330  
FXUS10 KWNH 031655  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1154 AM EST SUN DEC 03 2017  
 
VALID DEC 03/1200 UTC THRU DEC 07/0000 UTC  
 
...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR  
INGEST...  
 
12Z MODEL EVALUATION WITH PRELIMINARY PREFERENCES AND CONFIDENCE  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
 
...POSITIVELY-TILTED TROUGH DIGGING THROUGH THE INTERMOUNTAIN WEST  
AND REACHING THE PLAINS ON MONDAY...  
...INCREASING NEGATIVE TILT WITH ACCOMPANYING RAPID CYCLOGENESIS  
OVER THE WESTERN GREAT LAKES...  
...ACCOMPANYING COLD FRONT PUSHING EAST INTO THE EASTERN GREAT  
LAKES AND MID ATLANTIC...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: BLEND OF 12Z NAM / 00Z ECMWF ENS MEAN  
CONFIDENCE: AVERAGE  
 
OVER THE PAST SEVERAL MODEL CYCLES...THE MODELS HAVE CONTINUED TO  
CONVERGE BUT DETAIL DIFFERENCES REMAIN ON THE EXACT  
TRACK...TIMING...AND INTENSITY OF THE DEEPENING SURFACE LOW IN THE  
NORTH-CENTRAL US. THE MOST NOTEWORTHY TREND THUS FAR ON THE 12Z  
MODEL CYCLE IS THAT THE 12Z GFS HAS TRENDED STRONGER AND NORTHWEST  
WITH ITS SURFACE LOW AS IT MOVES IN THE VICINITY OF MINNESOTA ON  
MONDAY. THE 12Z NAM ALSO TRENDED SLIGHTLY TO THE  
NORTHWEST...ALTHOUGH NOT IN A PRONOUNCED FASHION LIKE THE GFS. THE  
QUESTION IS WHETHER THE GFS IS LEADING A SUBSTANTIVE TREND. THIS  
SHOULD BECOME CLEARER WITH THE ARRIVAL OF THE REST OF THE 12Z  
MODELS.  
 
THE GEFS AND ECMWF ENSEMBLE MEANS DO REVEAL A BROADER TREND TO THE  
NORTHWEST (AND DEEPER) WITH THE SURFACE LOW OVER THEIR PAST  
SEVERAL CYCLES...SHIFTING ABOUT 70-100KM NORTHWEST WITH THE LOW  
POSITION AND ABOUT 3-5MB DEEPER WITH THE PRESSURE SINCE THE 02/00Z  
MODEL CYCLE. HOWEVER...
THE 12Z GFS IS NOW POSITIONED NORTHWEST OF  
THE VAST MAJORITY OF 00Z ENSEMBLE MEMBERS (GEFS, ECMWF AND  
CMC)...AND IS ON THE LOW END OF PRESSURE VALUES AS WELL. IT ALSO  
CONTINUES TO BE AN OUTLIER WITH HOW FAST IT BRINGS THE EJECTING  
SHORTWAVE INTO THE WESTERN GREAT LAKES. THIS REDUCES CONFIDENCE IN  
THAT EXACT SOLUTION.  
 
ENSEMBLE SENSITIVITY ANALYSIS BASED ON THE 00Z MODEL CYCLE  
REVEALED THAT THE INTENSITY AND POSITION OF THE LOW WHEN IT IS  
NEAR LAKE SUPERIOR IS VERY LIKELY SENSITIVE TO THE DETAILS WITH  
THE TROUGH COMING INTO THE NORTHWEST NOW...AND SUGGESTED THAT  
HIGHER HEIGHTS IN THAT REGION COULD LEAD TO A WEAKER LOW FURTHER  
TO THE SOUTHEAST IN THE GREAT LAKES. IF ANYTHING...THE 12Z GFS  
CONSISTENTLY SHOWED INITIALLY LOWER HEIGHTS THAN UPPER AIR  
OBSERVATIONS (ALBEIT VERY SLIGHTLY) IN THAT REGION. THIS ALSO  
REDUCES CONFIDENCE IN THAT EXACT SOLUTION.  
 
HOWEVER...TO ACCOUNT FOR THE BROADER TREND TO THE NORTHWEST WITH  
THE LOW POSITION...THE PREFERENCE IS FOR A SOLUTION THAT  
REPRESENTS A REASONABLE LOW EVOLUTION GENERALLY ON THE NORTHWEST  
EDGE OF TIGHTER ENSEMBLE CLUSTERING. THIS APPEARS TO BE BEST  
REPRESENTED BY THE 12Z NAM AND 00Z ECMWF ENSEMBLE MEAN AT THIS  
POINT WHICH LIE TO THE NORTHWEST OF OTHER OPERATIONAL MODELS...BUT  
SOUTHEAST OF THE 12Z GFS. EXAMINING THE 12Z UKMET AS IT COMES  
IN...IT APPEARS TO BE VERY CLOSE TO THE 12Z NAM WITH ITS HEIGHT  
FIELDS ALOFT...BUT BECOMES AN OUTLIER WITH THE SURFACE LOW  
POSITION AS IT DEEPENS (JUMPS AROUND RELATIVE TO OTHER MODEL  
TRACKS) AND OCCLUDES (SLOWS DOWN SUBSTANTIALLY). FOR THIS  
REASON...THE PREFERENCE FOR THE MASS FIELDS WITH THIS SYSTEM WILL  
BE A BLEND OF THE 12Z NAM AND 00Z ECMWF ENSEMBLE...PENDING THE  
REST OF THE 12Z MODEL RUNS.  
 
   
..TRAILING LOBE OF VORTICITY FROM CENTRAL US TROUGH
 
 
...ASSOCIATED SOUTHERN EXTENT OF TRAILING COLD FRONT SLOWING DOWN  
AS IT MOVES INTO THE SOUTHERN US...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: NON-CMC BLEND  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
OVERALL MODELS SHOW FAIRLY GOOD AGREEMENT WITH THE SOUTHERN EXTENT  
OF THE TRAILING COLD FRONT AND ASSOCIATED ELONGATED TROUGH / LOBE  
OF VORTICITY EXTENDING ACROSS THE SOUTHERN US. ALTHOUGH SOME  
DETAIL DIFFERENCES EXIST...THESE COULD BE ACCOUNTED FOR REASONABLY  
WELL WITH A GENERAL MODEL BLEND. THE OUTLIER APPEARS TO BE THE 00Z  
CMC WHICH HAS A MUCH MORE COMPACT SHORTWAVE THAT IT ROTATES  
THROUGH THE SOUTHERN US...LEADING TO A FASTER SOUTHEAST  
PROGRESSION OF THE FRONT AND LESS SENSIBLE WEATHER IMPACTS ON THE  
STALLED PORTION OF THE FRONT EXTENDING WEST FROM THE GULF OF  
MEXICO INTO TEXAS BY MID-WEEK. FOR THIS REASON...A NON-CMC BLEND  
IS THE PREFERENCE.  
 
...LOW AMPLITUDE SHORTWAVE OVER TEXAS CONTINUING TO WEAKEN / GET  
ABSORBED INTO LARGER TROUGH AS BROAD HEIGHT FALLS COMMENCE...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: ABOVE AVERAGE  
 
MODELS SHOW A GREAT DEGREE OF SIMILARITY WITH THE MASS FIELDS FOR  
THIS SYSTEM. THEREFORE...A GENERAL MODEL BLEND IS PREFERRED.  
 
   
..TROUGH OVER THE NORTHEAST US CONTINUING TO AMPLIFY OFFSHORE
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: ABOVE AVERAGE  
 
MODELS SHOW A GREAT DEGREE OF SIMILARITY WITH THIS SYSTEM WHILE IT  
IS WITHIN REASONABLE PROXIMITY (ABOUT 1000KM OR SO) OF THE  
CONUS...THEREFORE A GENERAL MODEL BLEND IS PREFERRED.  
 
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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