485  
FXUS10 KWNH 271709  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
108 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 PRELIMINARY 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.  
 
...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 00Z 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 00Z ECMWF SHOWED THE  
LEAST AMOUNT OF DEVELOPMENT INITIALLY AND IS FURTHEST 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 00Z  
GEFS ENSEMBLE.  
 
...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 AND 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. 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 00Z ECMWF AND 00Z CMC WERE IN THE MIDDLE OF THESE  
TRENDS AND WERE BOTH VERY CONSISTENT WITH THE 00Z ECMWF ENSEMBLE  
MEAN. ADDITIONALLY...THE ECMWF (AND ITS ENSEMBLE IN PARTICULAR)  
HAS BEEN SHOWING THE MOST CONSISTENCY WITH THIS SYSTEM FOR THE  
PAST SEVERAL MODEL CYCLES. THE 06Z AND 12Z GFS RUNS HAVE TRENDED  
IN THIS DIRECTION...AND ARE NOW FAIRLY CLOSE WITH THE MASS FIELDS.  
 
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 A  
NON-UKMET BLEND BUT WEIGHTED TOWARD THE 00Z ECMWF AND 00Z ECMWF  
ENSEMBLE MEAN. ALTHOUGH THEY SHOW SOME DETAIL DIFFERENCES...THE  
MODELS ARE NOW FAIRLY SIMILAR IN BOTH THEIR DEPICTION OF THE MASS  
FIELDS AND MANY OF THE IMPACTS. THE LINGERING UNCERTAINTY BETWEEN  
THE MODELS WOULD MORE EFFECTIVELY BE CAPTURED BY A GENERAL MODEL  
BLEND WITH A WEIGHT TOWARD THE MORE CONSISTENT MODELS.  
HOWEVER...THE UKMET IS ENOUGH OF AN OUTLIER TO BE EXCLUDED AT THIS  
TIME.  
 
...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.  
 
...BREAKDOWN OF WESTERN UPPER RIDGE AND MID-UPPER LEVEL LOW  
APPROACHING CALIFORNIA COAST BY SUNDAY...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 12Z GFS / 00Z ECMWF  
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.  
 
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