280  
FXUS10 KWNH 281713  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
112 PM EDT SAT OCT 28 2017  
 
VALID OCT 28/1200 UTC THRU NOV 01/0000 UTC  
 
...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR  
INGEST...  
 
12Z MODEL EVALUATION...WITH PRELIMINARY PREFERENCES AND CONFIDENCE  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
 
   
..TROPICAL DEPRESSION EIGHTEEN
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
NHC BEST PROXY: 12Z NAM AND 00Z UKMET  
PLEASE REFER TO NHC FORECASTS  
 
THE 15Z ADVISORY FROM THE NATIONAL HURRICANE CENTER SHOWS TROPICAL  
DEPRESSION EIGHTEEN MOVING INTO THE FLORIDA STRAITS LATER TODAY  
AND TONIGHT...AND THEN ACCELERATING QUICKLY NORTHEAST WELL OFF THE  
EAST COAST OF THE UNITED STATES. THE ACCELERATION TREND DEPICTED  
BY THE NHC FORECAST IS GENERALLY BEST APPROXIMATED BY THE 12Z NAM  
AND 00 UKMET WHICH ARE A LITTLE SLOWER THAN THE OTHER MODEL  
GUIDANCE. THE NHC FORECAST DOES SHOW THE STORM BECOMING  
EXTRATROPICAL AND MERGING WITH THE FRONTAL SYSTEM AROUND 48 HOURS  
(12Z MONDAY) WHICH MATCHES WELL WITH THE EVOLUTION DEPICTED BY  
MODEL GUIDANCE.  
 
...KICKER SHORTWAVE DIGGING FROM GREAT LAKES INTO BASE OF THE  
LONGWAVE TROUGH OVER THE SOUTHEAST TODAY...  
...INCREASINGLY NEGATIVE TILT TROUGH EJECTING UP THE EAST COAST  
SUNDAY AND MONDAY WITH RAPID CYCLOGENESIS...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: BLEND OF 12Z GFS / 00Z ECMWF  
CONFIDENCE: AVERAGE  
 
TIMING DIFFERENCES WITH THE KICKER SHORTWAVE AS IT DIGS INTO THE  
BASE OF THE TROUGH CONTINUE TO PRODUCE SMALL BUT POTENTIALLY  
IMPORTANT DIFFERENCES LATER ON AS THE DEEPENING EXTRATROPICAL LOW  
MOVES INTO THE NORTHEAST. THE 12Z GFS AND 00Z ECMWF ARE GENERALLY  
FASTER WITH THE PROGRESSION...SHOWING THE LEADING EDGE OF THE WAVE  
AND MID-LEVEL VORT MAX REACHING WILMINGTON NC AROUND 18Z SUNDAY.  
THE 12Z NAM...00Z UKMET...AND 00Z CMC ARE SLOWER AND SLIGHTLY  
FURTHER WEST WITH THE PLACEMENT OF THE WAVE. THIS DIFFERENCE IS  
LIKELY DUE TO VERY SUBTLE VARIATIONS IN HOW THE MODELS ARE  
INITIALIZING THE WAVE (CURRENTLY OVER MN PER WATER VAPOR  
SATELLITE). UNTIL THIS WAVE REACHES THE BASE OF THE TROUGH...IT  
SHOULD HAVE A FAIRLY QUICK PROGRESSION...AT WHICH POINT IT WILL  
START TO INTENSIFY AS IT KICKS OUT ALONG THE EAST COAST. THE  
OPERATIONAL GFS AND ECMWF MODELS HAVE GOOD SUPPORT FROM THEIR  
ENSEMBLES (GEFS AND ECMWF ENS) WITH MANY MEMBERS SHOWING BOTH LOW  
PLACEMENT AND MINIMUM PRESSURE SIMILAR TO THAT OF THE GFS/ECMWF.  
THIS WOULD YIELD A SLIGHTLY FASTER ARRIVAL INTO THE NORTHEAST WITH  
A STRONGER CYCLONE (LOWER PRESSURES THAN THE NAM, CMC, AND UKMET  
AT COMPARABLE TIMES). IN GENERAL...MODELS HAVE CONTINUED TO  
CONVERGE FROM PREVIOUS CYCLES AND GENERALLY DEPICT THE EVOLUTION  
OF THE CYCLONE IN A SIMILAR FASHION...BUT WILL MAINTAIN THE  
PREFERENCE FOR THE GFS/ECMWF BLEND GIVEN THE STRONGER ENSEMBLE  
SUPPORT FOR THE DETAIL DIFFERENCES.  
 
...SHORTWAVE DIGGING INTO THE NORTHERN PLAINS BY SUNDAY AND  
PHASING WITH EAST COAST CYCLONE INTO MONDAY AND TUESDAY...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: ABOVE AVERAGE  
 
ALTHOUGH THERE ARE SOME SLIGHT DIFFERENCES BETWEEN THE  
MODELS...THEY CONTINUE TO SHOW GOOD OVERALL AGREEMENT WITH MANY OF  
THE MASS FIELDS AND A CONTINUATION OF THE GENERAL MODEL BLEND FROM  
THE PREVIOUS SHIFT IS PREFERRED.  
 
...BREAKDOWN OF WESTERN UPPER RIDGE AND MID-UPPER LEVEL LOW MOVING  
INTO THE SOUTHWEST MONDAY AND TUESDAY...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 12Z GFS / 00Z UKMET / 00Z ECMWF  
CONFIDENCE: ABOVE AVERAGE  
 
THE MODELS ARE ALMOST IDENTICAL IN THEIR PROGRESSION OF THE  
MID-LEVEL LOW INTO THE SOUTHERN CALIFORNIA COAST AND SUBSEQUENT  
EASTWARD MOTION THROUGH THE SOUTHWESTERN U.S. HOWEVER...LOOKING AT  
THE TEMPERATURE PROFILES AND THICKNESS FIELDS...THE NAM AND CMC  
ARE NOTABLY COOLER IN THE WESTERN STATES AND ARE OUTLIERS FROM THE  
REST OF THE GLOBAL MODELS. THEREFORE...A BLEND OF THE  
GFS...ECMWF...AND UKMET IS PREFERRED.  
 
   
..WEAK SHORTWAVE ENTERING PACIFIC NORTHWEST LATE SUNDAY
 
DIGGING  
TOWARD CENTRAL COASTAL CALIFORNIA BY TUES...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
~~  
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: AVERAGE  
 
MODELS DO SEEM TO SHOW SOME UNCERTAINTY WITH RESPECT TO THE  
PRECISE AMPLITUDE OF THIS WAVE...BUT ARE VERY SIMILAR WITH RESPECT  
TO TIMING. IN ORDER TO CAPTURE THE UNCERTAINTY AND GIVEN A LACK OF  
NOTABLE IMPACTS TO SENSIBLE WEATHER...A GENERAL MODEL BLEND IS  
RECOMMENDED.  
 
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