646  
FXUS10 KWNH 201846  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
146 PM EST SAT JAN 20 2018  
 
VALID JAN 20/1200 UTC THRU JAN 24/0000 UTC  
 
...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR  
INGEST...  
 
12Z MODEL EVALUATION INCLUDING PREFERENCES AND FORECAST CONFIDENCE  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
 
...TROUGH OVER THE INTERMOUNTAIN WEST KICKING INTO THE PLAINS ON  
SUNDAY WITH DEVELOPING CYCLONE TRACKING INTO THE GREAT LAKES BY  
MONDAY-TUESDAY...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
WEIGHTED MORE TOWARD THE GEFS/ECMWF ENS MEANS  
CONFIDENCE: AVERAGE  
 
---18Z UPDATE---  
NO CHANGE TO THE PRELIMINARY PREFERENCE. THE 12Z ECMWF DID TREND  
SLIGHTLY FASTER WITH THE PROGRESSION OF THE TROUGH AND LOW  
ALOFT...WHICH PLACES IT INTO SLIGHTLY BETTER AGREEMENT WITH THE  
GFS. A GENERAL MODEL BLEND IS STILL PREFERRED TO ACCOUNT FOR MODEL  
VARIABILITY.  
 
---PREV. DISCUSSION---  
OVERALL MODELS ARE REASONABLY CLOSELY CLUSTERED IN THEIR MASS  
FIELDS THROUGH THE COURSE OF THE CYCLOGENESIS PROCESS WITH THIS  
SYSTEM...WITH NORMAL MODEL VARIABILITY LEADING TO SLIGHTLY  
DIFFERENT TIMING AND POSITIONING OF KEY FEATURES. ONE IMPORTANT  
DIFFERENCE IS WITH TIMING OF THE WAVE AND ASSOCIATED SURFACE  
CYCLONE AS THEY SHIFT TO THE EAST/NORTHEAST OVER THE NEXT SEVERAL  
DAYS. THE 00Z ECMWF AND 12Z UKMET CONTINUE TO BE ON THE SLOW END  
OF MODEL SPREAD...WHILE THE 12Z NAM AND 12Z GFS ARE FASTER...WITH  
THE 00Z ECMWF ENS MEAN AND 06Z GEFS MEAN IN BETWEEN THESE  
EXTREMES. THESE DIFFERENCES HAVE ACTUALLY BECOME MORE PRONOUNCED  
OVER THE LATEST MODEL CYCLE. THE 12Z GFS IS SLIGHTLY FASTER WITH  
THE PROGRESSION OF THE MID-UPPER LEVEL TROUGH THAN ITS 06Z  
RUN...WHILE THE 00Z ECMWF WAS SLIGHTLY SLOWER THAN ITS 12Z RUN  
YESTERDAY. ENSEMBLE SENSITIVITY ANALYSIS ON THE 00Z MODEL CYCLE  
FOR THE NCEP/CMC ENSEMBLE MEMBERS SHOWED THE GREATEST AMOUNT OF  
VARIANCE WAS RELATED TO TIMING DIFFERENCES (APPROXIMATELY 70  
PERCENT OF THE VARIANCE BY MONDAY EVENING)...WITH MOST OF THE  
REMAINING UNCERTAINTY RELATED TO THE LOW POSITION AND HOW FAR  
NORTHWEST IT WILL TRACK.  
 
GIVEN THE LARGEST AMOUNT OF REMAINING UNCERTAINTY IS RELATED TO  
TIMING...THE PREFERENCE IS TO ACCOUNT FOR THIS WITH A GENERAL  
MODEL BLEND...
BUT WEIGHTED MORE TOWARD THE ENSEMBLE MEANS (ECMWF  
AND GEFS) WHICH STRIKE A BALANCE BETWEEN THE VARIABILITY OF THE  
OPERATIONAL MODELS. THIS PREFERENCE IS STRENGTHENED BY THE  
TENDENCY OF THE ECMWF TO BE SLOWER WITH THE PROGRESSION OF  
CYCLONES AND THE GFS TO BE FASTER. A BALANCE OF THESE MODEL  
TIMINGS MAY BE CLOSER TO THE MOST LIKELY SOLUTION.  
 
...BROAD GULF COAST TROUGH PUSHING QUICKLY EAST TO OVER THE  
ATLANTIC BY SUNDAY AFTERNOON...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: ABOVE AVERAGE  
 
---18Z UPDATE---  
THE 12Z CMC HAS MOVED INTO MUCH BETTER AGREEMENT WITH OTHER  
MODELS...SO A GENERAL MODEL BLEND IS NOW PREFERRED.  
 
---PREV. DISCUSSION---  
THE 00Z CMC SHIFTS THE TROUGH MORE QUICKLY TO THE EAST THAN ALL  
THE OTHER OPERATIONAL MODELS AND THE ECMWF AND GEFS ENSEMBLE  
MEANS...AND IT ALSO BUILDS THE RIDGE INTO THE SOUTHEAST MORE  
RAPIDLY THAN OTHER MODELS. GIVEN THE LACK OF SUPPORT FROM OTHER  
AVAILABLE MODELS...THE CMC IS EXCLUDED FROM THE PREFERENCE.  
OTHERWISE...THE MODELS ARE IN FAIRLY GOOD AGREEMENT AND A GENERAL  
BLEND IS PREFERRED TO ACCOUNT FOR SMALL DETAIL DIFFERENCES.  
 
   
..GULF OF ALASKA TROUGH WITH BROAD SURFACE CYCLONE
 
 
...STRONG EMBEDDED SHORTWAVE ROTATING AROUND TO THE SOUTHERN  
PERIPHERY OF THE TROUGH BY TUESDAY...   
..RESULTING SURFACE LOW DEVELOPMENT OFFSHORE PAC NW MON-TUE
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: BLEND OF THE 12Z GFS / 12Z ECMWF / 12Z CMC  
CONFIDENCE: SLIGHTLY BELOW AVERAGE  
 
---18Z UPDATE---  
NO CHANGE TO THE PRELIMINARY PREFERENCE. THE GFS...ECMWF...AND CMC  
CONTINUE TO BE IN THE BEST AGREEMENT OVERALL...AND ARE ALSO CLOSER  
TO THE 00Z ECMWF ENSEMBLE MEAN. THEREFORE...CONTINUITY OF THE  
PRELIMINARY PREFERENCE IS RECOMMENDED.  
 
---PREV. DISCUSSION---  
MODELS ARE IN FAIRLY GOOD AGREEMENT OVERALL THROUGH AROUND 23/00Z  
(MONDAY AFTERNOON)...BEFORE DIFFERENCES EMERGE WITH THE DEVELOPING  
CYCLONE IN THE VICINITY OF 140W. THE 12Z UKMET KICKS THE SHORTWAVE  
OUT TO THE EAST FASTER THAN ALL OTHER OPERATIONAL MODELS AND MANY  
OF THE ENSEMBLE MEMBERS...WHICH ALLOWS FOR EARLIER  
CYCLOGENESIS...ONE OF THE DEEPER SURFACE CYCLONES...AND A POSITION  
OF THE OCCLUDED LOW FURTHER NORTHWEST OF OTHER MODELS BY 24/00Z.  
MEANWHILE...THE 12Z NAM SHOWS FAR LESS DEEPENING OF THE SURFACE  
CYCLONE THAN OTHER MODELS...WHICH MAY BE RELATED TO THE STRENGTH  
OF THE SHORTWAVE ROUNDING THE BASE OF THE TROUGH...AND THE  
ASSOCIATED POSITIONING OF SYNOPTIC FEATURES LIKE JET STREAKS. THE  
PREFERENCE IS TO LEAN TOWARD THE 12Z GFS...00Z ECMWF...AND 00Z CMC  
WHICH ARE ALL VERY CLOSE IN THE POSITION AND INTENSITY OF THE  
EVOLVING SURFACE CYCLONE AND REASONABLY CLOSE WITH THE STRUCTURE  
OF THE TROUGH ALOFT.  
 
MODEL TRENDS AT WWW.WPC.NCEP.NOAA.GOV/HTML/MODEL2.SHTML  
500 MB FORECASTS AT WWW.WPC.NCEP.NOAA.GOV/H5PREF/H5PREF.SHTML  
 
LAMERS  
 

 
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab HPC Page Main Text Page