733  
FXUS10 KWNH 201835  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
134 PM EST MON NOV 20 2017  
 
VALID NOV 20/1200 UTC THRU NOV 24/0000 UTC  
 
...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR  
INGEST...  
 
12Z MODEL EVALUATION WITH FINAL PREFERENCES AND CONFIDENCE  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
 
...EVOLVING DEEP CYCLONE OVER THE EASTERN PACIFIC AND JUST  
OFFSHORE OF THE PACIFIC NORTHWEST THROUGH TUE NGT...   
..OPENING INTO WAVE AND EJECTING INTO THE NORTHWEST WED/THU
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
DESPITE SOME COMPLEXITIES WITH DETAILS SUCH AS EXACT PLACEMENT OF  
SURFACE LOWS AND HOW THOSE ARE DRIVEN BY EMBEDDED SHORTWAVES  
ROTATING AROUND A LARGER UPPER LEVEL LOW...THE MODELS ARE IN  
FAIRLY GOOD AGREEMENT WITH THEIR MASS FIELDS AND THE BIG PICTURE  
WITH THIS SYSTEM. NORMALIZED STANDARD DEVIATION OF 00Z ECMWF  
ENSEMBLE SPREAD OVER WESTERN NORTH AMERICA TUESDAY INTO WEDNESDAY  
IS BELOW NORMAL FOR 500MB HEIGHTS AND MSLP...WITH GREATER SPREAD  
OFFSHORE. THUS THE DETAIL DIFFERENCES WOULD LIKELY HAVE A GREATER  
FORECAST IMPACT OFFSHORE...BUT SENSIBLE WEATHER IN THE PACIFIC  
NORTHWEST DOES NOT APPEAR TO BE SUBSTANTIALLY AFFECTED BY THESE  
DIFFERENCES. THEREFORE...A GENERAL MODEL BLEND IS PREFERRED AT  
THIS TIME.  
 
18Z UPDATE: NO CHANGE TO THE PRELIMINARY PREFERENCE. MODEL  
DIFFERENCES REMAIN RELATIVELY SMALL WITH THIS SYSTEM ON THE 12Z  
MODEL CYCLE. THEREFORE A GENERAL MODEL BLEND IS STILL PREFERRED.  
 
   
..SHORTWAVE DIGGING THROUGH THE GREAT LAKES THROUGH TUESDAY
 
 
...BECOMING NEGATIVE TILT OVER THE NORTHEAST ON WEDNESDAY WITH  
SURFACE CYCLOGENESIS JUST OFFSHORE...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 40 PERCENT ECMWF ENSEMBLE MEAN  
20 PERCENT EACH 00Z ECMWF / 00Z UKMET / 00Z CMC  
CONFIDENCE: SLIGHTLY BELOW AVERAGE  
 
MORE SUBSTANTIAL DIFFERENCES EXIST WITH THIS SYSTEM...AND APPEAR  
TO BE LARGELY ROOTED IN THE SPEED OF THE SHORTWAVE DIGGING THROUGH  
THE GREAT LAKES. THE 12Z NAM AND GFS SHOW THE PRIMARY MID-LEVEL  
VORT MAX ASSOCIATED WITH THE WAVE SHIFTING THROUGH THE REGION MUCH  
FASTER...REACHING MAINE AND BECOMING INCREASINGLY NEGATIVELY  
TILTED BY 22/18Z. AT THE SAME TIME...THE 00Z ECMWF...CMC...AND  
UKMET SHOW THE VORT MAX CLOSER TO UPSTATE NEW YORK WITH A SLOWER  
PROGRESSION OVERALL. THIS DOES AFFECT THE FORECAST AS THE FASTER  
MODELS INDICATE THE SURFACE TROUGH AND LOW(S) FURTHER OFFSHORE AND  
DOWNSTREAM FROM THE NORTHEASTERN CONUS. MEANWHILE...THE SLOWER  
MODELS SHOW A SLIGHTLY STRONGER SURFACE WAVE...WITH THE SURFACE  
TROUGH AND LOW(S) TUCKED IN CLOSER TO THE NORTHEAST. THIS IS ALSO  
EVIDENT IN ENSEMBLE SENSITIVITY PLOTS...WITH A STRONG RELATIONSHIP  
BETWEEN A SLOWER DIGGING WAVE LEADING TO LOWER SURFACE PRESSURES  
OFF THE NORTHEAST.  
 
00Z HIGHER PROBABILITY ENSEMBLE CLUSTERS (GEFS AND ECMWF) TEND TO  
ALIGN WITH THE SLOWER WAVE...WITH THE TWO CLUSTERS THAT ARE  
COMPRISED OF THE GREATEST NUMBER OF ENSEMBLE MEMBERS IN GOOD  
AGREEMENT WITH THAT SOLUTION. FOR THIS REASON...THE PREFERENCE IS  
FOR THE SLOWER SOLUTION AT THIS TIME. WITHIN THAT CAMP OF  
MODELS...THE ECMWF...UKMET...AND CMC DO SHOW QUITE DIFFERENT  
SURFACE LOW LOCATIONS AND INTENSITIES. THE OPERATIONAL 00Z ECMWF  
MAY BE TOO INTENSE WITH ITS SURFACE LOW...AS IT IS AT LEAST 10MB  
DEEPER THAN ANY OTHER GLOBAL MODEL. HOWEVER...WE CANNOT DISCOUNT  
THAT IT MAY BE MORE ACCURATE WITH RESPECT TO LOCATION. FOR THIS  
REASON...PREFER A BLEND OF THE CMC...UKMET...AND ECMWF TO  
INCORPORATE THESE DIFFERENCES AND THE INHERENT UNCERTAINTY...WITH  
A LARGE COMPONENT OF THE ECMWF ENSEMBLE MEAN INCLUDED AS WELL.  
 
18Z UPDATE: NO CHANGE TO THE PRELIMINARY PREFERENCE. THE 12Z  
ECMWF...UKMET...AND CMC CONTINUE TO REPRESENT A SLOWER SOLUTION  
WITH THE WAVE DIGGING THROUGH THE GREAT LAKES AND THUS THE PRIOR  
REASONING HAS NOT CHANGED APPRECIABLY AT THIS TIME.  
 
   
..BROAD UPPER LEVEL TROUGH OVER THE GULF OF MEXICO
 
   
..REINFORCING SHORTWAVE DIGGING FROM THE ROCKIES
 
   
..POSSIBLE WEAK LOW-LEVEL CYCLOGENESIS BY THURSDAY
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 12Z ECMWF / 12Z GFS / 12Z CMC  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
THE 00Z CMC AND UKMET SHOW HIGHER HEIGHTS THROUGH MUCH OF THE GULF  
OF MEXICO WITH THIS TROUGH...AND LOWER HEIGHTS IN THE INTERIOR  
SOUTHEAST...BY THURSDAY. THIS IS DUE TO A STRONGER SECONDARY  
DIGGING WAVE AND RESULTS IN SURFACE CYCLOGENESIS FURTHER NORTHEAST  
OFF THE SOUTHEAST ATLANTIC COAST. MEANWHILE...THE 12Z NAM SHOWS A  
SLIGHTLY DEEPER TROUGH OVERALL WITH THE TROUGH AXIS ALIGNED  
FURTHER WEST OF MODEL CONSENSUS. THE 12Z GFS AND 00Z ECMWF ARE  
MORE CLOSELY ALIGNED WITH THEIR ENSEMBLE MEANS (12Z GEFS AND 00Z  
ECMWF ENS) WHICH ALSO SEEM TO BE THE MORE LIKELY SOLUTIONS. THESE  
GENERATE A BROAD WEAK BAROCLINIC LOW IN THE GULF OF MEXICO BY  
THURSDAY. THEREFORE...GIVEN THE SUPPORT FROM ENSEMBLES...THE  
PREFERENCE IS FOR A BLEND OF THE ECMWF AND GFS.  
 
18Z UPDATE: ADD THE CMC TO THE BLEND. THE 12Z UKMET CONTINUES TO  
SHOW A STRONGER SECONDARY WAVE OVER THE INTERIOR SOUTHEAST BY  
THURSDAY...AND THE UKMET AND 12Z NAM SHOW STRONGER SURFACE LOW  
DEVELOPMENT OFF THE SOUTHEAST ATLANTIC COAST (WITH LIMITED SUPPORT  
FROM OTHER MODELS). THE 12Z UKMET IS ALSO THE DEEPEST WITH THE  
SURFACE LOW OVER THE GULF OF MEXICO...AND ONE OF THE MODELS WITH A  
STRONGER UPSTREAM SURFACE RIDGE...WHICH MAY RESULT IN STRONGER  
LOW-LEVEL FLOW ACROSS THE REGION. THE 12Z CMC HAS MOVED INTO  
BETTER AGREEMENT WITH THE GFS AND ECMWF OVER BOTH THE GULF OF  
MEXICO AND THE INTERIOR SOUTHEAST...AND THUS IT HAS BEEN ADDED TO  
THE PREFERENCE.  
 
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