795  
FXUS10 KWNH 261718  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
117 PM EDT THU OCT 26 2017  
 
VALID OCT 26/1200 UTC THRU OCT 30/0000 UTC  
 
...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR  
INGEST...  
 
12Z MODEL EVALUATION...WITH PRELIMINARY PREFERENCES AND CONFIDENCE  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
 
...NEGATIVE TILT TROUGH EJECTING INTO EASTERN CANADA AND  
DEAMPLIFYING WITH SECONDARY SURFACE LOW DEVELOPMENT NEAR THE GULF  
OF MAINE...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: BLEND OF 12Z GFS / 12Z NAM / 00Z ECMWF  
CONFIDENCE: ABOVE AVERAGE  
 
THE CURRENT NEGATIVE TILT TROUGH EXTENDING FROM SOUTHERN ONTARIO  
INTO THE MID ATLANTIC WILL CONTINUE TO EJECT NORTH-NORTHEAST INTO  
EASTERN CANADA OVER THE NEXT 24 HOURS OR SO...AND WILL GRADUALLY  
DEAMPLIFY IN THE PROCESS. THE MODELS SHOW A GREAT DEAL OF  
SIMILARITY WITH THIS SYSTEM...ALTHOUGH THE 00Z UKMET DEEPENS THE  
SURFACE LOW MORE THAN THE OTHER MODELS DESPITE A SIMILAR INTENSITY  
OF THE MID-UPPER LEVEL VORT MAX. GIVEN THAT IT IS AN OUTLIER IN  
THIS REGARD...AND MORE SUBSTANTIAL DEEPENING GENERALLY DOESN'T FIT  
EXPECTATIONS WITH A DEAMPLIFYING TROUGH ALOFT...WILL EXCLUDE THE  
UKMET FROM THE PREFERENCE FOR THIS SYSTEM. THE 00Z CMC IS  
DISPLACED SLIGHTLY EAST WITH ITS FEATURES IN THE CORE OF THE  
TROUGH AND THE LOW LEVELS AND IT IS ALSO EXCLUDED AT THIS TIME.  
THE 12Z NAM AND GFS...AND 00Z ECMWF SEEM TO BE IN EXCELLENT  
AGREEMENT.  
 
 
...SHORTWAVE TROUGH DIGGING INTO THE WESTERN GREAT LAKES WITH  
CLOSED LOW DEVELOPMENT AND CYCLONE OCCLUSION BY FRIDAY...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: BLEND OF 12Z GFS / 00Z ECMWF / 00Z CMC  
CONFIDENCE: ABOVE AVERAGE  
 
MODELS HAVE CONTINUED TO CONVERGE WITH RESPECT TO THIS  
SYSTEM...AND ARE NOW IN VERY GOOD AGREEMENT WITH MANY OF THE  
ASSOCIATED FEATURES AND MASS FIELDS. THE 12Z NAM IS SHOWING A  
COMMON BIAS TO BE ON THE COOL SIDE WITH THE TEMPERATURE PROFILE  
AND THE 00Z UKMET IS ACTUALLY COLDER THAN THE NAM IN CERTAIN  
AREAS. GIVEN THE EXCELLENT AGREEMENT BETWEEN THE GFS...ECMWF...AND  
CMC TO BE SLIGHTLY WARMER...THESE MODELS ARE PREFERRED IN THE  
BLEND ALTHOUGH THE DIFFERENCES BETWEEN ALL THE MODELS ARE STILL  
RELATIVELY MINOR.  
 
...TROPICAL WAVE/DISTURBANCE CURRENTLY IN THE WESTERN CARIBBEAN  
AND APPROACHING THE FLORIDA STRAITS BY THIS WEEKEND...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: BLEND OF 12Z GFS / 00Z ECMWF  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
NHC AND TAFB CONTINUE TO ANALYZE THE SURFACE LOW CENTER WITH THIS  
DISTURBANCE VERY CLOSE TO THE COAST OF NORTHERN NICARAGUA...AND  
THE PREFERENCE IS TO LEAN TOWARD THE MODELS THAT HAVE THE SURFACE  
LOW CLOSER TO THE COAST OF CENTRAL AMERICA OVER THE NEXT 24-36  
HOURS. THE 12Z GFS AND 00Z ECMWF SHOW THIS...SO WILL PREFER A  
BLEND OF THE TWO. THE 12Z NAM AND 00Z CMC ARE NOT SIGNIFICANTLY  
DIFFERENT...BUT THE 00Z UKMET SHOWS A SURFACE LOW MUCH STRONGER  
AND FURTHER EAST. THIS SEEMS LIKE A LESS LIKELY SOLUTION GIVEN THE  
DISORGANIZATION OF THE SYSTEM AT THE MOMENT.  
 
...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: BLEND OF 00Z ECMWF / 12Z GFS / 00Z CMC  
LEAN 50 PERCENT ECMWF...25 PERCENT EACH GFS/CMC  
CONFIDENCE: AVERAGE  
 
A SHORTWAVE WILL RAPIDLY DIG SOUTH-SOUTHEAST ALONG THE BACKSIDE OF  
THE GREAT LAKES TROUGH ON SATURDAY...AND REACH THE SOUTHEAST BY  
SUNDAY. THE INTENSIFYING VORT MAX ALOFT DIGGING INTO THE BASE OF  
THE TROUGH WILL BE ACCOMPANIED BY A TRANSITION TO NEGATIVE TILT  
AND SUBSEQUENT EJECTION NORTH-NORTHEAST ALONG THE EAST COAST. THE  
COMBINATION OF THE DIGGING WAVE ALOFT...INCREASING BAROCLINIC ZONE  
WITH CAA APPROACHING THE COAST...AND PLUME OF TROPICAL MOISTURE  
JUST OFFSHORE SHOULD PROMOTE RAPID DEEPENING OF A SURFACE LOW  
ALONG THE EAST COAST.  
 
THE 12Z NAM DIGS THE SHORTWAVE FURTHER WEST INITIALLY...WHICH  
LEADS TO ABOUT A 6-12HR SLOWER TRANSITION TO NEGATIVE TILT WITH  
THE TROUGH...AND A MEAN TROUGH POSITION THAT IS FURTHER WEST. THE  
DIFFERENCES GROW AFTER ABOUT T+72HR AND THE NAM WAS EXCLUDED FROM  
THE PREFERENCE FOR THIS SYSTEM. FOR THE 00Z UKMET...GIVEN IT IS  
LIKELY TOO STRONG WITH THE SURFACE LOW FOR THE TROPICAL  
DISTURBANCE APPROACHING FLORIDA (AND UNCERTAINTIES AS TO HOW THAT  
WOULD PLAY INTO EVENTUAL CYCLOGENESIS FURTHER NORTH)...IT WAS ALSO  
EXCLUDED FROM THE PREFERENCE FOR THIS SYSTEM. THERE ARE SOME  
DIFFERENCES BETWEEN THE 12Z GFS AND THE 00Z ECMWF AND CMC...BUT IN  
GENERAL THEY ARE FAIRLY CLOSE IN THEIR DEPICTION OF THE  
CYCLOGENESIS PROCESS AND ASSOCIATED WEATHER IMPACTS. A BLEND OF  
THESE MODELS IS PREFERRED. GIVEN THE ANOMALOUS FEATURES AT PLAY  
(NAEFS 700-500MB HEIGHTS ABOVE 90TH PERCENTILE FOR LOWEST HEIGHTS  
IN APPROACHING WAVE; NAEFS 200MB JET STREAK OVER THE SOUTHEAST AT  
99TH PERCENTILE; NAEFS PWATS IN MOISTURE PLUME ABOVE 95TH  
PERCENTILE) WILL ALSO LEAN IN THE DIRECTION OF THE SOLUTION WITH  
MORE RAPID DEEPENING...WHICH IS THE 00Z ECMWF IN THIS CASE.  
 
...SHORTWAVE CURRENTLY NEAR THE ALEUTIANS ROUNDING THE TOP OF THE  
RIDGE AND DIGGING INTO THE NORTHERN PLAINS BY SUNDAY...   
..ASSOCIATED SURFACE LOW AND COLD FRONT
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 12Z GFS / 00Z ECMWF / 00Z CMC  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
THE 00Z UKMET SHOWED A DIFFERENT STRUCTURE TO THIS WAVE AS IT WAS  
DIGGING SOUTHEAST...WITH A WEAKER TROUGH OVERALL THAT LEADS TO A  
WEAKER SURFACE LOW FURTHER TO THE NORTHEAST OF THE OTHER MODELS.  
THE NAM SHOWS A MUCH STRONGER LOW ALOFT...BUT IT IS CONFINED MUCH  
FURTHER TO THE NORTHEAST COMPARED TO OTHER MODELS. THESE ARE THE  
LARGEST DIFFERENCES. THE GFS...ECMWF...AND CMC HAVE SOME DETAIL  
DIFFERENCES...BUT ARE CLOSE ENOUGH TO BE INCLUDED IN THE  
PREFERENCE FOR THIS SYSTEM.  
 
...BREAKDOWN OF WESTERN UPPER RIDGE AND MID-UPPER LEVEL LOW  
APPROACHING CALIFORNIA COAST BY SUNDAY...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 12Z GFS / 00Z ECMWF / 00Z CMC  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
THE 00Z UKMET WAS MUCH STRONGER AND FURTHER SOUTH WITH THE CLOSED  
LOW ALOFT...AND IS A CLEAR OUTLIER WITH THIS SYSTEM. THE 12Z NAM  
WAS NOT QUITE AS STRONG...BUT STILL STRONGER THAN THE REMAINING  
MODEL GUIDANCE (AND ALSO DISPLACED FURTHER SOUTH). THE PREFERENCE  
IS FOR THE OTHER MODELS (GFS, ECMWF, CMC) WHICH ARE IN GOOD  
AGREEMENT.  
 
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