630  
FXUS10 KWNH 151658  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1157 AM EST THU FEB 15 2018  
 
VALID FEB 15/1200 UTC THRU FEB 19/0000 UTC  
   
..SEE NOUS42 KWNO (ADMNFD) FOR STATUS OF THE UPPER AIR INGEST
 
 
   
..12Z MODEL EVALUATION WITH PRELIMINARY PREFERENCES/CONFIDENCE
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
 
...SEVERAL LOW-AMPLITUDE SHORTWAVES MOVING QUICKLY EAST THROUGH  
THE GREAT LAKES AND NORTHEAST THROUGH FRIDAY NIGHT...  
...DEEPENING SURFACE LOW IN SOUTHEAST CANADA AND TRAILING COLD  
FRONT SWEEPING THROUGH THE NORTHEAST AND OHIO VALLEY...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
MODELS DO SHOW SOME MINOR DIFFERENCES WITH THIS SYSTEM,  
PARTICULARLY WITH A SECONDARY SHORTWAVE THAT PUSHES THROUGH THE  
NORTHEAST LATE ON FRIDAY. ALL MODELS SHOW THE LOBE OF VORTICITY  
ASSOCIATED WITH THIS WAVE GETTING ELONGATED AND SHEARED OUT WITH  
TIME, BUT THE 12Z NAM AND 00Z UKMET SHOW A MUCH FLATTER APPEARANCE  
BY 17/00Z, WHILE THE 12Z GFS AND 00Z CMC/ECMWF SHOW MORE  
DEFINITION. THESE SUBTLE CHANGES AMOUNT TO SMALL DIFFERENCES IN  
THE MASS FIELDS, BUT THERE IS NO CLEAR PREFERENCE FOR ANY  
PARTICULAR SMALL CHANGE. THEREFORE, THE PREFERENCE WITH THIS  
SYSTEM IS A GENERAL MODEL BLEND.  
 
...SHORTWAVE CURRENTLY OVER THE GULF OF ALASKA DIGGING INTO THE  
NORTHERN ROCKIES FRIDAY NIGHT...   
..KICKING EAST TO THE OHIO VALLEY BY SATURDAY EVENING
 
   
..ASSOCIATED SURFACE CYCLOGENESIS OFF NORTHEAST COAST SUNDAY
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 40% ECMWF, 20% EC ENS MEAN, 40% OTHER MODELS  
CONFIDENCE: SLIGHTLY BELOW AVERAGE  
 
DIFFERENCES WITH THIS SYSTEM APPEAR TO PARTICULARLY RELATE TO THE  
STRENGTH AND TIMING OF THE SHORTWAVE AS IT PUSHES THROUGH THE  
MIDWEST ON SATURDAY. THE MODELS THAT HAVE A SLOWER AND STRONGER  
SHORTWAVE (12Z NAM AND 00Z UKMET) TEND TO HAVE A STRONGER SURFACE  
LOW OFF THE NORTHEAST COAST THAT IS POSITIONED FURTHER TO THE  
NORTHEAST. MEANWHILE, THE MODELS WITH A FASTER AND SLIGHTLY WEAKER  
SHORTWAVE (12Z GFS, 00Z ECMWF/CMC) HAVE A SURFACE LOW THAT IS  
FURTHER OFFSHORE OF THE NORTHEAST COAST. AN EXAMINATION OF  
ENSEMBLE LOW POSITIONS OVER THE PAST FOUR MODEL CYCLES SHOWS THAT  
ALTHOUGH SW-NE DISPLACEMENT (RELATED TO TIMING) HAS NARROWED, THE  
NW-SE SPREAD IS ROUGHLY THE SAME AS IT WAS A COUPLE DAYS AGO.  
 
ADDING COMPLEXITY IS THE FACT THAT THE SHORTWAVE THAT REACHES THE  
EAST COAST WILL LIKELY RESULT FROM SOME PHASING OF THE NORTHERN  
STREAM WITH THE LINGERING TROUGH OVER THE SOUTHWEST. THIS MAY  
RESULT IN CONTINUED UNCERTAINTY FOR SEVERAL MORE MODEL CYCLES.  
 
MODELS ALSO SHOW INDICATIONS THAT THE LOW-LEVEL CYCLONE WILL BE  
INFLUENCED BY DIABATIC HEATING EFFECTS, WITH THE GFS AND ECMWF  
SHOWING A RAMP-UP IN LOW-MID LEVEL PV AS THE SURFACE LOW DEVELOPS  
OVER THE ATLANTIC. THE GFS IS PERHAPS MORE AGGRESSIVE WITH THIS,  
PARTICULARLY INITIALLY (SATURDAY NIGHT) AND TIES THE SURFACE LOW  
VERY CLOSELY TO THE QPF MAX, WHICH EVENTUALLY RESULTS IN A SURFACE  
LOW ON THE SOUTHEASTERN EXTENT OF ENSEMBLE LOW POSITIONS.  
 
THE MODEL PREFERENCE FOR THIS SYSTEM, THEREFORE, LEANS TOWARD A  
GREATER WEIGHT ON THE 00Z ECMWF WHICH SITS CLOSEST TO THE ENSEMBLE  
MEANS AND ROUGHLY IN THE MIDDLE OF THE NW-SE LOW POSITION SPREAD  
OFF THE EAST COAST. REGARDING THE SHORTWAVE UPSTREAM, ALL THE  
AVAILABLE MODELS SIT WELL WITHIN THE ENSEMBLE SPREAD FOR UPPER  
LEVEL HEIGHTS, SO ANY OF THEM COULD BE REASONABLE POSSIBILITIES.  
THEREFORE, A GENERAL BLEND IS PREFERRED WITH A 40% WEIGHT ON THE  
OPERATIONAL ECMWF, 20% ON THE ECMWF ENSEMBLE MEAN, AND 40% ON THE  
REMAINING MODELS.  
 
   
..NEXT TROUGH DIGGING INTO THE NORTHWEST THIS WEEKEND
 
 
...SURFACE LOW AND FRONT IN THE INTERMOUNTAIN WEST WITH LEE  
CYCLOGENESIS IN THE PLAINS BY SUNDAY EVENING...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: NON-UKMET BLEND  
CONFIDENCE: AVERAGE  
 
MODEL SPREAD WITH THE HEIGHT FIELDS ALOFT HAS CONTINUED TO  
GRADUALLY NARROW OVER THE PAST SEVERAL MODEL CYCLES, ALTHOUGH SOME  
SPREAD STILL EXISTS -- ESPECIALLY WITH THE AMPLITUDE OF THE  
TROUGH. ALTHOUGH SPREAD REMAINS, THERE IS NO CLEAR PREFERENCE FOR  
ONE OPERATIONAL MODEL OVER THE OTHER, AND ALL MODELS FIT WITHIN  
THE ENVELOPE OF ENSEMBLE SCENARIOS. THE ONE EXCEPTION IS WITH THE  
00Z UKMET WHICH HAS LOWER HEIGHTS OVER THE INTERIOR NORTHWEST THAN  
MOST OF THE OTHER MODELS, AND THE MORE EXPANSIVE TROUGH MAY HAVE  
AN EFFECT ON SENSIBLE WEATHER.  
 
MODEL TRENDS AT WWW.WPC.NCEP.NOAA.GOV/HTML/MODEL2.SHTML  
500 MB FORECASTS AT WWW.WPC.NCEP.NOAA.GOV/H5PREF/H5PREF.SHTML  
 
LAMERS  
 

 
 
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