169  
FXUS10 KWNH 271704  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1203 PM EST WED DEC 27 2017  
 
VALID DEC 27/1200 UTC THRU DEC 31/0000 UTC  
 
...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR  
INGEST...  
 
...AN EVALUATION OF THE 12Z NAM/GFS ALONG WITH PRELIMINARY  
PREFERENCES/FORECAST CONFIDENCE...  
 
...PROGRESSIVE SHORTWAVE ADJOINING EXITING HEIGHT FALLS ACROSS NEW  
ENGLAND ON WED/THU...  
 
PRELIMINARY PREFERENCE: GENERAL MODEL COMPROMISE  
FORECAST CONFIDENCE: ABOVE AVERAGE  
 
THE HEMISPHERIC FLOW ACROSS THE CONTINENT WILL FAVOR BELOW AVERAGE  
HEIGHTS OVER MUCH OF CENTRAL/EASTERN CANADA WITH LOBES OF  
VORTICITY PEELING OFF AND MOVING THROUGH THE NORTHEASTERN U.S. ONE  
EMBEDDED SHORTWAVE SHOULD ACCELERATE FROM THE LOWER GREAT LAKES  
TOWARD SOUTHERN NEW ENGLAND THIS AFTERNOON WHILE LARGER SCALE  
HEIGHT FALLS DESCEND FROM EXTREME SOUTHERN QUEBEC. OVERALL  
DIFFERENCES ARE QUITE SMALL WHICH AFFORDS A GENERAL MODEL  
COMPROMISE.  
 
   
..SWIFT MOVING SYSTEM CROSSING THE UPPER MIDWEST BY 28/1800Z
 
 
...TRAILING AMPLIFIED FLOW ENCOMPASSING THE NORTHEASTERN U.S. ON  
SATURDAY...   
..INDUCED SURFACE CYCLOGENESIS OFF THE CAROLINAS
 
 
PRELIMINARY PREFERENCE: BLEND OF THE 00Z ECMWF/ECMWF ENSEMBLE MEAN  
FORECAST CONFIDENCE: SLIGHTLY BELOW AVERAGE  
 
WITHIN THE ACTIVE SHORTWAVE TRAIN...A PAIR OF FEATURES SHOULD  
SWING THROUGH THE NORTH-CENTRAL U.S. INTO THE GREAT  
LAKES/NORTHEASTERN STATES DURING THE SHORT RANGE PERIOD. AS THE  
INITIAL SYSTEM SWINGS THROUGH THE GREAT LAKES EARLY FRIDAY...THE  
12Z GFS/06Z GEFS MEAN WERE A BIT MORE AMPLIFIED THAN THE MODEL  
CONSENSUS...PARTICULARLY THE GFS. LOOKING BACK AT THE NEXT ROUND  
OF AMPLIFIED FLOW...THERE ARE DISCERNIBLE TIMING DIFFERENCES  
WITHIN THE REGION OF LOWER HEIGHTS. INTERESTINGLY...THE 12Z GFS  
HAS ACTUALLY SHOWED DOWN SOMEWHAT RELATIVE TO ITS PREVIOUS RUNS.  
SOME SURFACE WAVE DEVELOPMENT IS LIKELY OVER THE WESTERN ATLANTIC  
THIS WEEKEND WITH ECMWF ENSEMBLE MEMBERS MUCH MORE ADAMANT ABOUT  
THIS EVOLUTION THAN THE GEFS. THE ECMWF ENSEMBLES ALSO HAVE  
SUPPORT FROM THE CMC MEMBERS. GIVEN ITS SHOWN MORE  
CONSISTENCY...THIS WILL STAY CLOSER TO A 00Z ECMWF/ECMWF ENSEMBLE  
MEAN BLEND.  
 
...SHEARED SHORTWAVE CROSSING INTO WESTERN BRITISH COLUMBIA BY  
THURSDAY AFTERNOON...  
 
PRELIMINARY PREFERENCE: BLEND OF THE 12Z GFS/00Z ECMWF  
FORECAST CONFIDENCE: AVERAGE  
 
LOW-AMPLITUDE RIDGING INITIALLY OVER THE FAR EASTERN PACIFIC WILL  
BRIEFLY CAVE TO A SHORTWAVE WHICH PEELS OFF FROM THE NORTHEASTERN  
PACIFIC TROUGH. THERE ARE SOME INTENSITY DIFFERENCES AS THE SYSTEM  
MOVES INTO COASTAL BRITISH COLUMBIA ON THURSDAY AFTERNOON. THE  
12Z/06Z/00Z GFS AND 00Z UKMET ARE STRONGER ALOFT WHILE ALSO BEING  
A HAIR SLOWER THAN THE CONSENSUS. THE SHORTWAVE QUICKLY SHEARS  
THEREAFTER AS IT ENCOUNTERS CONFLUENT MID-LEVEL FLOW DOWNSTREAM.  
AT THIS POINT WILL TAKE A SPLIT BETWEEN THE AMPLIFIED 12Z GFS AND  
FLATTER 00Z ECMWF UNTIL ADDITIONAL GUIDANCE ARRIVES.  
 
...MORE ROBUST SYSTEM TRACKING THROUGH THE PACIFIC NORTHWEST BY  
30/0600Z...   
..ATTENDANT FRONTAL/SURFACE FEATURES
 
 
PRELIMINARY PREFERENCE: A NON-00Z CMC MODEL COMPROMISE  
FORECAST CONFIDENCE: AVERAGE  
 
A MORE POTENT SHORTWAVE TAKES AIM AT THE PACIFIC NORTHWEST LATE IN  
THE FORECAST PERIOD WHICH WILL ULTIMATELY MAINTAIN A WET PATTERN  
ACROSS THE NORTHWESTERN U.S. INTO THE UPPER INTERMOUNTAIN  
WEST/NORTHERN ROCKIES. MULTI-DAY ENSEMBLE SPAGHETTI PLOT  
COMPARISONS HAVE SHOWN A MARKED DECREASE IN FORECAST SPREAD.  
PREVIOUS MODEL CYCLES SHOWED THE UKMET/CMC SOLUTIONS OFTEN  
DIVERGING FROM THE CONSENSUS OF THE ENSEMBLE MEANS. HOWEVER...THE  
00Z ENSEMBLES CLUSTER MUCH BETTER ALTHOUGH THE 00Z CMC DOES BECOME  
A SLOWER OUTLIER BY EARLY SATURDAY. REGARDING THE SURFACE LOW  
TRACK...ALL SOLUTIONS CLUSTER WELL INITIALLY ALTHOUGH ENSEMBLE  
SPREAD DOES INCREASE BY 30/0000Z OFFSHORE OF WA. WHILE OPERATIONAL  
MODELS ARE IN AGREEMENT...THE 00Z ECMWF ENSEMBLE MEAN IS DISPLACED  
WESTWARD AS NOTED BY MANY OF ITS MEMBERS BEING WEST OF THE  
OPERATIONAL RUN. AS A WHOLE...WILL STAY AWAY FROM THE SLOWER 00Z  
CMC AND FAVOR A CONSENSUS OF OTHER SOLUTIONS. CONFIDENCE WILL NOT  
BE RAISED ABOVE AVERAGE THOUGH GIVEN HOW MUCH THE SPREAD HAS  
CHANGED THE PAST FEW MODEL CYCLES.  
 
PREVIOUS MODEL TRENDS AT WWW.WPC.NCEP.NOAA.GOV/HTML/MODEL2.SHTML  
PREVIOUS 500 MB FORECASTS AT  
WWW.WPC.NCEP.NOAA.GOV/H5PREF/H5PREF.SHTML  
 
RUBIN-OSTER  
 

 
 
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