997  
FXUS10 KWNH 291659  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1259 PM EDT SUN OCT 29 2017  
 
VALID OCT 29/1200 UTC THRU NOV 02/0000 UTC  
 
...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR  
INGEST...  
 
12Z MODEL EVALUATION...WITH PRELIMINARY PREFERENCES AND CONFIDENCE  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
   
..TROPICAL STORM PHILIPPE
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
NHC BEST PROXY: 00Z ECMWF  
PLEASE REFER TO NHC FORECASTS  
 
THE 00Z ECMWF IS CLOSEST TO THE 15Z NHC FORECAST TRACK. THE ECMWF  
WAS GENERALLY FURTHEST SOUTH WITH THE POSITION OF THE LOW CENTER  
RELATIVE TO OTHER MODEL GUIDANCE...AND RECENT VISIBLE SATELLITE  
LOOPS SUGGEST THAT A FURTHER SOUTH POSITION IS LIKELY MORE CORRECT  
AT THIS TIME. OTHER MODELS ARE FURTHER NORTH AND GENERALLY SHOW A  
FASTER ABSORPTION INTO THE EXTRATROPICAL CYCLONE DEVELOPING TO ITS  
NORTHWEST.  
 
...NEGATIVE TILT SHORTWAVE EJECTING UP THE EAST COAST WITH RAPID  
CYCLOGENESIS JUST OFFSHORE OF THE EAST COAST...   
..ABSORPTION OF PHILIPPE AS IT TRANSITIONS TO EXTRATROPICAL
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
60 PERCENT 12Z GFS/NAM...40 PERCENT 00Z ECMWF/CMC/UKMET  
CONFIDENCE: ABOVE AVERAGE  
 
AVAILABLE MODELS DO SHOW SIMILARITY WITH THIS SYSTEM IN THE LARGE  
SCALE SENSE...BUT THERE REMAIN SOME DETAIL DIFFERENCES RELATED TO  
THE LOCATION OF INITIAL LOW DEVELOPMENT...HOW THE REMNANTS OF  
PHILIPPE EVENTUALLY GET ABSORBED INTO THE CYCLONE...AND OTHER  
MESOSCALE LOWS AND FRONTAL WAVES THAT DEVELOP TO THE SOUTHEAST OF  
THE DEVELOPING CYCLONE. THE REGIONAL IMPACTS SHOULD BE FAIRLY  
SIMILAR IN ANY GIVEN MODEL SOLUTION -- THE LOCATION OF THE IMPACTS  
MAY JUST VARY SLIGHTLY AND THIS WOULD MAKE A DIFFERENCE FOR  
LOCALIZED AREAS.  
 
REGARDING THE PLACEMENT OF THE INITIAL EXTRATROPICAL LOW...MODELS  
ALL SHOW THAT THIS SHOULD BE OCCURRING NOW AND ROUGHLY IN THE  
VICINITY OF THE COASTAL CAROLINAS. EXAMINATION OF SURFACE  
OBSERVATIONS SHOWS THE LARGEST 3-HR PRESSURE FALLS ARE FROM  
MOREHEAD CITY NC TO CAPE HATTERAS NC AND SURFACE WIND FIELDS DO  
SEEM TO INDICATE AN INCREASING CYCLONIC CURVATURE IN THIS AREA.  
THE MODELS THAT ARE CLOSEST TO THE AFOREMENTIONED AREA WITH A  
SURFACE LOW AT 18Z ARE THE 12Z GFS...12Z NAM...AND 00Z ECMWF. THE  
ECMWF WAS MUCH DEEPER WITH THE LOW AND THERE WAS LESS EVIDENCE IN  
THE SURFACE OBSERVATIONS FOR THAT AT THE MOMENT.  
 
FOR THE LARGE SCALE PATTERN...THE 00Z CMC WAS SLIGHTLY HIGHER WITH  
ITS HEIGHTS THROUGH MUCH OF THE EASTERN CONUS TROUGH AND HAD ITS  
INITIAL SURFACE LOW DEVELOPMENT SLIGHTLY FURTHER EAST THAN THE  
OTHER MODELS. GIVEN LIKELY CYCLOGENESIS CLOSER TO THE COAST...THIS  
IS A SLIGHT OUTLIER.  
 
THEREFORE...THE 12Z NAM AND 12Z GFS APPEAR TO BE PERFORMING BEST  
AT THE MOMENT. HOWEVER...GIVEN THAT THE MODELS ARE ALL FAIRLY  
CLOSE WITH THEIR OVERALL REPRESENTATIONS OF THE SYSTEM...THE  
PREFERENCE IS FOR A GENERAL MODEL BLEND TO ACCOUNT FOR THE SMALL  
DIFFERENCES IN LOCAL AND MESOSCALE DETAILS. THE PREFERENCE WILL BE  
SLIGHTLY WEIGHTED TOWARD THE NAM AND GFS.  
 
   
..DIGGING TROUGH INTO NORTHERN PLAINS TODAY
 
PUSHING INTO GREAT  
LAKES ON MONDAY AND PHASING WITH EAST COAST CYCLONE...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: ABOVE AVERAGE  
 
DESPITE COMPLEXITIES WITH THE PHASING OF TWO STRONG WAVES/LOWS  
ALOFT AND EMBEDDED SHORTWAVES IN THE FLOW...THE MODELS ARE IN  
QUITE GOOD AGREEMENT WITH HOW THIS PROCESS UNFOLDS. THERE ARE SOME  
SLIGHT DIFFERENCES IN THE EXACT PLACEMENT OF HEIGHT AND  
TEMPERATURE FIELDS...BUT THERE IS NOT A SUFFICIENT BIAS TO EXCLUDE  
ANY MODEL FROM THE PREFERENCE FOR THIS SYSTEM. THEREFORE...A  
GENERAL BLEND IS PREFERRED.  
 
...MULTIPLE SHORTWAVES MOVING THROUGH THE WESTERN CONUS  
ACCELERATING THE BREAKDOWN OF THE RIDGE AND LEADING TO BROAD  
TROUGH DEVELOPMENT...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: NON-NAM BLEND  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
OVERALL THERE IS GOOD AGREEMENT AMONG THE MODELS IN THEIR HANDLING  
OF THE MASS FIELDS ACROSS MUCH OF THE WESTERN CONUS OVER THE NEXT  
FEW DAYS. THE SOLUTIONS FOR THE CLOSED LOW MOVING INTO THE DESERT  
SOUTHWEST AND SHEARING OUT INTO THE SOUTHERN PLAINS ARE VERY  
SIMILAR. SOME SMALL DIFFERENCES REMAIN WITH THE AMPLITUDE OF THE  
SHORTWAVE DROPPING DOWN THE CALIFORNIA COAST MONDAY AND  
TUESDAY...BUT THE TIMING IS VERY SIMILAR AND THERE DOES NOT APPEAR  
TO BE MUCH CHANGE TO SENSIBLE WEATHER IMPACTS. MORE NOTABLE  
DIFFERENCES ARE LIKELY TO ENTER THE PICTURE WITH THE SHORTWAVES  
DIGGING INTO THE NORTHWEST ON WEDNESDAY...AS PRECIPITATION WILL BE  
SPREADING INTO THE NORTHERN TIER OF STATES AHEAD OF THAT. THE 12Z  
NAM IS GENERALLY STRONGER WITH THESE WAVES AND ADVANCES THE COLD  
AIR IN A LITTLE FURTHER/FASTER. THIS IS A TYPICAL BIAS. FOR THIS  
REASON THE PREFERENCE IS FOR A NON-NAM BLEND...MORE SO FOR THE  
LONGER TIME RANGES (BEYOND 48 HOURS) AS IT IS STILL IN FAIRLY GOOD  
AGREEMENT IN THE NEXT DAY OR SO.  
 
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