447  
FXUS10 KWNH 231836  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
135 PM EST TUE JAN 23 2018  
 
VALID JAN 23/1200 UTC THRU JAN 27/0000 UTC  
 
...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR  
INGEST...  
 
12Z MODEL EVALUATION INCLUDING PREFERENCES AND FORECAST CONFIDENCE  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
 
...BROAD OCCLUDED CYCLONE AND ASSOCIATED TROUGH OVER THE  
NORTHEAST, PROGRESSING TO NEAR LABRADOR BY WEDNESDAY...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: ABOVE AVERAGE  
 
---18Z UPDATE---  
NO CHANGE TO THE PRELIMINARY PREFERENCE. MODELS REMAIN IN GOOD  
AGREEMENT OVERALL.  
 
---PREVIOUS DISCUSSION---  
MODELS ARE IN EXCELLENT AGREEMENT WITH THE EVOLUTION OF THIS  
SYSTEM OVER THE NEXT 24 HOURS AS IT MOVES OUT OF THE NORTHEAST  
REGION. AVAILABLE OPERATIONAL MODELS SHOW TIGHT CLUSTERING WITH  
THE LOW POSITION AND LIMITED SPREAD WITH MANY OF THE MASS FIELDS.  
THEREFORE, A GENERAL MODEL BLEND IS PREFERRED WITH ABOVE AVERAGE  
CONFIDENCE.  
 
...NORTHERN PLAINS SHORTWAVE AND ASSOCIATED SECONDARY COLD FRONT  
MOVING QUICKLY EAST TO THE EAST COAST BY WEDNESDAY NIGHT...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
---18Z UPDATE---  
NO CHANGE TO THE PRELIMINARY PREFERENCE. MODELS REMAIN IN GOOD  
AGREEMENT OVERALL, AND SEVERAL OF THE NEW 12Z MODEL RUNS (UKMET,  
ECMWF) INITIALIZED CLOSER TO THE GFS.  
 
---PREVIOUS DISCUSSION---  
THERE ARE SOME INITIAL DIFFERENCES ACROSS THE NORTHERN PLAINS, BUT  
IT APPEARS TO HAVE VERY LITTLE IMPACT ON THE MODEL SCENARIOS  
THROUGH THE NEXT 24-36 HOURS AS THE TROUGH PROGRESSES QUICKLY TO  
THE EAST. THE 12Z GFS INITIALIZED WITH SLIGHTLY LOWER HEIGHTS  
ACROSS MUCH OF THE NORTHERN PLAINS THAN OTHER MODELS, BUT IT STILL  
SHOWS A VERY SIMILAR AMPLITUDE AND TIMING OF THE WAVE BY 25/06Z  
(WEDNESDAY NIGHT). IT DOES SHOW SLIGHTLY FASTER TIMING OF THE  
COLDER AIR OVER THE MID ATLANTIC, BUT OVERALL IS REASONABLY CLOSE  
TO OTHER MODELS. THEREFORE, A GENERAL MODEL BLEND IS PREFERRED  
WITH THIS SYSTEM.  
 
...PROGRESSIVE TROUGH PUSHING ONTO THE WEST COAST WEDNESDAY, AND  
DEAMPLIFYING AS IT PUSHES EAST TO GREAT LAKES BY FRIDAY...  
...ASSOCIATED SURFACE LOW NEAR THE CANADIAN BORDER AND TRAILING  
COLD FRONT PUSHING INTO THE CENTRAL US...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: BLEND OF 12Z GFS, 00Z ECMWF  
LEAN TO ECMWF (65%) VS GFS (35%)  
CONFIDENCE: AVERAGE  
 
---18Z UPDATE---  
NO CHANGE TO THE PRELIMINARY PREFERENCE. THE 12Z UKMET AND 12Z CMC  
HAVE NOT TRENDED ENOUGH FROM THEIR 00Z RUNS TO BE INCLUDED IN THE  
PREFERENCE AT THIS TIME. THE UKMET IN PARTICULAR STILL SHOWS A  
STRONGER SURFACE LOW THAT IS MUCH FURTHER SOUTH OF THE OTHER  
MODELS AND ENSEMBLE MEANS OVER THE NORTH-CENTRAL US.  
 
---PREVIOUS DISCUSSION---  
THE 12Z GFS AND 00Z ECMWF OFFER FAIRLY SIMILAR SCENARIOS THROUGH  
THE FIRST THREE DAYS OF THE FORECAST, WITH THE MOST NOTABLE  
DIFFERENCES EMERGING LATE IN THE PERIOD OVER THE PLAINS. DESPITE A  
SIMILAR LOW POSITION BY 27/00Z (FRIDAY EVENING), THE GFS HAS  
FASTER TIMING OF THE COLD FRONT IN THE PLAINS, WHILE THE ECMWF IS  
SLOWER, AND IN BETTER AGREEMENT WITH MOST OF THE OTHER MODELS.  
EVEN THE GEFS MEAN (06Z RUN) IS SLOWER THAN THE OPERATIONAL GFS.  
GIVEN THE NEUTRAL OR DEAMPLIFYING TREND OF THE TROUGH THROUGH THE  
PERIOD, DO NOT ANTICIPATE AN ACCELERATING COLD FRONT DOWN THE  
PLAINS AS WE SOMETIMES SEE IN WINTER. THEREFORE, THE BLEND  
PREFERENCE IS WEIGHTED MORE TOWARD THE ECMWF, PARTICULARLY AT  
LATER HOURS.  
 
THE 00Z CMC WAS NOT INCLUDED AS IT SUPPRESSES THE HEIGHT FALLS  
FURTHER WEST AND OFFSHORE INITIALLY ALONG THE PACIFIC NORTHWEST  
COAST, WHICH LEADS TO A FLATTER WAVE, A WEAKER SURFACE LOW THAT IS  
SITUATED ON THE SOUTHERN END OF MODEL SPREAD. THE 00Z UKMET WAS  
ALSO NOT INCLUDED, AS IT SHOWS A STRONGER SURFACE LOW (POSITION  
FURTHEST SOUTH OF ANY MODEL), WITH NOTEWORTHY TROUGH AMPLIFICATION  
OVER THE PLAINS -- UNSUPPORTED BY ANY OTHER MODEL. AND ALTHOUGH  
THE 12Z NAM IS NOT SIGNIFICANTLY DIFFERENT WITH THE TIMING OR  
POSITION OF THE TROUGH OR SURFACE LOW, IT DOES HAVE A COLD BIAS IN  
THE NORTHWEST BEHIND THE COLD FRONT, WHICH COULD LEAD TO LOWER  
FORECAST SNOW LEVELS THAN WOULD BE SUGGESTED BY OTHER MODELS.  
 
MODEL TRENDS AT WWW.WPC.NCEP.NOAA.GOV/HTML/MODEL2.SHTML  
500MB FORECASTS AT WWW.WPC.NCEP.NOAA.GOV/H5PREF/H5PREF.SHTML  
 
LAMERS  
 

 
 
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