719  
FXUS10 KWNH 011846  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
146 PM EST FRI DEC 01 2017  
 
VALID DEC 01/1200 UTC THRU DEC 05/0000 UTC  
 
...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR  
INGEST...  
 
12Z MODEL EVALUATION WITH FINAL PREFERENCES AND CONFIDENCE  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
 
   
..NORTHERN STREAM TROUGH EXITING THE NORTHEAST TODAY
 
   
..AMPLIFICATION AND WEAK CYCLOGENESIS OFFSHORE OVER ATLANTIC
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: ABOVE AVERAGE  
 
THROUGH 03/00Z...OR THE FIRST 36 HOURS OF THE FORECAST...THE  
MODELS SHOW SIMILARITY WITH THIS SYSTEM. AFTER THAT POINT...THE  
TROUGH IS AT LEAST 1000 KM OFFSHORE AND THE SENSIBLE WEATHER  
IMPACTS TO THE CONUS SHOULD BE NEGLIGIBLE. THEREFORE...A GENERAL  
MODEL BLEND IS PREFERRED FOR THIS SYSTEM.  
 
18Z UPDATE: NO CHANGE TO PRELIMINARY PREFERENCE. MODELS CONTINUE  
TO SHOW SIMILARITY WITH THIS SYSTEM WHEN IT IS CLOSE TO THE  
ATLANTIC COAST.  
 
...LOW AMPLITUDE SHORTWAVES OVER THE CENTRAL U.S. IN ZONAL FLOW  
QUICKLY PROGRESSING EAST BY LATE SATURDAY TO SUNDAY...   
..POSSIBLE WEAK SURFACE WAVE DEVELOPING NEAR COASTAL NC
 
   
..AMPLIFYING TROUGH OFF THE NORTHEAST ON SUNDAY INTO MONDAY
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: BLEND OF THE 12Z GFS / 12Z ECMWF / 12Z UKMET  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
ACROSS MUCH OF THE DOWNSTREAM REGION IN THE EASTERN CONUS AND INTO  
THE WESTERN ATLANTIC...THE 00Z CMC APPEARS TO BE BIASED HIGH WITH  
ITS HEIGHTS ALOFT CONSISTENTLY COMPARED TO OTHER MODELS. THIS DOES  
EVENTUALLY LEAD TO SOME DIFFERENCES IN THE OVERALL PATTERN AS THE  
NORTHERN STREAM TROUGH UNDERGOES AMPLIFICATION OVER THE NW  
ATLANTIC. OVERALL THE TIMING OF THE NORTHERN STREAM TROUGH IS NOT  
TOO DIFFERENT FROM OTHER MODELS...BUT IN TERMS OF POSITIONING AND  
AMPLITUDE...IT MAY NOT REPRESENT THE MOST LIKELY SOLUTION. THE 00Z  
UKMET IS SLOWER AND MORE AMPLIFIED WITH THE NORTHERN STREAM  
TROUGH...AND DOES NOT HAVE MUCH MODEL SUPPORT FOR THIS IDEA. THIS  
DIFFERENCE EMERGES QUICKLY IN THE FIRST 24-36 HOURS OF THE  
FORECAST OVER THE NORTHERN PLAINS AND GROWS FROM THERE. THE 12Z  
NAM MORE AGGRESSIVELY AMPLIFIES THE TROUGH AND ENDS UP EVEN MORE  
AMPLIFIED THAN THE UKMET DESPITE SOME INITIAL SIMILARITY WITH THE  
GFS AND ECMWF. HOWEVER THE 12Z GFS AND 00Z ECMWF ARE FAIRLY CLOSE  
TO THE MULTI-MODEL ENSEMBLE MEAN AND SHOULD REPRESENT THE MOST  
LIKELY SOLUTION. THEREFORE A BLEND OF THE ECMWF AND GFS IS  
PREFERRED.  
 
18Z UPDATE: ADD UKMET TO THE PREFERENCE. THE 12Z CMC CONTINUES TO  
HAVE HIGHER HEIGHTS ACROSS MUCH OF THE EAST COAST AS THE  
SHORTWAVES APPROACH AND MOVE INTO THE ATLANTIC...AND THE 12Z NAM  
CONTINUES TO SHOW FASTER AMPLIFICATION THAN THE OTHER MODELS.  
HOWEVER THE 12Z UKMET HAS MOVED INTO BETTER AGREEMENT WITH THE  
ECMWF AND GFS AND IT HAS BEEN ADDED TO THE PREFERENCE. GREATER  
DIFFERENCES EMERGE BEYOND 04/12Z BUT THE NORTHERN STREAM TROUGH IS  
WELL OFFSHORE AT THAT POINT.  
 
   
..CLOSED LOW JUST WEST OF NORTHERN BAJA CALIFORNIA
 
 
...DEAMPLIFYING AND ACCELERATING ENE WITH THE SUBTROPICAL JET INTO  
TEXAS BY SUNDAY...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: ABOVE AVERAGE  
 
MODELS CONTINUE TO CONVERGE WITH THE TIMING OF THIS SYSTEM AND ARE  
GENERALLY IN GOOD AGREEMENT AT THIS POINT. HOWEVER...AS THE WAVE  
DEAMPLIFIES THERE ARE STILL MINOR TIMING DIFFERENCES. THE 00Z  
UKMET AND 00Z CMC CONTINUE TO BE SLIGHTLY SLOWER THAN THE MORE  
CONSISTENT ECMWF...AND ARE SLIGHTLY OUT OF PHASE WITH THE TROUGH.  
THE 12Z NAM AND 12Z GFS ARE A LITTLE SLOWER AS WELL...WHILE THE  
06Z GEFS MEAN IS VERY WELL ALIGNED WITH THE ECMWF AND ECMWF  
ENSEMBLE MEAN. GIVEN THE STRONG CONSISTENCY OF THE ECMWF AND ECMWF  
ENSEMBLE SOLUTIONS THUS FAR OVER THE PAST SEVERAL MODEL  
CYCLES...AND THE AGREEMENT FROM THE GEFS MEAN...WILL LEAN IN THAT  
DIRECTION AT THE MOMENT.  
 
18Z UPDATE: CHANGE TO A GENERAL MODEL BLEND. WITH THE COMPLETION  
OF THE 12Z MODEL CYCLE...THE OPERATIONAL MODELS AND 12Z GEFS MEAN  
ARE ALL IN EXCELLENT AGREEMENT NOW WITH THE TIMING AND AMPLITUDE  
OF THE DEAMPLIFYING WAVE AS IT MOVES INTO NM/TX. GIVEN THAT THE  
SMALL CHANGES FROM THE 00Z CYCLE HAVE RESULTED IN GOOD  
AGREEMENT...A GENERAL MODEL BLEND IS NOW PREFERRED.  
 
...POSITIVELY-TILTED GULF OF ALASKA TROUGH DIGGING INTO PACIFIC  
NORTHWEST BY SATURDAY AND ASSOCIATED COLD FRONT...   
..LEE CYCLOGENESIS IN THE CENTRAL PLAINS BY LATE SUNDAY
 
   
..STRENGTHENING CYCLONE INTO THE UPPER MISS. RIVER VALLEY
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: BLEND OF 12Z GFS / 12Z CMC / 12Z GEFS / 12Z ECMWF ENS  
CONFIDENCE: AVERAGE  
 
FAIRLY SUBSTANTIAL DIFFERENCES EMERGE ACROSS MUCH OF THE WESTERN  
CONUS BY 03/00Z (36HR FCST) AS THE 00Z ECMWF BRINGS THE TROUGH IN  
FASTER AND DIG THE TROUGH FURTHER SOUTHEAST. THIS LEADS TO SOME  
40-50M HEIGHT DIFFERENCES AT 500MB ACROSS PORTIONS OF THE GREAT  
BASIN THIS WEEKEND BETWEEN THE ECMWF AND SOME OF THE OTHER  
OPERATIONAL MODELS. ENSEMBLE SPAGHETTI PLOTS AT  
500MB...PARTICULARLY AROUND 546-552DKM...REVEAL THE 00Z ECMWF ON  
THE FAST/SE EXTREME OF ALL AVAILABLE ENSEMBLE MEMBERS FROM THE  
GEFS...CMC...AND ECMWF ENSEMBLES. THIS LEADS TO STRONGER  
CYCLOGENESIS ACROSS THE MIDWEST WITH A FASTER OCCLUSION AND A  
SURFACE LOW POSITION FURTHER TO THE SW AS COMPARED TO OTHER  
MODELS. THE 00Z UKMET...WHICH IS INITIALLY SIMILAR TO THE OTHER  
MODELS...DOES HAVE A STRONGER SHORTWAVE (COMPARED TO OTHER MODELS)  
AT THE BASE OF THE TROUGH AS IT EMERGES FROM THE ROCKIES...WHICH  
ULTIMATELY GIVES IT A STRONGER CYCLONE IN THE MIDWEST AS WELL.  
BOTH OF THESE SOLUTIONS APPEAR TO BE OUTLIERS TO A CERTAIN EXTENT  
RIGHT NOW...SO THE PREFERENCE IS TO LEAN TOWARD THE 12Z GFS...00Z  
CMC...AND 06Z GEFS MEAN WHICH ARE FAIRLY SIMILAR. THE 00Z ECMWF  
ENSEMBLE MEAN IS ALSO INCORPORATED. IT DOES SHOW THE FASTER  
APPROACH OF THE TROUGH OVER THE WEST LIKE THE OPERATIONAL  
ECMWF...BUT IS NOT AS ROBUST WITH ITS CYCLOGENESIS OVER THE  
MIDWEST AND EVENTUALLY COMES INTO BETTER ALIGNMENT WITH THE HEIGHT  
FIELDS ALOFT. THIS WOULD GIVE SOME WEIGHT TO THE POSSIBILITY OF  
ECMWF TROUGH TIMING BEING MORE ACCURATE...WHILE LIMITING THE  
INFLUENCE OF THE OUTLIER SURFACE LOW INTENSITY/POSITION SOLUTIONS  
ON MONDAY.  
 
18Z UPDATE: NO CHANGE TO PRELIMINARY PREFERENCE. THE 12Z UKMET  
TRENDED CLOSER TO THE TROUGH EVOLUTION DEPICTED BY THE PREVIOUS  
(00Z) ECMWF RUN...AND IT STILL HAS A STRONGER LOW INTENSITY AND  
FURTHER WEST LOW POSITION RELATIVE TO THE OTHER MODELS. HOWEVER  
THE 12Z ECMWF HAS MADE A NOTABLE SHIFT CLOSER TO THE OTHER GLOBAL  
MODELS...AND THE PRELIMINARY PREFERENCE. THIS MAKES MORE SENSE  
BASED ON THE ENSEMBLE MEMBERS FROM THE 00Z CYCLE AS WELL AS THE  
NEW 12Z GEFS MEAN. THE 12Z ECMWF LOW POSITION STILL LAGS THE  
GFS/CMC AND ENSEMBLE MEANS TO THE SOUTHWEST...WHILE THOSE MODELS  
ARE NOW IN EXCELLENT AGREEMENT ON THE LOW POSITION. WILL MAINTAIN  
THE PRELIMINARY PREFERENCE...WHICH DOES NOT INCLUDE THE  
OPERATIONAL ECMWF...AS ITS LOW POSITION IS STILL A LITTLE OUT OF  
ALIGNMENT. THE OPERATIONAL GFS AND CMC PROBABLY DEPICT A MORE  
REASONABLE LOW INTENSITY THAN THE ENSEMBLE MEANS.  
 
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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