137  
FXUS10 KWNH 271842  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
241 PM EDT FRI OCT 27 2017  
 
VALID OCT 27/1200 UTC THRU OCT 31/0000 UTC  
 
...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR  
INGEST...  
 
12Z MODEL EVALUATION...WITH FINAL PREFERENCES AND CONFIDENCE  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
 
...OCCLUDING CYCLONE OVER THE WESTERN GREAT LAKES GRADUALLY  
WEAKENING AND LIFTING NNE TO NEAR HUDSON BAY...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: ABOVE AVERAGE  
 
MODELS ARE IN EXCELLENT AGREEMENT WITH THIS SYSTEM...SHOWING VERY  
LITTLE DIFFERENCE AMONG A VARIETY OF MASS FIELDS. THEREFORE...A  
GENERAL MODEL BLEND IS PREFERRED WITH HIGH CONFIDENCE.  
 
UPDATE FROM PRELIMINARY PREFERENCE: NONE. MODELS REMAIN IN GOOD  
AGREEMENT.  
 
...TROPICAL WAVE/DISTURBANCE CURRENTLY IN THE WESTERN CARIBBEAN  
AND APPROACHING THE FLORIDA STRAITS BY THIS WEEKEND...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 12Z GFS  
CONFIDENCE: AVERAGE  
 
THE CONVECTION AROUND THIS AREA OF LOW PRESSURE IN THE WESTERN  
CARIBBEAN HAS BECOME INCREASINGLY ORGANIZED...AND MOST OF THE  
MODELS NOW SHOW A STRONGER LOW MOVING IN THE GENERAL DIRECTION OF  
THE FLORIDA STRAITS AND THE NORTHWEST BAHAMAS. THE 12Z NAM JOINS  
THE 12Z UKMET AND CMC IN SHOWING THE LOWEST SURFACE PRESSURES BY  
T+48HR BUT THESE SOLUTIONS ARE NOT GROUPED IN THEIR TRACKS OF THE  
LOW (UKMET/CMC NORTH AND NAM SOUTH). THE 12Z ECMWF CONTINUED TO  
SHOW A WEAKER LOW FURTHER TO THE NORTHWEST. PREFER THE 12Z GFS IN  
THIS CASE...AS IT DOES NOT DEEPEN THE LOW QUITE AS FAST AS SOME  
OTHER MODELS (BUT STILL MORE THAN THE ECMWF)...AND IS RIGHT ALONG  
THE MODEL CONSENSUS TRACK AS WELL AS HIGHER PROBABILITY CLUSTERS  
FROM THE 00Z ECMWF ENSEMBLE AND 12Z GEFS ENSEMBLE.  
 
UPDATE FROM PRELIMINARY PREFERENCE: NONE. THE 12Z GFS REMAINS THE  
MODEL CLOSEST TO THE CONSENSUS TRACK AND THE HIGHER PROBABILITY  
CLUSTERS FROM THE 00Z ECMWF ENSEMBLE AND 12Z GEFS ENSEMBLE. OTHER  
TRENDS GENERALLY REMAINED CONSISTENT FROM THE 00Z CYCLE.  
 
...KICKER SHORTWAVE DIGGING FROM GREAT LAKES INTO BASE OF THE  
LONGWAVE TROUGH OVER THE SOUTHEAST THIS WEEKEND...  
...INCREASINGLY NEGATIVE TILT TROUGH EJECTING UP THE EAST COAST  
SUNDAY WITH RAPID CYCLOGENESIS JUST OFFSHORE...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: NON-UKMET BLEND  
WEIGHTED TOWARD THE ECMWF ENSEMBLE MEAN  
CONFIDENCE: AVERAGE  
 
MODEL DEPICTIONS OF THE RAPID CYCLOGENESIS ALONG THE EASTERN  
SEABOARD CONTINUE TO SHOW SOME DIFFERENCES DUE TO VARIABILITY IN  
THE TIMING OF THE KICKER SHORTWAVE AND MECHANISMS THAT LEAD TO  
INITIAL SURFACE LOW DEVELOPMENT. WITH RESPECT TO THE TIMING  
DIFFERENCES OF THE SHORTWAVE AS IT TRANSITIONS TO A NEGATIVE TILT  
AND BEGINS TO EJECT...THE 00Z MODEL RUNS SHOWED MORE VARIABILITY.  
THE 00Z UKMET WAS SLOWEST...IT HAD THE STRONGEST MID-LEVEL VORT  
MAX AS IT DIGS INTO THE BASE OF E TROUGH...AND AS A RESULT CLOSED  
OFF A FAIRLY STRONG LOW OVER THE CAROLINAS. THIS RESULTED IN THE  
MODEL BEING AT LEAST 6 HOURS SLOWER WITH THE PROGRESSION OF THE  
WAVE AS COMPARED TO OTHER MODELS. THE 12Z UKMET CONTINUES TO  
DEPICT THE SHORTWAVE IN THIS WAY...ALTHOUGH THE 12Z NAM AND 12Z  
ECMWF HAVE TRENDED IN THAT DIRECTION WITH A SLIGHTLY STRONGER  
KICKER WAVE THAT DIGS A LITTLE FURTHER WEST INITIALLY. THE NAM AND  
ECMWF ARE NOT QUITE AS SLOW AS THE UKMET...BUT DO INDICATE A  
SURFACE LOW TRACK FURTHER TO THE WEST THAN THEIR 00Z RUNS.  
 
MEANWHILE...THE 00Z GFS AND MANY 00Z GEFS MEMBERS WERE FASTER AND  
THE LEADING EDGE OF THE WAVE KICKED OUT A LITTLE FURTHER EAST INTO  
THE ATLANTIC. THIS LED TO MANY DEVELOPING A SURFACE LOW ON THE  
EASTERN EDGE OF THE MODEL SPREAD. THE 12Z GFS AND GEFS MEAN BOTH  
SHIFTED BACK TO THE WEST. THIS IS VERY CLOSE TO THE CONSISTENT  
PLACEMENT OF THE 00Z ECMWF ENSEMBLE MEAN OVER THE PAST SEVERAL  
MODEL CYCLES. THE 12Z CMC LIES ROUGHLY IN THE MIDDLE OF THE NEW  
GFS AND ECMWF RUNS.  
 
REGARDING THE INITIAL SURFACE LOW DEVELOPMENT AND FIRST 6-12HR OF  
DEEPENING...MODELS ALSO STILL SHOW QUITE A BIT OF VARIABILITY WITH  
THIS OFF THE EAST COAST. WITHIN THE DEEP PLUME OF TROPICAL  
MOISTURE AND ASSOCIATED CORRIDOR OF LOWER STATIC STABILITY...MANY  
MODELS APPEAR TO BE DEVELOPING MULTIPLE LOW CENTERS TIED TO BURSTS  
OF CONVECTION ALONG THE EFFECTIVE FRONT. THIS IS REASONABLE  
OVERALL...HOWEVER SOME MODELS MAY BE OVERDOING THE NUMBER OF LOW  
CENTERS OR THEIR INTENSITY. ON SUNDAY AFTERNOON AND EVENING...THE  
12Z NAM AND GFS BOTH SHOW AT LEAST FOUR SEPARATE LOW CENTERS. THIS  
INTRODUCES SOME UNCERTAINTY...AS THE TRACK OF THE LOW THAT  
EVENTUALLY CONSOLIDATES AND PUSHES INTO THE NORTHEASTERN U.S.  
WOULD BE AFFECTED BY THE LOCATION OF INITIAL DEVELOPMENT AND  
GROWTH.  
 
TAKING ALL THAT INTO ACCOUNT...THE PREFERENCE FOR THIS SYSTEM IS  
FOR A NON-UKMET BLEND WEIGHTED TOWARD THE 12Z GFS...12Z CMC...AND  
PARTICULARLY THE 00Z ECMWF ENSEMBLE. THE LINGERING UNCERTAINTY  
BETWEEN THE MODELS WOULD MORE EFFECTIVELY BE CAPTURED BY A GENERAL  
MODEL BLEND WITH A WEIGHT TOWARD THE MORE CONSISTENT MODELS. THE  
ECMWF HAS CONTINUED TO GRADUALLY SHOW THE SURFACE LOW AND RELATED  
FIELDS FURTHER WEST OVER THE PAST SEVERAL MODEL CYCLES...AND NOW  
GENERALLY LIES ON THE FAR WESTERN EDGE OF GUIDANCE. OUT OF RESPECT  
FOR HOW IT IS HAS BEEN LEADING THE TREND THUS FAR...IT IS INCLUDED  
IN THE BLEND...HOWEVER THE WEIGHT WILL BE PLACED TOWARD THE 00Z  
ECMWF ENSEMBLE MEAN WHICH HAS BEEN FAR MORE CONSISTENT OVER THE  
PAST COUPLE DAYS. THE UKMET IS ENOUGH OF AN OUTLIER TO BE EXCLUDED  
AT THIS TIME.  
 
UPDATE FROM PRELIMINARY PREFERENCE: CHANGE IN WEIGHT TOWARD THE  
ECMWF ENSEMBLE MEAN. THE OPERATIONAL ECMWF CONTINUES TO SHOW  
SURFACE LOW PLACEMENT FURTHER WEST AND IS NOW ON THE WESTERN EDGE  
OF MODEL SPREAD. THE PREFERENCE WILL BE WEIGHTED TOWARD THE ECMWF  
ENSEMBLE MEAN WHICH HAS BEEN VERY CONSISTENT THE PAST SEVERAL DAYS.  
 
...GULF OF ALASKA SHORTWAVE ROUNDING THE TOP OF THE RIDGE AND  
DIGGING INTO THE NORTHERN PLAINS BY SUNDAY...   
..ASSOCIATED COLD FRONT PUSHING INTO NORTHERN TIER OF STATES
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
MULTI-MODEL SPAGHETTI PLOTS OVER THE PAST SEVERAL MODEL CYCLES  
INDICATE THAT MODEL SPREAD HAS CONTINUED TO DIMINISH WITH THIS  
SYSTEM...AND THERE IS NOW FAIRLY GOOD AGREEMENT. ALTHOUGH SOME  
SMALL DIFFERENCES STILL EXIST...THE MODELS ALL SHOW SIMILAR TIMING  
OF THE SHORTWAVE AND FRONT...STRENGTH OF THE SURFACE LOW...AND  
TIMING OF THE COLD FRONT. THEREFORE...A GENERAL MODEL BLEND IS  
PREFERRED.  
 
UPDATE FROM PRELIMINARY PREFERENCE: NONE. MODELS REMAIN IN GOOD  
AGREEMENT.  
 
...BREAKDOWN OF WESTERN UPPER RIDGE AND MID-UPPER LEVEL LOW  
APPROACHING CALIFORNIA COAST BY SUNDAY...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 12Z GFS / 12Z ECMWF / 12Z CMC  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
OVERALL...THE MODELS SHOW DECENT AGREEMENT WITH THIS SYSTEM WITH  
THE PRIMARY DIFFERENCES BEING IN PRECISELY WHICH LATITUDE THE  
UPPER LEVEL LOW APPROACHES THE SOUTHERN CALIFORNIA COAST. THIS MAY  
BE TIED TO THE STRENGTH OF THE SUBTROPICAL RIDGE IN INDIVIDUAL  
MODEL SOLUTIONS. THE 00Z UKMET SHOWED A STRONGER RIDGE AND SHOWS A  
FLATTER WAVE AND WEAKER LOW APPROACHING THE COAST FURTHER NORTH.  
HOWEVER THE 12Z NAM SHOWED A WEAKER RIDGE...WITH THE LOW  
APPROACHING FURTHER SOUTH. THE MODELS THAT ARE MOST CONSISTENT  
WITH THE 00Z GEFS AND 00Z ECMWF ENSEMBLE MEANS ARE THE 12Z GFS AND  
00Z ECMWF. ALL ARE IN EXCELLENT AGREEMENT. THEREFORE...THE  
PREFERENCE IS TOWARD A BLEND OF THE GFS AND ECMWF FOR THIS SYSTEM.  
 
UPDATE FROM PRELIMINARY PREFERENCE: INCLUDE THE 12Z CMC. THE  
CANADIAN HAS MOVED INTO BETTER AGREEMENT WITH THE GFS AND  
ECMWF...WITH ONLY SLIGHT TIMING DIFFERENCES.  
 
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