708  
FXUS10 KWNH 061832  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
131 PM EST TUE FEB 06 2018  
 
VALID FEB 06/1200 UTC THRU FEB 10/0000 UTC  
 
...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR  
INGEST...  
   
..12Z MODEL EVALUATION WITH PRELIMINARY PREFERENCES/CONFIDENCE
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
 
...WEAK CYCLOGENESIS FROM MID-ATLANTIC INTO THE NORTHEAST WITH  
TRAILING COLD FRONT SWEEPING THROUGH THE SOUTHEAST WED-THU...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: BLEND OF 12Z GFS, ECMWF, UKMET  
CONFIDENCE: AVERAGE  
 
---18Z UPDATE---  
NO CHANGE TO THE PRELIMINARY PREFERENCE. THE 12Z CMC CONTINUES TO  
HAVE THE WEAKEST DEPICTION OF THE TROUGH, AND THE ECMWF AND UKMET  
REMAIN FAIRLY CLOSE TO THE GFS IN TIMING AND INTENSITY OF THE  
SURFACE LOW (SLIGHTLY CLOSER TO THE COAST THAN THE GFS). WITH THE  
BEST CLUSTERING CONTINUING TO BE WITH THE GFS, UKMET, AND ECMWF,  
THAT CONTINUES TO BE THE PREFERRED BLEND.  
 
---PREV. DISCUSSION---  
AS THE UPSTREAM WAVE CONTRIBUTING TO THE GRADUAL CYCLOGENESIS  
PUSHES THROUGH THE GREAT LAKES REGION EARLY WEDNESDAY, SMALL  
DIFFERENCES BEGIN TO EMERGE. THE 12Z NAM SHOWS A SLIGHTLY DEEPER  
TROUGH WHICH EVENTUALLY LEADS TO EARLIER SURFACE LOW DEVELOPMENT,  
WITH A STRONGER SURFACE LOW THAT IS ON THE WESTERN PERIPHERY OF  
MODEL SPREAD AS IT MOVES INTO THE NORTHEAST. THE 00Z CMC, BY  
CONTRAST, SHOWS SLIGHTLY LESS AMPLIFICATION OF THE TROUGH IN THE  
NORTHERN GREAT LAKES, WITH MORE DOWNSTREAM RIDGING. THIS LEADS TO  
ONE OF THE WEAKER SURFACE LOWS DEPICTED BY OPERATIONAL MODELS. THE  
PREFERENCE IS TO LEAN CLOSER TO THE GFS, ECMWF, AND UKMET WHICH  
ARE MORE TIGHTLY CLUSTERED AND THEIR MASS FIELDS ARE REASONABLY  
CLOSE TO THE 00Z ECMWF ENSEMBLE MEAN. OVERALL DIFFERENCES WITH  
THIS SYSTEM ARE RELATIVELY SMALL, BUT EVEN MINOR FLUCTUATIONS IN  
THE STRENGTH OR TRACK OF THE SURFACE LOW WOULD HAVE IMPLICATIONS  
ON THE PRECIPITATION TYPE FORECAST ACROSS PORTIONS OF THE  
NORTHEAST.  
 
...GRADUAL DEEPENING OF A BROAD TROUGH WITH SEVERAL DIGGING WAVES  
ACROSS THE NORTHERN TIER OF STATES THURSDAY TO FRIDAY...   
..SOUTHWARD PROGRESSION OF AN ARCTIC HIGH INTO NORTHERN PLAINS
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: BLEND OF 12Z GFS, ECMWF  
CONFIDENCE: SLIGHTLY BELOW AVERAGE  
 
---18Z UPDATE---  
NO CHANGE TO THE PRELIMINARY PREFERENCE. DESPITE ALL MODELS  
AGREEING ON BROAD TROUGH AMPLIFICATION, THERE REMAIN SUBSTANTIAL  
DIFFERENCES IN THE DETAILS. THE PREFERENCE IS TO MAINTAIN  
CONTINUITY WITH THE ORIGINAL ASSESSMENT.  
 
---PREV. DISCUSSION---  
SEVERAL DIGGING WAVES WILL CONTRIBUTE TO HEIGHT FALLS AND A  
GRADUAL DEEPENING OF THE BROAD TROUGH ACROSS THE NORTHERN TIER OF  
STATES LATE THIS WEEK. ONE WAVE, CURRENTLY NORTH OF THE PRIMARY  
MID-UPPER LEVEL LOW OVER NUNAVUT, WILL ROUND THE LOW AND BECOME  
INCREASINGLY SHEARED OUT AS IT KICKS INTO THE WESTERN GREAT LAKES  
ON FRIDAY. MODEL AGREEMENT IS GENERALLY GOOD WITH THAT PARTICULAR  
WAVE, HOWEVER GREATER DIFFERENCES ARE NOTED WITH A SECONDARY WAVE  
THAT WILL BE DIGGING FROM THE GULF OF ALASKA INTO THE  
INTERMOUNTAIN WEST. THIS DIGGING WAVE WILL BE THE RESULT OF  
COMPLEX PHASING OF SEVERAL ELEMENTS IN THE VICINITY OF ALASKA OVER  
THE NEXT 24 HOURS -- ONE SHORTWAVE ROUNDING THE TOP OF AN EASTERN  
PACIFIC RIDGE, AND ANOTHER SHORTWAVE UNDERCUTTING THE NORTHWARD  
EXTENSION OF THE RIDGE OVER ALASKA. AS THESE TWO ELEMENTS PHASE,  
MODEL SPREAD MARKEDLY INCREASES, WHICH HAS DOWNSTREAM IMPLICATIONS.  
 
THE 12Z NAM PRODUCES A MUCH DEEPER TROUGH OVER PORTIONS OF THE  
WEST, WITH THE LEAD PORTION OF THE WAVE ACCELERATING INTO THE  
PLAINS BY 10/00Z, WELL AHEAD OF ANY OTHER OPERATIONAL MODEL. WITH  
A LACK OF SUPPORT FROM OTHER MODELS AND ENSEMBLES, THE NAM IS NOT  
INCLUDED IN THIS PREFERENCE. THE NEXT FASTEST SOLUTION IS  
REPRESENTED BY THE 00Z UKMET/CMC WHICH ARE NOTABLY AHEAD OF THE  
ENSEMBLE MEANS. THE 12Z GFS AND 00Z ECMWF, BY CONTRAST, ARE  
CLOSEST TO THE ENSEMBLE MEANS, AND IN PARTICULAR THE 06Z GEFS AND  
00Z ECMWF ENSEMBLE. THEREFORE, THE PREFERENCE IS FOR A BLEND OF  
THE GFS AND ECMWF, WITH A SLIGHT LEAN IN THE DIRECTION OF THE  
ECMWF. THE GFS HAS HIGHER HEIGHTS ACROSS PORTIONS OF THE WEST AT  
THE BASE OF THE TROUGH THAN MANY OTHER OPERATIONAL MODELS, AS WELL  
AS ITS ENSEMBLE MEAN, WHILE THE ECMWF IS BETTER SUPPORTED BY A  
RANGE OF OPERATIONAL MODELS AND IS MORE SIMILAR TO BOTH THE GEFS  
AND ECMWF ENSEMBLE MEAN.  
 
...BROAD SOUTHWESTERLY RETURN FLOW AND ASSOCIATED INCREASING  
PRECIPITATION CHANCES FRIDAY OVER THE SOUTHEAST...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: BLEND OF 12Z GFS, ECMWF  
CONFIDENCE: AVERAGE  
 
---18Z UPDATE---  
NO CHANGE TO PRELIMINARY PREFERENCE. THE 12Z ECMWF TRENDED CLOSER  
TO THE GFS IN TERMS OF DEPICTING A SLOWER EROSION OF THE LOW-LEVEL  
RIDGE, WITH ASSOCIATED CHANGES IN THE PRECIPITATION ACROSS THE  
SOUTHEAST.  
 
---PREV. DISCUSSION---  
DESPITE SOME SIMILARITY IN THE MASS FIELDS, THERE ARE DIFFERENCES  
IN HOW THE MODELS HANDLE PRECIP ACROSS THE SOUTHEAST ON FRIDAY.  
DIFFERENCES APPEAR TO BE PARTIALLY RELATED TO THE STRENGTH OF THE  
RESIDUAL HIGH AND SURFACE RIDGE ALONG THE EAST COAST AND DOWN THE  
LEE OF THE APPALACHIANS, AS WELL AS HOW MODELS HANDLE THE  
DEEPENING OF THE UPSTREAM TROUGH. GIVEN THE INFLUENCE OF THE TWO  
PRIMARY SHORT-TERM SYSTEMS ON CONDITIONS IN THE SOUTHEAST, THE  
PREFERENCE IS LEANING TOWARD THE MODELS THAT HANDLE THOSE SYSTEMS  
BEST: THE GFS AND ECMWF. THE 12Z GFS SHOWS A STRONGER RIDGE OVER  
THE SOUTHEAST AND RESTRICTS THE RETURN FLOW A LITTLE LONGER. DRIER  
AIR AND A SLOWER TIMING OF THE RAMP UP IN RETURN FLOW RESULT IN  
LESS PRECIP THAN THE ECMWF, WHICH ERODES THE INFLUENCE OF THE  
RIDGE FASTER. GIVEN UNCERTAINTIES AT THIS TIME RANGE, THE  
PREFERENCE IS FOR A BLEND OF THE TWO AT THIS TIME.  
 
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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