959  
FXUS10 KWNH 131910  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
209 PM EST WED DEC 13 2017  
 
VALID DEC 13/1200 UTC THRU DEC 17/0000 UTC  
 
...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR  
INGEST...  
 
12Z MODEL EVALUATION...WITH LATEST PREFERENCES AND CONFIDENCE  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
   
..DEEP CYCLONE OVER THE NORTHEAST
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: NON-UKMET CONSENSUS  
CONFIDENCE: ABOVE AVERAGE  
 
THE MODELS OVER THE NEXT 24 HOURS ALLOW THE VERY DEEP COLD  
CYCLONIC LOW OVER THE NORTHEAST U.S. TO LIFT NORTH UP ACROSS  
SOUTHEAST CANADA INCLUDING NEW BRUNSWICK AND THEN ACROSS EASTERN  
QUEBEC LATER TODAY AND TONIGHT. THE 12Z UKMET AGAIN TENDS TO WRAP  
ITS SURFACE LOW PROBABLY A LITTLE TOO FAR BACK TO THE WEST OVER  
QUEBEC TONIGHT AND THURSDAY...AND ENDS UP A BIT LEFT OF THE  
OTHERWISE WELL CLUSTERED MODEL SUITE. WILL PREFER A NON-UKMET  
CONSENSUS AS A RESULT FOR NOW.  
 
   
..CLIPPER SHORTWAVE/LOW CENTER DIGGING SOUTHEAST FROM CANADA
 
   
..CROSSING THE UPPER MIDWEST/OH VALLEY/MID-ATLANTIC TODAY/THURS
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: ABOVE AVERAGE  
 
THE GUIDANCE SHOWS A STRONG CLIPPER TYPE SHORTWAVE AND ASSOCIATED  
SURFACE LOW DIGGING SOUTHEAST OUT OF SOUTHERN CANADA AND QUICKLY  
MOVING SOUTHEAST TOWARD THE UPPER MIDWEST. THIS SYSTEM WILL  
RAPIDLY CROSS PARTS OF THE LOWER GREAT LAKES...OH VALLEY AND  
MID-ATLANTIC REGION TODAY THROUGH THURSDAY. THE LATEST GUIDANCE IS  
IN GOOD AGREEMENT WITH THE MASS FIELD DETAILS OF THIS SYSTEM...SO  
A GENERAL MODEL BLEND WILL BE PREFERRED.  
 
   
..SHORTWAVE DIGGING SOUTH ACROSS THE PLAINS THURS
 
   
..ENERGY CROSSING THE LOWER MS/TN VALLEYS ON FRI
 
   
..POSSIBLE SURFACE LOW DEVELOPMENT OFF THE MID-ATLANTIC ON FRI
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: BLEND OF THE 12Z GFS AND 12Z UKMET  
CONFIDENCE: AVERAGE  
 
A SHORTWAVE WILL DROP SOUTH DOWN ACROSS THE PLAINS THURSDAY BEFORE  
SWEEPING ACROSS THE LOWER MS AND TN VALLEY REGION ON FRIDAY. THIS  
ENERGY IS EXPECTED TO HELP RELOAD A LARGER SCALE LONGWAVE TROUGH  
ACROSS THE EASTERN U.S. BY THE END OF THE WEEK AND WILL ENCOURAGE  
A SURFACE LOW DEVELOPING OFFSHORE THE MID-ATLANTIC BY FRIDAY  
NIGHT. THE GUIDANCE SHOWS GOOD AGREEMENT OVERALL WITH THE  
SHORTWAVE DETAILS...WITH SOME MINOR TIMING DIFFERENCES.  
HOWEVER...AT THE SURFACE...THE 12Z ECMWF REFLECTS A STRONGER  
SURFACE LOW OFFSHORE THE MID-ATLANTIC BY FRIDAY NIGHT AND WITH A  
TRACK A LITTLE LEFT OF THE MODEL CONSENSUS. THE 12Z CMC ALSO LEANS  
STRONGER AND A BIT SLOWER THAN THE CONSENSUS. THE REMAINING  
GUIDANCE IS A BIT MORE ELONGATED AND MODESTLY FLATTER BY  
COMPARISON UNTIL THE LOW IS WELL OFFSHORE. WILL SUGGEST A BLEND OF  
THE 12Z GFS AND 12Z UKMET AT THIS POINT WHICH TEND TO OVERALL HAVE  
A BIT BETTER SUPPORT FROM THE 12Z GEFS MEAN AND 00Z ECENS MEAN.  
 
   
..NEW UPPER LOW DROPPING SOUTH TOWARD THE GREAT LAKES BY FRI
 
   
..ENERGY ADVANCING INTO NEW ENGLAND ON SAT
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: BLEND OF THE 12Z GFS/12Z UKMET AND 12Z ECMWF  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
THE MODELS TAKE ARCTIC SHORTWAVE ENERGY CURRENTLY UP OVER NUNAVUT  
IN NORTHERN CANADA DOWN TO THE SOUTH OVER THE NEXT COUPLE OF DAYS  
WHICH RESULTS IN A NEW UPPER LOW DROPPING SOUTH OVER HUDSON BAY  
AND THEN ACROSS ONTARIO BY THE END OF THE WEEK. THIS WILL RESULT  
IN HEIGHT FALLS ENCROACHING ON THE GREAT LAKES REGION AND A WAVE  
OF LOW PRESSURE IMPACTING THIS REGION. THEREAFTER GOING INTO THE  
WEEKEND...THE ENERGY WILL PIVOT ACROSS NEW ENGLAND. THERE ARE SOME  
MINOR TIMING DIFFERENCES WITH THE ARRIVAL OF HEIGHT FALLS AS THE  
12Z NAM AND 12Z CMC ARE A LITTLE FASTER THAN THE 12Z GFS/12Z UKMET  
AND 12Z ECMWF TO PIVOT THE TROUGH AXIS ACROSS NEW ENGLAND. THE 12Z  
GEFS MEAN AND 00Z ECENS MEAN ARE BOTH SUPPORTIVE OF THE SLIGHTLY  
SLOWER CONSENSUS...AND SO A BLEND OF THE GFS/UKMET AND ECMWF WILL  
BE PREFERRED AT THIS TIME.  
 
   
..SHORTWAVE TROUGH ARRIVING ACROSS THE PACIFIC NORTHWEST ON FRI
 
   
..ENERGY DIGGING SOUTHEAST ACROSS THE INTERMOUNTAIN WEST/PLAINS
 
   
..COLD FRONT SETTLING OVER THE PLAINS/MIDWEST BY SAT
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: BLEND OF THE 12Z GFS AND 12Z ECMWF...AT THE SURFACE  
BLEND OF THE 12Z UKMET AND 12Z ECMWF...ALOFT W/TROUGH  
CONFIDENCE: SLIGHTLY BELOW AVERAGE  
 
THE MODELS BREAK DOWN THE WESTERN NORTH AMERICA RIDGE BY THE END  
OF THE WEEK AND THIS ALLOWS A SHORTWAVE TROUGH AND ASSOCIATED COLD  
FRONT TO ARRIVE ACROSS THE PACIFIC NORTHWEST BY THE END OF THE  
WEEK. AS ALL OF THE ENERGY INITIALLY COMES INTO THE PACIFIC  
NORTHWEST...THE 12Z ECMWF IS GENERALLY THE SLOWEST WITH THE  
SHORTWAVE TROUGH...WITH THE 12Z NAM THE MOST PROGRESSIVE OVERALL  
FOLLOWED BY THE 12Z UKMET. THE 12Z GFS AND 12Z CMC SPLIT THE  
DIFFERENCE...BUT THE MODEL SPREAD WITH THE INITIAL ARRIVAL OF THE  
HEIGHT FALLS IS RATHER MINIMAL.  
 
ALL OF THIS ENERGY ALONG WITH A COLD FRONT WILL ADVANCE SOUTHEAST  
ACROSS THE INTERMOUNTAIN WEST AND THEN OUT ACROSS THE PLAINS AND  
MIDWEST FRIDAY NIGHT THROUGH SATURDAY. BY SATURDAY...THE 12Z NAM  
HAS ENERGY SHIFTING TOO QUICKLY OFF TO THE EAST ACROSS THE  
NORTHERN PLAINS WHICH ALLOWS THE NORTHERN PORTION OF A COLD FRONT  
AND A SURFACE WAVE TO ADVANCE MORE QUICKLY TOWARD THE UPPER MS  
VALLEY. THE REMAINING GLOBAL MODELS ARE ALL CLUSTERED TOWARD A  
SLOWER SOLUTION AND HAVE MUCH OF THE UPPER TROUGHING BACK OVER THE  
NORTHERN HIGH PLAINS AND DOWN ACROSS THE FOUR CORNERS REGION.  
HOWEVER...THE 12Z GFS IS THE SLOWEST SOLUTION WITH MUCH MORE  
ENERGY LINGERING BACK OVER THE GREAT BASIN.  
 
REGARDING THE FRONT AND A SURFACE LOW OVER THE SOUTHERN HIGH  
PLAINS ON SATURDAY...THE NAM IS FARTHEST SOUTH WITH THE  
FRONT...WITH THE CMC THE FARTHEST NORTH. THE UKMET ALSO TENDS TO  
BE A LITTLE FARTHER NORTH AND WEST WITH AT LEAST THE NORTHERN  
PORTION OF THE FRONT. THE GFS AND ECMWF SHOW MUCH BETTER  
CLUSTERING BY COMPARISON AND ARE CLOSE TO THE MODEL CONSENSUS. THE  
12Z GEFS MEAN AND 00Z ECENS MEAN SUPPORT THE MODEL CONSENSUS WITH  
THE GFS/ECMWF CLUSTER AT THE SURFACE. HOWEVER...ALOFT WITH THE  
UPPER TROUGH...THE GFS APPEARS TO HAVE TOO MUCH ENERGY TOO FAR  
BACK TO THE WEST...AND THE LATEST 12Z GEFS MEAN AND THE 00Z ECENS  
MEAN FAVOR THE RELATIVELY WELL CLUSTERED UKMET AND ECMWF WITH  
THIS. SO THEREFORE WILL FAVOR A GFS/ECMWF BLEND AT THE SURFACE  
WITH THE FRONT AND SURFACE WAVE ACTIVITY...BUT A BLEND OF THE  
UKMET AND ECMWF ALOFT WITH THE HEIGHT FALL EVOLUTION.  
 
   
..UPPER LEVEL LOW OVER THE BAJA PENINSULA/GULF OF CALIFORNIA
 
   
..PORTION OF ENERGY LIFTING TOWARD THE RIO GRANDE VALLEY
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: BLEND OF THE 12Z GFS AND 12Z ECMWF  
CONFIDENCE: AVERAGE  
 
A WEAK SHORTWAVE DROPPING SOUTH DOWN THE SPINE OF THE ROCKIES  
TODAY WILL TEND TO MERGE UP WITH THE UPPER LOW CURRENTLY OVER THE  
BAJA PENINSULA ON THURSDAY...WITH THE RESULTANT PHASED ENERGY THEN  
RESULTING IN NEW CLOSED LOW EVOLUTION THAT DROPS SOUTHEAST ACROSS  
WESTERN MAINLAND MEXICO BEFORE THEN LIFTING NORTHEAST TOWARD THE  
RIO GRANDE VALLEY BY SATURDAY IN RESPONSE TO UPSTREAM HEIGHT FALLS  
DROPPING INTO THE SOUTHWEST U.S. AND ADJACENT FOUR CORNERS REGION.  
12Z NAM IS A FAST OUTLIER WITH THE ENERGY EJECTING TOWARD THE RIO  
GRANDE VALLEY ON SATURDAY. THE 12Z UKMET HAS TRENDED SLOWER AND IS  
ONLY A LITTLE MORE PROGRESSIVE THAN THE WELL CLUSTERED 12Z GFS/12Z  
ECMWF AND 12Z CMC. GIVEN STRONG ENSEMBLE SUPPORT AWAY FROM THE NAM  
AND TOWARD THE GFS/ECMWF CLUSTER...A GFS/ECMWF BLEND WILL AGAIN BE  
PREFERRED.  
 
...UPPER LOW DEVELOPMENT OVER CALIFORNIA ON SAT..   
..ENERGY DIGGING TOWARD THE NORTHERN BAJA PENINSULA
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 12Z NAM  
CONFIDENCE: BELOW AVERAGE  
 
THE MODELS TAKE A PORTION OF THE UPPER TROUGH CROSSING THE PACIFIC  
NORTHWEST ON FRIDAY AND DROP IT SOUTH DOWN ACROSS CALIFORNIA ON  
SATURDAY WHERE MOST OF THE MODELS SHOW IT AMPLIFYING AND  
SEPARATING OUT INTO A COMPACT CLOSED LOW THAT ARRIVES TOWARD  
SOUTHERN CALIFORNIA BY LAT SATURDAY. THE 12Z GFS AND 12Z CMC ARE  
STRONG OUTLIERS WITH THIS SYSTEM...WITH THE CMC ALSO THE FARTHEST  
EAST WITH THE ENERGY. THE 12Z UKMET TRENDED FARTHER WEST WITH ITS  
UPPER LOW...BUT IS STILL EAST OF THE 12Z ECMWF WHICH TAKES ITS  
UPPER LOW MUCH FARTHER WEST AND OFFSHORE CALIFORNIA. THE 12Z NAM  
SPLITS THE DIFFERENCE BETWEEN THE UKMET/ECMWF CAMP TO THE WEST AND  
THE GFS/CMC CAMP TO THE EAST. THE NAM IS NOT NEARLY AS STRONG AS  
THE GFS AND CMC AND APPEARS REASONABLE AS A COMPROMISE SOLUTION  
FOR NOW.  
 
MODEL TRENDS AT WWW.WPC.NCEP.NOAA.GOV/HTML/MODEL2.SHTML  
500 MB FORECASTS AT WWW.WPC.NCEP.NOAA.GOV/H5PREF/H5PREF.SHTML  
 
ORRISON  
 

 
 
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