221  
FXUS10 KWNH 171835  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
134 PM EST SUN DEC 17 2017  
 
VALID DEC 17/1200 UTC THRU DEC 21/0000 UTC  
 
...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR  
INGEST...  
 
12Z MODEL EVALUATION...WITH LATEST PREFERENCES AND CONFIDENCE  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
   
..DEAMPLIFYING SHORTWAVE OVER LOWER/MID MS AND OH VALLEY TODAY
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: ABOVE AVERAGE  
 
THE GUIDANCE IS IN VERY GOOD AGREEMENT WITH THE DETAILS OF THIS  
ENERGY SHEARING OUT OVER THE NEXT 12 TO 24 HOURS. A GENERAL MODEL  
BLEND WILL BE PREFERRED.  
 
   
..CLOSED LOW OVER THE DESERT SOUTHWEST/NW MEXICO
 
   
..EJECTING OUT ACROSS THE SOUTHERN PLAINS ON TUESDAY
 
   
..SHEARING OUT ACROSS THE LOWER MS/TN VALLEYS ON WEDNESDAY
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: BLEND OF THE 12Z GFS/12Z UKMET AND 12Z ECMWF  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
THE MODELS ARE IN VERY GOOD AGREEMENT TAKING THE UPPER LOW OVER  
THE DESERT SOUTHWEST AND FAR NORTHWEST PORTION OF MEXICO OFF TO  
THE EAST OVER THE NEXT COUPLE OF DAYS. THE ENERGY WILL EJECT OUT  
ACROSS THE SOUTHERN PLAINS THROUGH TUESDAY AND THEN BEGIN TO SHEAR  
OUT DOWNSTREAM OVER THE LOWER MS/TN VALLEYS THROUGH WEDNESDAY. THE  
HEIGHT FALLS WILL INDUCE AN AREA OF LOW PRESSURE ACROSS THE  
SOUTHEAST TX COASTAL PLAIN BY LATE TUESDAY WHICH WILL THEN MOVE  
QUICKLY DOWNSTREAM ACROSS THE TN VALLEY AND INTERIOR SOUTHEAST ON  
WEDNESDAY. THE 12Z NAM APPEARS TO BE A STRONG OUTLIER WITH THE  
EVOLUTION OF THIS SYSTEM BY 36 HOURS AND THIS REMAINS THE CASE AS  
IT ADVANCES DOWNSTREAM ACROSS THE SOUTH AND ULTIMATELY LEADS TO A  
SURFACE LOW TRACKING A BIT NORTH OF THE MODEL CONSENSUS. THE 12Z  
CMC FOR ITS PART APPEARS TO BE STILL A TAD TOO SLOW WITH IT ENERGY  
PROPAGATION FROM 60 HOURS ONWARD AND ALSO APPEARS TO GENERALLY BE  
TOO WEAK AND TOO FAR SOUTH WITH ITS LOW TRACK AND FRONTAL  
PLACEMENT. A COMPROMISE OF THE BETTER CLUSTERED 12Z GFS/12Z UKMET  
AND 12Z ECMWF WILL BE PREFERRED.  
 
   
..SHORTWAVE TRAVERSING THE NORTHERN PLAINS TODAY
 
   
..ENERGY DAMPENING OUT OVER THE GREAT LAKES/NORTHEAST BY MONDAY
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
 
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: ABOVE AVERAGE  
 
THE MODELS ARE IN GOOD AGREEMENT WITH THE DETAILS OF THIS SYSTEM  
AS IT SHEARS OUT OVER THE NEXT 24 HOURS OR SO. A WEAK AREA OF LOW  
PRESSURE OVER THE MIDWEST WILL CROSS THE LOWER GREAT LAKES AND  
NORTHEAST THROUGH MONDAY BEFORE DAMPENING OUT. A GENERAL MODEL  
BLEND WILL BE PREFERRED.  
 
   
..AMPLIFYING NORTHERN STREAM TROUGH OVER SOUTHERN CANADA MONDAY
 
   
..ENERGY DIGGING ACROSS THE MIDWEST/GREAT LAKES BY TUESDAY
 
   
..TROUGHING OVER SOUTHEAST CANADA/NORTHEAST U.S. BY WEDNESDAY
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND...THROUGH 60 HOURS  
BLEND OF THE 12Z GFS AND 12Z ECMWF...AFTER 60 HOURS  
CONFIDENCE: ABOVE AVERAGE  
 
THE MODELS GRADUALLY AMPLIFY A NORTHERN STREAM LONGWAVE TROUGH  
ACROSS AREAS OF SOUTHERN CANADA AND THE NORTHERN U.S. WITH IMPACTS  
ACROSS THE NORTHERN PLAINS/MIDWEST AND GREAT LAKES THROUGH MONDAY  
AND TUESDAY. A STRONG COLD FRONT WILL ACCOMPANY THESE HEIGHT  
FALLS. THE MODELS ARE IN RATHER GOOD AGREEMENT WITH THE LARGE  
SCALE DETAILS OF THE TROUGH AND THE COLD FRONT. THE 12Z NAM  
BECOMES A DEEPER OUTLIER IN TIME WITH THE LARGER SCALE TROUGH  
THOUGH...MAINLY AFTER 60 HOURS...OVER SOUTHEAST CANADA. THE 12Z  
UKMET ALSO TENDS TO PIVOT ENERGY A BIT MORE AGGRESSIVELY OFFSHORE  
OF NEWFOUNDLAND BY THE END OF THE PERIOD COMPARED TO THE REMAINING  
GUIDANCE. WILL SUGGEST A GENERAL MODEL BLEND THROUGH ABOUT 60  
HOURS...AND THEN A BLEND OF THE WELL CLUSTERED 12Z GFS AND 12Z  
ECMWF THEREAFTER WHICH IS ALSO SUPPORTED BY THE 12Z CMC..  
 
   
..DEEP UPPER TROUGH/CLOSED LOW CROSSING THE GULF OF AK MONDAY
 
   
..ENERGY DIGGING SHARPLY INTO THE PACIFIC NORTHWEST ON TUESDAY
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: BLEND OF THE 12Z GFS AND 12Z ECMWF...THROUGH 60 HOURS  
BLEND OF THE 12Z UKMET AND 12Z ECMWF...AFTER 60 HOURS  
CONFIDENCE: ABOVE AVERAGE  
 
THE MODELS TAKE A NORTHERN STREAM SHORTWAVE TROUGH OVER THE GULF  
OF AK AND AMPLIFY IT SOUTHEAST TOWARD THE PACIFIC NORTHWEST ON  
TUESDAY WHICH ALLOWS FOR SOME STRONG HEIGHT FALLS TO ARRIVE IN  
CONJUNCTION WITH A DEEPENING AREA OF LOW PRESSURE. THIS LOW  
PRESSURE WAVE MOVES ACROSS NORTHWEST WA BY LATE TUESDAY. LOW  
PRESSURE AND A RATHER STRONG COLD FRONT WILL THEN TRAVERSE THE  
INTERMOUNTAIN WEST THROUGH EARLY WEDNESDAY. LOW PRESSURE WILL BE  
REDEVELOPING OUT ACROSS THE HIGH PLAINS BY LATE WEDNESDAY ALONG  
WITH A STRENGTHENING WARM FRONT NEAR PARTS OF THE NORTH-CENTRAL  
PLAINS AND MIDWEST. THE 12Z NAM IS A BIT OF A DEEPER OUTLIER WITH  
THE UPPER LOW THAT DIGS IN ACROSS THE NORTHWEST U.S. TUESDAY AND  
WEDNESDAY AND GRADUALLY APPEARS TO BE A BIT TOO SLOW. THE  
REMAINING GUIDANCE IS A TAD FASTER WITH THE 12Z ECMWF THE MOST  
PROGRESSIVE...HOWEVER THE 12Z UKMET AND 12Z CMC ARE CLUSTERED  
RATHER CLOSE TO THE ECMWF SOLUTION. THE 12Z GFS IS A LITTLE SLOWER  
THAN THIS CONSENSUS. AT THE SURFACE...THE 12Z UKMET APPEARS TO BE  
A BIT TOO STRONG WITH THE LOW PRESSURE THAT IMPACTS WESTERN WA AND  
VANCOUVER ISLAND. THERE ARE SOME MODEST TIMING DIFFERENCES WITH  
THE EVOLUTION OF LOW PRESSURE OVER THE HIGH PLAINS...WITH THE  
ECMWF A BIT QUICKER TO CONSOLIDATE LOW PRESSURE OVER THE CENTRAL  
HIGH PLAINS VERSUS THE SLIGHTLY SLOWER GFS...BUT THE UKMET AND CMC  
ARE CLOSER TO THE ECMWF SOLUTION. THE 00Z ECENS MEAN AND 12Z GEFS  
MEAN BOTH SUGGEST THE GFS IS A TAD TOO SLOW AND ARE MORE  
SUPPORTIVE OF THE ECMWF. SO WILL SUGGEST A BLEND OF THE GFS AND  
ECMWF THROUGH ABOUT 60 HOURS BASED ON EXCELLENT MODEL  
CLUSTERING...BUT THEN A BLEND OF THE UKMET AND ECMWF THEREAFTER  
SINCE THE GFS APPEARS A LITTLE TOO SLOW.  
 
MODEL TRENDS AT WWW.WPC.NCEP.NOAA.GOV/HTML/MODEL2.SHTML  
500 MB FORECASTS AT WWW.WPC.NCEP.NOAA.GOV/H5PREF/H5PREF.SHTML  
 
ORRISON  
 

 
 
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