228  
FXUS10 KWNH 041648  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1147 AM EST MON DEC 04 2017  
 
VALID DEC 04/1200 UTC THRU DEC 08/0000 UTC  
 
...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR  
INGEST...  
 
12Z MODEL EVALUATION WITH PRELIMINARY PREFERENCES AND CONFIDENCE  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
 
   
..STRONG OCCLUDING CYCLONE OVER THE WESTERN GREAT LAKES
 
 
...ACCOMPANYING COLD FRONT PUSHING THROUGH MUCH OF THE EASTERN  
CONUS BY WEDNESDAY EVENING...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: BLEND OF 12Z GFS / 12Z NAM / 00Z ECMWF  
CONFIDENCE: ABOVE AVERAGE  
 
MODELS HAVE COME INTO MUCH BETTER AGREEMENT WITH THIS  
SYSTEM...BOTH SHOWING TIGHT CLUSTERING OF SURFACE LOW POSITIONS  
AND INTENSITIES...AS WELL AS SIMILARITY WITH MANY OF THE MASS  
FIELDS ALOFT. DUE TO SOME SLIGHT DIFFERENCES IN THE CMC AND UKMET  
THAT MAY MAKE A DIFFERENCE WITH SOME SENSIBLE WEATHER  
IMPACTS...THEY WERE EXCLUDED FROM THE PREFERENCE AT THIS TIME. THE  
00Z CMC IS FURTHER TO THE LEFT AND FASTER WITH THE SURFACE  
LOW...WHICH DOES NOT HAVE MUCH SUPPORT FROM THE ENSEMBLES OR OTHER  
GLOBAL MODELS. IT DOES SHOW THE SHORTWAVE ALOFT ADVANCING SLIGHTLY  
FASTER...WITH THE AXIS ABOUT 100KM FURTHER NORTH THAN THE  
CONSENSUS AGREED TO BY THE OTHER MODELS. THE 00Z CMC MAY ALSO BE  
SLIGHTLY FASTER WITH THE COLD FRONT ON THE EAST COAST.  
MEANWHILE...THE 00Z UKMET CONTINUES TO SHOW A SLIGHT SLOW BIAS  
WITH THE LOW AS IT OCCLUDES...WHICH HAS SHOWN UP IN THE PAST  
SEVERAL MODEL CYCLES. OTHER MODELS DO SLOW THE PROGRESSION OF THE  
LOW...BUT NOT TO THE SAME DEGREE. THE UKMET ALSO ADVANCES THE COLD  
FRONT MORE SLOWLY ON THE EAST COAST THAN THE OTHER MODELS.  
 
THE 12Z GFS...12Z NAM...AND 00Z ECMWF ARE OTHERWISE IN GOOD TO  
EXCELLENT AGREEMENT AND HAVE STRONG SUPPORT FROM THE ENSEMBLE  
MEANS (GEFS AND ECMWF). THEREFORE...THE PREFERENCE LEANS TOWARD A  
BLEND OF THOSE THREE. SOME SMALL DETAIL DIFFERENCES FROM THE NAM  
MAY ARGUE FOR SLIGHTLY LESS WEIGHT -- IT IS SLIGHTLY SLOWER WITH  
THE COLD FRONT OVER THE EAST (NOT TO THE SAME DEGREE AS THE  
UKMET)...AND IS ALSO SLOWER WITH AN EMBEDDED SHORTWAVE ROUNDING  
THE PRIMARY CYCLONE AND REACHING THE NORTHEAST ON WEDNESDAY. THIS  
COULD AFFECT THE ORIENTATION OF SOME OF THE WIND FIELDS.  
HOWEVER...IT IS CLOSE ENOUGH TO THE GFS AND ECMWF THAT IT COULD BE  
INCORPORATED INTO A BLEND.  
 
   
..TRAILING LOBE OF VORTICITY FROM CENTRAL US TROUGH
 
 
...ASSOCIATED SOUTHERN EXTENT OF TRAILING COLD FRONT STALLING IN  
THE GULF AND INTO SRN TX AND NRN MEXICO...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: BLEND OF 12Z GFS / 12Z NAM / 00Z ECMWF  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
AS DELINEATED IN THE DISCUSSION ABOUT THE PRIMARY CENTRAL NOAM  
CYCLONE...THE CMC AND UKMET HAVE THE GREATEST DIFFERENCES WITH THE  
TIMING OF THE COLD FRONT FROM THE ENSEMBLE MEANS AND OTHER  
AVAILABLE OPERATIONAL MODELS. OTHERWISE...THERE ARE DETAIL  
DIFFERENCES OVERALL BUT THEY ARE NOT SUBSTANTIAL ENOUGH IN ANY ONE  
MODEL TO LEAN IN A PARTICULAR DIRECTION. GIVEN THE TIMING  
DIFFERENCES WITH THE UKMET AND CMC...WILL PREFER A BLEND OF THE  
GFS...NAM AND ECMWF WITH THE STALLING FRONT AS WELL.  
 
...SECONDARY SHORTWAVES ON BACKSIDE OF OCCLUDING CYCLONE DIGGING  
FROM CANADIAN PRAIRIES INTO THE PLAINS...  
...GRADUAL AMPLIFICATION OF THE TAIL END OF THE TROUGH OVER THE  
FOUR CORNERS REGION...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: NON-UKMET BLEND  
CONFIDENCE: AVERAGE  
 
THERE ARE DETAIL DIFFERENCES WITH HOW THE MODELS HANDLE SEVERAL  
LOW AMPLITUDE SHORTWAVES THAT WILL DIG DOWN THE BACKSIDE OF THE  
TROUGH INTO THE PLAINS. OVERALL THOUGH THESE ALL FIT WITHIN  
ENSEMBLE SPREAD...WITH THE EXCEPTION OF THE 00Z UKMET WHICH SHOWS  
CONSIDERABLY MORE AMPLIFICATION OVER THE FOUR CORNERS REGION THAN  
ALMOST ANY AVAILABLE ENSEMBLE MEMBER OR OPERATIONAL MODEL. AS IT  
IS A SIGNIFICANT OUTLIER...IT HAS BEEN EXCLUDED FROM THE  
PREFERENCE. THE REMAINING VARIANCE APPEARS TO BE EXPLAINED WELL BY  
MODEL CENTER -- WITH THE ECMWF AND ITS ENSEMBLE MEMBERS SHOWING  
THE MOST AMPLIFICATION (AND SLOWEST TIMING OF THE TROUGH  
AXIS)...THE GFS AND GEFS IN THE MIDDLE...AND THE CMC AND ITS  
ENSEMBLE SHOWING THE LEAST AMPLIFICATION (AND FASTEST TIMING OF  
THE TROUGH AXIS). THIS MAY PRODUCE SOME DIFFERENCES IN SENSIBLE  
WEATHER IMPACTS FROM NORTHERN MEXICO INTO TEXAS. THROUGH  
07/12Z...MOST OF THE OPERATIONAL MODELS (EXCEPT THE UKMET) SIT IN  
THE MIDDLE OF THE SPREAD...SO A GENERAL NON-UKMET BLEND SHOULD  
CAPTURE A REASONABLE CONSENSUS SOLUTION.  
 
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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