Prognostic Meteorological Discussion
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000 FXUS10 KWNH 280644 PMDHMD MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 243 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 INCLUDING THE UKMET/CANADIAN/ECMWF AND PREFERENCES SHORTWAVE TROUGHS OVER SOUTH DAKOTA AND NEBRASKA... PREFERENCE: NAM CONFIDENCE: ABOVE AVERAGE MODEL INITIALIZATION AND SHORT TERM TRENDS THROUGH 06Z SUPPORT A SOLUTION CLOSEST TO THE NAM...WITH THE GFS TRENDING SLOWER BUT STILL APPEARING TOO FAST...AND THE ECMWF OVER-AMPLIFIED AFTER 12Z COMPARED WITH OTHER GUIDANCE AND THEREFORE PIVOTING THE ASSOCIATED SURFACE TOO FAR NORTH AND SLOWLY. OVERALL...THE NAM FITS THE CONSENSUS THE BEST...WITH THE 12Z ECMWF PROBABLY A CLOSER FIT TO THE NAM OVERALL COMPARED TO ITS 00Z RUN. 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 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 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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