542  
FXUS10 KWNH 071838  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
138 PM EST THU DEC 07 2017  
 
VALID DEC 07/1200 UTC THRU DEC 11/0000 UTC  
 
...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR  
INGEST...  
 
12Z MODEL EVALUATION WITH FINAL PREFERENCES AND CONFIDENCE  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
 
   
..STALLED FRONT ACROSS THE SOUTHERN CONUS / GULF COAST
 
 
...DIGGING SHORTWAVE IN NM/TX ROUNDING THE BASE OF THE TROUGH AND  
MOVING ALONG THE GULF COAST FRI NGT AND SAT...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: BLEND OF 12Z ECMWF / 12Z NAM / 12Z UKMET  
GREATEST WEIGHT PLACED ON THE 12Z ECMWF  
CONFIDENCE: AVERAGE  
 
DESPITE THE MAIN IMPACTS FROM THIS SYSTEM ONLY BEING ABOUT 12-36  
HOURS AWAY...THERE IS STILL A FAIR AMOUNT OF MODEL DISAGREEMENT ON  
SOME OF THE SPECIFICS -- PRIMARILY RELATED TO THE INTENSITY OF THE  
CORE OF THE SHORTWAVE AND TIMING. THE 12Z GFS HAS TRENDED VERY  
SLIGHTLY SLOWER WITH THE EJECTING SHORTWAVE...BUT STILL REMAINS ON  
THE FAST END OF GUIDANCE AND APPEARS TO INCREASINGLY BE AN OUTLIER  
IN THAT REGARD. MORE OF THE MODELS SEEM TO SHOW A TIMING ABOUT 6  
HOURS SLOWER...SO THE PREFERENCE IS TO LEAN IN THAT DIRECTION WITH  
RESPECT TO TIMING.  
 
OVERALL THERE HAS BEEN A CONVERGENCE OF MODEL SPREAD IN TERMS OF  
THE EXPECTED AMPLITUDE OF THE WAVE AS IT EJECTS EAST ALONG THE  
GULF COAST. THE NAM AND CMC WERE ON THE MORE AMPLIFIED END OF  
GUIDANCE SEVERAL MODEL CYCLES AGO AND HAVE SLIGHTLY TRENDED TOWARD  
THE CONSENSUS. MEANWHILE THE GFS WAS ON THE LESS AMPLIFIED END OF  
GUIDANCE AND HAS ALSO TRENDED TOWARD THE CONSENSUS. THAT BEING  
SAID...THERE STILL REMAINS SOME DISAGREEMENT IN THIS RESPECT IN  
ADDITION TO THE TIMING DIFFERENCES. THE NEW 12Z UKMET...12Z NAM  
AND 00Z ECMWF ARE VERY SIMILAR IN THEIR TIMING...BUT OFFER A  
REASONABLE SPREAD OF POSSIBLE SOLUTIONS WITH RESPECT TO THE  
STRENGTH OF THE WAVE AS IT TRENDS EAST...IN ADDITION TO OTHER MASS  
FIELDS. THE NAM IS CURRENTLY THE MOST AMPLIFIED WITH COLDEST  
LOW-MID LEVEL TEMPERATURE PROFILE...THE UKMET IN THE MIDDLE...AND  
THE ECMWF ON THE LESS AMPLIFIED END OF GUIDANCE.  
 
18Z UPDATE: NO CHANGE TO THE PRELIMINARY PREFERENCE. MODELS TEND  
TO AGREE MORE ON THE OVERALL AMPLITUDE OF THE WAVE NOW...WITH THE  
GREATEST DIFFERENCES NOW FOCUSED AROUND TIMING. THE 12Z ECMWF IS  
SLIGHTLY FASTER NOW AND LIES BETWEEN THE FASTEST GFS SOLUTION AND  
THE SLOWER NAM/UKMET/CMC SOLUTIONS. STILL PREFER TO INCLUDE THE  
NAM IN THE PREFERENCE AS THE TREND OVERALL HAS BEEN FOR A MORE  
AMPLIFIED WAVE IN MOST OF THE AVAILABLE MODELS. ALSO PREFER TO  
MAINTAIN THE INCLUSION OF THE ECMWF DESPITE THE SLIGHTLY FASTER  
TREND AS THIS WILL BETTER REPRESENT THE SPREAD IN THE DIRECTION OF  
THE GFS/GEFS. TOGETHER...A BLEND OF THE UKMET...ECMWF...AND NAM  
SHOULD YIELD SOMETHING CLOSE TO A CONSENSUS SOLUTION AS SPREAD  
CONTINUES TO NARROW.  
 
...SHORTWAVE DIGGING FROM THE ARCTIC CIRCLE IN NW CANADA TO THE  
WESTERN GREAT LAKES BY SATURDAY MORNING...   
..PHASING WITH SHORTWAVE EJECTING UP THE EAST COAST
 
   
..SURFACE LOW DEVELOPMENT JUST OFF THE EASTERN SEABOARD
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: BLEND OF 12Z ECMWF / 12Z NAM / 12Z UKMET  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
THE AFOREMENTIONED SYSTEM ALONG THE GULF COAST WILL PLAY A ROLE UP  
THE EAST COAST AS THE ASSOCIATED EJECTING WAVE TRANSLATES  
NORTHEAST AND BEGINS TO PHASE WITH A DIGGING SHORTWAVE THROUGH THE  
GREAT LAKES REGION. THE RESULT SHOULD BE SURFACE CYCLOGENESIS  
ABOUT 250-300KM OFF THE EAST COAST. THE OVERALL TREND WITH THIS  
LOW OVER THE PAST SEVERAL MODEL CYCLES HAS BEEN TO NUDGE THE LOW  
TRACK CLOSER TO THE COAST. THIS IS EVIDENT IN THE GEFS AND ECMWF  
ENSEMBLE MEANS...AS WELL AS THE OPERATIONAL MODELS. ALL OF THE 12Z  
OPERATIONAL RUNS THAT ARE AVAILABLE SO FAR (NAM, GFS, UKMET) LOOK  
VERY SIMILAR IN TERMS OF LOW TRACK...BUT THE GFS IS STILL FASTER  
WITH TIMING BY APPROXIMATELY 6 HOURS. THE 00Z CMC AND 00Z ECMWF  
ARE STILL ABOUT A DEGREE OR TWO OF LONGITUDE EAST...BUT GIVEN THE  
OVERALL TRENDS...THEY MAY SHOW MORE SIMILARITY ON THE 12Z RUNS  
WHEN THEY ARRIVE. THE PREFERENCE WILL BE FOR A BLEND OF THE  
NAM...UKMET...AND ECMWF...WITH A SLIGHTLY HIGHER WEIGHT GIVEN TO  
THE NAM/UKMET SOLUTION OVERALL AS THE TREND IS IN THAT DIRECTION.  
 
18Z UPDATE: NO CHANGE TO THE PRELIMINARY PREFERENCE. MODEL SPREAD  
ALSO CONTINUES TO NARROW WITH THE SYSTEM AS IT MOVES UP THE EAST  
COAST...AS WELL AS WITH THE DIGGING SHORTWAVE THROUGH THE GREAT  
LAKES. THE WESTWARD TREND OF THE SURFACE LOW TRACK HAS CONTINUED  
ON ALL THE MODELS...INCLUDING THE NEW 12Z ECMWF AND 12Z CMC. THE  
GFS CONTINUES TO LOOK LIKE A SLIGHTLY FASTER SOLUTION...DESPITE  
THE ECMWF TRENDING A LITTLE FASTER. A BLEND OF THE ECMWF/UKMET/NAM  
SHOULD YIELD A REASONABLE CONSENSUS SOLUTION AT THIS TIME...AS  
STATED WITH THE AFOREMENTIONED SYSTEM...WITH THE NEW ECMWF  
CAPTURING A SLIGHTLY FASTER POSSIBILITY ADVERTISED BY THE  
GFS/GEFS...AND THE CMC STILL SITTING ON THE EASTERN END OF MODELS  
WITH SURFACE LOW POSITION (AGAINST THE OVERALL TREND).  
 
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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