Prognostic Meteorological Discussion
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000
FXUS10 KWNH 280443
PMDHMD

MODEL DIAGNOSTIC DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1242 AM EDT FRI AUG 28 2015

VALID AUG 28/0000 UTC THRU AUG 31/1200 UTC

...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR
INGEST...

00Z NAM/GFS EVALUATION AND PRELIMINARY PREFERENCES

SHORTWAVE TROUGHS OVER SOUTH DAKOTA AND NEBRASKA...

PREFERENCE: NAM
CONFIDENCE: ABOVE AVERAGE

MODEL INITIALIZATION AND SHORT TERM TRENDS THROUGH 05Z SUPPORT A
SOLUTION CLOSEST TO THE NAM...WITH THE GFS TRENDING SLOWER BUT
STILL APPEARING TOO FAST.  THROUGH THE REMAINDER OF THE
PERIOD...THE NAM REMAINS ABOUT MIDWAY BETWEEN THE FASTER GFS AND
THE SLOWER/STRONGER 12Z ECMWF WHICH APPEARS TOO DEEP WITH RESPECT
TO THE REST OF THE GUIDANCE OVER THE GREAT LAKES BY 12Z SUN.
THUS...RECOMMEND THE NAM.

UPPER TROUGH OVER THE GULF OF MEXICO...

PREFERENCE: 2/3 18Z GEFS MEAN TO 1/3 00Z GFS
CONFIDENCE: BELOW AVERAGE

CRUCIAL STRUCTURE AND PLACEMENTS OF INDIVIDUAL SHORTWAVE TROUGH
EMBEDDED OR FORMING WITHIN THE LOW WILL DEPEND IN PART ON
CONVECTION THAT FORMS UPSTREAM AND DROPS INTO THE BASE OF THE
TROUGH AS WELL AS CONVECTIVE AMPLIFICATION OF PERTURBATIONS OVER
THE GULF OF MEXICO.  THE CURRENT DETERMINISTIC AND ENSEMBLE MEMBER
SPREAD IS QUITE HIGH...INDICATIVE OF THE STRUGGLE THE MODELS ARE
HAVING.  ONE OF THE BEST WAYS TO ADDRESS THE SPREAD IS TO RELY
UPON AN ENSEMBLE MEAN...WITH THE 18Z GEFS MEAN THE MOST
REPRESENTATIVE SOLUTION THAT BALANCES THE PREFERRED LONGWAVE
PATTERN WITH A SOLUTION CLOSEST TO THE CONSENSUS.  ONE ADVANTAGE
THOUGH THAT THE 00Z NAM/GFS MIGHT HAVE IS WITH THE SLIGHTLY SLOWER
AND STRONGER DEPICTION OF A SHORTWAVE TROUGH CROSSING THE
CAROLINAS ON DAY 3...WHICH IS DEPICTED MUCH FASTER IN THE 12Z
ECMWF...AND IS POSSIBLY TOO FAST.  THUS...IT MIGHT BE WORTH
INCLUDING A LITTLE OF THE GFS ALONG WITH THE GEFS MEAN.

SHORTWAVE TROUGHS CROSSING THE PACIFIC NORTHWEST THIS WEEKEND...

PREFERENCE: NAM
CONFIDENCE: ABOVE AVERAGE

MODEL AGREEMENT IS EXCELLENT GIVEN STRONG LARGE-SCALE FORCING
MECHANISMS...WITH THE NAM SLIGHTLY SLOWER THAN THE GFS/12Z ECMWF
BY SUNDAY...PERHAPS DUE TO ITS SLOW BIAS.  OTHERWISE...MODEL
DIFFERENCES ARE SMALL AND SUB GRIDSCALE...WITH THE NAM`S HIGHER
RESOLUTION LIKELY AN ADVANTAGE GIVEN THE ROLE OF TERRAIN...DESPITE
BEING SLIGHTLY SLOWER ALOFT.

TROPICAL STORM ERICA...

THE GFS IS CLOSEST TO THE OFFICIAL NHC TRACK...WITH THE NAM AND
12Z ECMWF FASTER AND SLIGHTLY SOUTH.  REFER TO THE NHC FOR THE
OFFICIAL TRACK AND INTENSITY FORECAST.

...MODEL TREND GRAPHICS AT
www.wpc.ncep.noaa.gov/html/model2.shtml...

...500 MB FORECASTS AT
www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml...

JAMES

$$





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