957  
FXUS10 KWNH 271902  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
202 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 MODEL GUIDANCE ALONG WITH FINAL  
PREFERENCES/FORECAST CONFIDENCE...  
 
...PROGRESSIVE SHORTWAVE ADJOINING EXITING HEIGHT FALLS ACROSS NEW  
ENGLAND ON WED/THU...  
 
FINAL 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 ON THE SMALLER SIDE 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
 
 
FINAL PREFERENCE: BLEND OF THE 12Z/00Z ECMWF  
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/GEFS MEAN HAVE MADE SOME FLATTER ADJUSTMENTS BRINGING THEM  
CLOSER TO THE MODEL CONSENSUS. 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 SLOWED DOWN SOMEWHAT RELATIVE TO ITS PREVIOUS RUNS.  
ADDITIONALLY...THE PREVIOUSLY MORE AMPLIFIED 00Z ECMWF HAS MOVED  
TOWARD A MORE SUPPRESSED SOLUTION. THERE IS SOME ENSEMBLE SUPPORT  
ALTHOUGH A VAST MAJORITY OF THE ECMWF ENSEMBLES FAVOR MORE  
AMPLIFIED FLOW. 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. WILL CONTINUE TO  
STICK WITH THE ECMWF SUITE HERE BUT WILL TAKE AN AVERAGE BETWEEN  
THE TWO ECMWF RUNS AS IT COULD EASILY MOVE BACK TOWARD AN  
AMPLIFIED PATTERN AGAIN TONIGHT.  
 
...SHEARED SHORTWAVE CROSSING INTO WESTERN BRITISH COLUMBIA BY  
THURSDAY AFTERNOON...  
 
FINAL PREFERENCE: BLEND OF THE 12Z GFS/ECMWF  
FORECAST CONFIDENCE: SLIGHTLY ABOVE 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 12Z/00Z UKMET ARE STRONGER ALOFT WHILE ALSO  
BEING A HAIR SLOWER THAN THE CONSENSUS. THE 12Z ECMWF HAS ALSO  
TRENDED DEEPER RELATIVE TO ITS PRECEDING RUN. THE SHORTWAVE  
QUICKLY SHEARS THEREAFTER AS IT ENCOUNTERS CONFLUENT MID-LEVEL  
FLOW DOWNSTREAM. ALTHOUGH THE 12Z GFS/ECMWF DIFFER SOMEWHAT WITH  
TIMING...THEY AT LEAST BOTH AGREE ON THE INITIAL INTENSITY OF THE  
UPPER TROUGH. THUS...WILL MAINTAIN THE PREVIOUS PREFERENCE WHILE  
INCLUDING THE LATEST 12Z ECMWF INTO THE MIX WITH FORECAST  
CONFIDENCE RAISED A BIT.  
 
...MORE ROBUST SYSTEM TRACKING THROUGH THE PACIFIC NORTHWEST BY  
30/0600Z...   
..ATTENDANT FRONTAL/SURFACE FEATURES
 
 
FINAL PREFERENCE: A NON-12Z/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 12Z/00Z CMC DO  
BECOME SLOWER OUTLIERS 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. THIS DOES SEEM TO BE DRIVEN BY THE 00Z ECMWF  
ENSEMBLE MEAN AVERAGING A NEIGHBORING WAVE TO THE NORTHWEST...THUS  
LEADING TO THIS OFFSET. AS A WHOLE...WILL CONTINUE TO STAY AWAY  
FROM THE SLOWER 12Z00Z 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.  
THINGS DO LOOK BETTER RESOLVED HOWEVER AS THE 12Z MODELS HAVE  
REMAINED FAIRLY CONSISTENT WITH THE PREVIOUS RUNS.  
 
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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