELL AS THE GEFS AND ECMWF ENSEMBLE
MEANS. THE NAM APPEARS TO MOVE THE LOW NORTHWARD TOO QUICKLY. THE
ECMWF AND UKMET BOTH TRENDED A LITTLE CLOSER TO THE COAST BY 12Z
TUESDAY RELATIVE TO THEIR PREVIOUS RUNS...AND TO THE GFS. WITH
THIS TAKEN INTO ACCOUNT...WILL RECOMMEND A BLEND OF THE
GFS/ECMWF/UKMET.


...UPPER-LEVEL LOW NORTH OF MINNESOTA THIS MORNING...
...LONGWAVE TROUGH/UPPER-LEVEL LOW EVOLVING ACROSS THE CENTRAL
U.S...

PREFERENCE: GFS/ECMWF/UKMET BLEND (ECMWF/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 TUESDAY AS ANOTHER SHORT WAVE
MOVES SOUTHWARD INTO THE NORTHERN PLAINS BY LATE TUESDAY. THE GFS
SHOWS A MORE AMPLIFIED WAVE WHILE THE ECMWF/UKMET/CMC DEPICT 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...

PREFERENCE: ECMWF
CONFIDENCE: AVERAGE

THE 12Z ECMWF SOLUTION IS ALIGNED VERY CLOSELY WITH BOTH THE 00Z
ECMWF ENSEMBLE MEAN AND THE 12Z GEFS MEAN. THE SURFACE LOW IN THE
DETERMINISTIC GFS SOLUTION IS A LITTLE FARTHER SOUTH. GIVEN THE
GOOD AGREEMENT OF THE ECMWF WITH THE MEANS...WILL RECOMMEND THE
ECMWF.


...LOW-PRESSURE SYSTEM REACHING THE PACIFIC NORTHWEST EARLY
TUESDAY...

PREFERENCE: GFS/ECMWF/UKMET BLEND
CONFIDENCE: AVERAGE

THERE APPEARS TO BE A REASONABLE CLUSTERING WITH RESPECT TO THE
TIMING OF THIS SYSTEM BETWEEN THE GFS/UKMET/ECMWF AND THE 00Z
ECMWF ENSEMBLE MEAN/12Z GEFS MEAN. WHILE THE NAM AND CMC APPEAR TO
BE OUTLIERS ON THE SLOW SIDE.


...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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