653  
FXUS10 KWNH 111915  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
214 PM EST WED DEC 11 2019  
 
VALID DEC 11/1200 UTC THRU DEC 15/0000 UTC  
 
...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR  
INGEST...  
 
12Z MODEL EVALUATION WITH PREFERENCES AND FORECAST CONFIDENCE  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
 
...SOUTHERN STREAM/SOUTHEAST TO MID ATLANTIC AND NORTHEAST SURFACE  
LOW FRI/SAT...  
 
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: AVERAGE  
 
19Z UPDATE: OVERALL THE 12Z SUITE OF GUIDANCE HAS TRENDED TOWARD A  
SIMILAR EVOLUTION OF THIS SYSTEM. THE GFS PROBABLY ENDS UP WITH  
TOO MUCH SHORTWAVE ENERGY HANGING BACK IN THE TRAILING WAVE OVER  
THE OH/TN VALLEY SATURDAY, AND TOO LITTLE WITH THE LEAD WAVE OVER  
THE SOUTHEAST. THIS HAS SOME IMPACT ON THE LOW AND QPF FORECAST,  
BUT PROBABLY NOT ENOUGH TO REMOVE IT FROM THE BLEND AT THIS POINT.  
THE UKMET REMAINS A BIT MORE ENERGETIC WITH THE PHASING OF THE  
ENERGY RESULTING IN A BIT SLOWER/WEST EVOLUTION OF THE LOW...BUT  
EVEN THESE DIFFERENCES NO LONGER APPEAR BIG ENOUGH TO REMOVE IT  
FROM THE BROADER MODEL CONSENSUS. THUS AT THIS POINT A GENERAL  
MODEL CONSENSUS SHOULD FARE PRETTY WELL WITH THIS SYSTEM. THE  
TREND TODAY HAS BEEN FOR THE TWO STREAMS OF ENERGY TO INTERACT A  
BIT MORE, RESULTING IN A FURTHER INLAND LOW TRACK UP INTO NEW YORK  
BY SATURDAY.  
   
..ORIGINAL DISCUSSION
 
 
ALL THE MODELS HAVE THE SAME GENERAL IDEA OF AN AREA OF LOW  
PRESSURE DEVELOPING OVER THE SOUTHEAST AND MOVING UP THE COAST  
FRIDAY NIGHT INTO SATURDAY...HOWEVER DIFFERENCES WITH THE DETAILS  
OF THE EVOLUTION REMAIN. QUITE A FEW PIECES OF SHORTWAVE ENERGY  
INTERACTING WITHIN A QUICK ZONAL FLOW, THUS MAKES SENSE THAT  
MODELS WOULD STRUGGLE A BIT WITH THE DETAILS OF HOW EVERYTHING  
COMES TOGETHER. THE INITIAL WAVE THAT TRIGGERS THE LOW DEVELOPMENT  
SEEMS PRETTY WELL FORECAST, HOWEVER IT IS THE TRAILING, STRONGER,  
ENERGY THAT EVENTUALLY CATCHES THIS LOW THAT IS A BIT LESS  
CERTAIN. THE 12Z GFS/NAM AND 00Z ECMWF SEEM PRETTY WELL CLUSTERED  
WITH THE SYSTEM, AND WHILE DIFFERENCES STILL EXIST WITHIN THESE  
SOLUTIONS, A COMPROMISE OF THE THREE SEEMS LIKE A PRETTY GOOD  
STARTING POINT. THE 06Z GFS WAS AN OUTLIER IN HOW IT HANDLED THE  
EVOLUTION, BUT THE 12Z RUN APPEARS MUCH MORE ALIGNED WITH THE  
BETTER CONSENSUS. THE 00Z CMC APPEARS TOO STRONG WITH THE LEAD  
WAVE, WHICH ALLOWS FOR A STRONGER LOW INITIALLY MOVING UP THE  
COAST. WHILE THIS IS NOT IMPOSSIBLE, IT APPEARS LIKE A LOWER  
PROBABILITY EVENT. THE 00Z UKMET IS MUCH STRONGER WITH THE  
TRAILING SHORTWAVE ENERGY CAUSING AN EARLIER FORMATION OF A  
NEGATIVELY TILTED TROUGH AND A FURTHER WEST LOW TRACK...AND THIS  
TOO SEEMS LESS LIKELY AT THIS POINT. THUS BOTH THE 00Z CMC AND  
UKMET ARE NOT IN OUR PREFERRED BLEND AT THIS TIME.  
 
...NORTHERN STREAM SHORTWAVES ACROSS THE NORTHERN PLAINS TO THE  
UPPER MS VALLEY AND UPPER GREAT LAKES...  
 
PREFERENCE: 12Z GFS/NAM AND 12Z ECMWF  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
THE NORTHERN STREAM 500 MB SHORTWAVE DEPARTS THE NORTHERN ROCKIES  
ACROSS THE NORTHERN PLAINS TONIGHT AND INTO THE UPPER MS VALLEY  
THU. THE WAVE DEAMPLIFIES AND TURNS NORTHEAST ACROSS THE UPPER  
GREAT LAKES THU NIGHT AND ACROSS THE CANADIAN BORDER. MINOR  
DIFFERENCES IN WAVE AMPLITUDE AND PHASING AND GENERAL GOOD  
CLUSTERING OF SOLUTIONS LEAD TO A RECOMMENDATION TO USE A  
MULTI-MODEL BLEND.  
 
THE NEXT SHORTWAVE DROPS OUT OF SOUTHWEST CANADA INTO THE NORTHERN  
PLAINS THU AFTERNOON AND NIGHT.  
THIS WAVE GRADUALLY AMPLIFIES AS IT CROSSES THE NORTHERN PLAINS  
INTO THE UPPER MS VALLEY FRI WITH THE SOUTHERN STREAM PORTION  
NOTED ABOVE DEVELOPING IN THE MID TO LOWER MS VALLEY. THE 12Z  
ECMWF HAS SLIGHTLY AMPLIFIED THIS WAVE TO REDUCE THE DIFFERENCES.  
 
THE NORTHERN STREAM PORTION OF THE TROUGH IS SLOW TO MOVE OUT OF  
THE UPPER MS VALLEY WITH A SLOW DRIFT TOWARDS THE UPPER GREAT  
LAKES ON SAT. THERE IS GOOD AGREEMENT ON THE LOCATION AND TIMING  
AMONG THE NAM, GFS, AND ECMWF AS A RESULT OF THE SLOW MOVEMENT,  
SUPPORTED BY THE 12Z GEFS MEAN AND 00Z ECMWF ENSEMBLE MEAN.  
 
...PACIFIC NORTHWEST TO THE NORTHERN GREAT BASIN, NORTHERN TO  
CENTRAL ROCKIES/CENTRAL PLAINS...  
 
PREFERENCE: 06Z GEFS MEAN IN THE PACIFIC NORTHWEST, IN BETWEEN THE  
12Z NAM/GFS OVER THE GREAT BASIN TO THE ROCKIES AND CENTRAL PLAINS  
CONFIDENCE: AVERAGE  
 
FOR DAYS 1-2 ACROSS THE PACIFIC NORTHWEST, CA, AND THE GREAT  
BASIN, A PARADE OF SMALL AMPLITUDE SHORTWAVES IN THE STRONG ZONAL  
UPPER JET GRAPHIC IS BEST HANDLED BY A GENERAL MODEL BLEND .  
STILL, TOWARD THE END OF THE TRAIN, A STRONGER/GREATER AMPLITUDE  
SHORTWAVE TRACKS THROUGH WITH SOME MODERATE NEUTRAL TO NEGATIVE  
TILT ORIENTATION LATE FRIDAY INTO EARLY MORNING SAT WITH AN  
ASSOCIATED SURFACE CYCLONE. REMARKABLY, THE GLOBAL GUIDANCE IS  
FAIRLY AGREEABLE WITH THE TIMING AND MASS FIELDS AS A WHOLE,  
PROVIDING SOME CONFIDENCE. THE LARGE DIFFERENCES ARE THE WIDTH OF  
THE UPPER LEVEL SYSTEM, WITH THE GFS A BIT MORE SLOWER AND BROADER  
SUPPORTED MORE BY THE CMC WHILE THE NAM/ECMWF ARE WELL  
PLACED/SHAPED BUT NARROWER AND THE UKMET IS MOST NARROW AND  
NEGATIVELY TILTED. WILL HEDGE MORE OF A COMPROMISE BETWEEN THE  
00Z NAM/GFS AND 12Z ECWMF.  
 
THE LATTER HALF OF DAY 2 THROUGH DAY 3, DIFFERENCES DEVELOP IN THE  
700 MB WAVE STRUCTURE FROM THE CENTRAL GREAT BASIN ACROSS THE  
CENTRAL ROCKIES AND HIGH CENTRAL PLAINS. THE 00Z-12Z ECMWF IS  
FASTER THAN THE NAM/GFS/UKMET, WITH THE SLOWER NAM AND UKMET ALSO  
MORE AMPLIFIED. CONFIDENCE IS LOWER HERE AS THE GFS TRENDED FASTER  
ON THE 12Z RUN. I RECOMMEND INTERMEDIATE TIMING AND AMPLITUDE  
BETWEEN THE FASTER/LESS AMPLIFIED ECMWF/GFS AND THE SLOWER/MORE  
AMPLIFIED NAM/UKMET. THE 12Z ECMWF HAS INCREASED THE AMPLITUDE OF  
THE CENTRAL PLAINS 700 MB TROUGH SAT TO NARROW THE GAP IN WAVE  
AMPLITUDE, SO THE 12Z RUN LOOKS GOOD.  
 
BY DAY 3 IN THE PACIFIC NORTHWEST, THE UPSTREAM CLOSED LOW OVER  
THE NORTHEAST PACIFIC DRIFTS SLOWLY EAST, WITH MOST SOLUTIONS  
OPENING THE CLOSED LOW AT 500 MB INTO AN OPEN WAVE DURING THE DAY  
SATURDAY, WITH THE 500 MB WAVE POSSIBLY APPROACHING THE CA COAST  
LATE SAT. THE GFS IS SLOWER OPENING UP THE CLOSED LOW INTO A WAVE,  
RESULTING IN STRONGER HEIGHT FALLS FROM WESTERN OR INTO NORTHWEST  
CA SAT AFTERNOON. THE 12Z GEFS MEAN CLUSTERS WELL WITH THE 12Z NAM  
500 MB WAVE TIMING AND A BIT LESS AMPLIFIED THAN THE OPERATIONAL  
GFS. A BLEND OF THE 12Z ECMWF AND UKMET MESHES WELL WITH THE 12Z  
GEFS MEAN.  
 
MODEL TRENDS AT WWW.WPC.NCEP.NOAA.GOV/HTML/MODEL2.HTML  
500 MB FORECASTS AT WWW.WPC.NCEP.NOAA.GOV/H5PREF/H5PREF.SHTML  
 
PETERSEN/CHENARD  
 

 
 
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