041  
FXUS10 KWNH 261849  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
248 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 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 12Z 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 12Z 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.  
 
CHANGE FROM PRELIMINARY PREFERENCE: NONE. THE UKMET AND CMC  
CONTINUED THEIR RESPECTIVE TRENDS FROM THE 00Z RUN...ALTHOUGH THEY  
CAME INTO SLIGHTLY BETTER 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 12Z 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.  
 
CHANGE FROM PRELIMINARY PREFERENCE: NONE. THE 12Z NAM AND 12Z CMC  
CONTINUE TO BE A LITTLE COLDER WITH THE TEMPERATURE  
PROFILE...PARTICULARLY FROM 27/00Z TO 28/00Z. THE DIFFERENCE IS  
RELATIVELY MINOR...BUT MAY MAKE AN IMPACT ON PRECIPITATION TYPE  
FORECASTS.  
 
...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 12Z ECMWF SHOW THIS...SO WILL PREFER A  
BLEND OF THE TWO. THE 12Z NAM AND 12Z CMC ARE NOT SIGNIFICANTLY  
DIFFERENT...BUT THE 12Z 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.  
 
CHANGE FROM PRELIMINARY PREFERENCE: NONE. THE UKMET CONTINUES TO  
SHOW A MUCH STRONGER SURFACE LOW DEVELOPING AND TRACKING BETWEEN  
FLORIDA AND THE BAHAMAS. GIVEN THE LACK OF SUPPORT FOR THIS FROM  
OTHER MODELS...THE PREFERENCE FOR A WEAKER AND FURTHER WEST  
SOLUTION CONTINUES.  
 
...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 12Z ECMWF / 12Z GFS  
LEAN 65 PERCENT ECMWF...35 PERCENT GFS  
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 12Z 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. THE 12Z CMC LOOKS  
SLIGHTLY DIFFERENT NOW (AS COMPARED TO THE 00Z RUN)...WITH THE  
WAVE DIGGING FASTER AND REACHING THE SOUTHEAST COAST ABOUT 6HR  
SOONER. THIS LEADS TO A FASTER PROGRESSION OF THE SURFACE LOW TO  
THE NORTH (AND A WEAKER SURFACE LOW OVERALL). GIVEN THE  
INCONSISTENCY WITH PREVIOUS RUNS AND THE MORE CONSISTENT  
ECMWF/GFS...THE CMC WAS EXCLUDED.  
 
THERE ARE SOME DIFFERENCES BETWEEN THE 12Z GFS AND THE 12Z  
ECMWF...BUT IN GENERAL THEY ARE FAIRLY CLOSE IN THEIR DEPICTION OF  
THE CYCLOGENESIS PROCESS AND ASSOCIATED WEATHER IMPACTS. THE GFS  
IS JUST FURTHER EAST...WHILE THE ECMWF IS FURTHER WEST (IN FACT IT  
TRENDED FURTHER WEST ON THIS RUN). 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 12Z ECMWF IN THIS CASE. VERY STRONG  
BAROCLINIC ZONE AND STRONG JET DYNAMICS SHOULD ALSO FAVOR RAPID  
DEEPENING.  
 
CHANGE FROM PRELIMINARY PREFERENCE: REMOVE THE CMC. THE CANADIAN  
CAME IN WITH A SLIGHTLY DIFFERENT SOLUTION THAT PLACES IT IN  
LARGER DISAGREEMENT WITH THE GFS AND ECMWF. CONTINUE TO FAVOR THE  
GFS/ECMWF WHICH ARE BOTH MORE CONSISTENT AND FIT THE SYNOPTIC  
PATTERN RECOGNITION A LITTLE BETTER.  
 
...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 12Z UKMET TRENDED CLOSER TO THE OTHER GLOBAL MODELS WITH ITS  
STRUCTURE OF THE WAVE...BUT DIFFERENCES REMAIN. THE NAM SHOWS A  
STRONGER LOW ALOFT...BUT IT IS CONFINED 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.  
 
CHANGE FROM PRELIMINARY PREFERENCE: NONE.  
 
...BREAKDOWN OF WESTERN UPPER RIDGE AND MID-UPPER LEVEL LOW  
APPROACHING CALIFORNIA COAST BY SUNDAY...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: ABOVE AVERAGE  
 
MODELS SHOW GOOD AGREEMENT WITH THE MASS FIELDS FOR THIS SYSTEM  
AND THUS A GENERAL MODEL BLEND IS PREFERRED WITH ABOVE AVERAGE  
CONFIDENCE.  
 
CHANGE FROM PRELIMINARY PREFERENCE: TREND TO GENERAL BLEND. THE  
AGREEMENT BETWEEN THE MODELS HAS IMPROVED ON THE 12Z MODEL CYCLE  
SO A GENERAL BLEND IS NOW PREFERRED.  
 
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