250  
FXUS10 KWNH 270414  
PMDHMD  
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1214 AM EDT FRI OCT 27 2017  
 
VALID OCT 27/0000 UTC THRU OCT 30/1200 UTC  
 
...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR  
INGEST...  
 
00Z MODEL EVALUATION...WITH PREFERENCES AND CONFIDENCE  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
   
..SURFACE LOW IN MAINE LIFTING INTO EASTERN CANADA TODAY
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: ABOVE AVERAGE  
 
MASS FIELDS APPEAR TIGHT ENOUGH THROUGH THE MID/LOWER ATMOSPHERE  
TO SUPPORT A GENERAL MODEL BLEND AT ABOVE AVERAGE CONFIDENCE AS  
THE LOW SLIDES OUT OF MAINE INTO SOUTHEAST CANADA BY THE END OF  
TODAY.  
 
 
...SHORTWAVE TROUGH DIGGING INTO THE WESTERN GREAT LAKES WITH  
CLOSED LOW DEVELOPMENT AND CYCLONE OCCLUSION BY FRIDAY...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: ABOVE AVERAGE  
 
MODEL CONVERGENCE TO A COMMON SOLUTION CONTINUES EVEN AS THE  
OCCLUDED LOW TRANSITIONS TOWARD THE TRIPLE POINT LOW DEEPENING AND  
ACCELERATING DUE NORTH TOWARD JAMES BAY BE SUNDAY. THE 00Z NAM  
WAS THE GREATEST SHIFT BUT NOW SOLIDLY FITS INTO THE CLUSTER WELL  
ENOUGH AND THE 00Z GFS SHOWS LITTLE CHANGE. MINOR DIFFERENCES IN  
SPACING EXIST WITH THE APPROACHING DEEPER SHORTWAVE/BASE OF THE  
TROF THROUGH THE OHIO/TN VALLEY...AS THE 00Z NAM/GFS SHOW CLOSER  
SPACING COMPARED TO THE NON-NCEP SOLUTION BUT THIS IS VERY MINOR  
TOWARD THE EVOLUTION TO THE SYSTEM ACROSS THE GREAT LAKES TO  
DEVIATE FROM GENERAL MODEL PREFERENCE.  
 
...TROPICAL WAVE/DISTURBANCE CURRENTLY IN THE WESTERN CARIBBEAN  
AND APPROACHING THE FLORIDA STRAITS BY THIS WEEKEND...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 00Z NAM/GFS AND 12Z ECMWF BLEND  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
LARGE CONVECTIVE CLUSTER CURRENTLY EXISTS OVER NORTHEAST  
HONDURAS...WITH THE MEAN TROPICAL WAVE FURTHER WEST. THIS FAVORS  
SOLUTIONS INITIALIZING FURTHER WEST OFF THE N. HONDURAN COAST.  
THE 12Z UKMET IS FURTHEST EAST AND DOES NOT APPEAR TO SHOW THE  
LIMITING EFFECTS OF THE COASTAL INTERACTION AS IT CONTINUES TO  
DEEPEN THE SYSTEM GREATER/EARLIER AND FURTHER EAST THAN THE BULK  
OF GUIDANCE INCLUDING ENSEMBLE SOLUTIONS. THE 12Z CMC WHILE  
INITIALLY SETTING UP WELL NEAR HONDURAS...LIFTS NORTHWARD WITH A  
SIGNIFICANT DEEPENING AS WELL SHOWING SOME SYMMETRIC EVOLUTION  
THAT SLOWS ITS NORTHEASTWARD PROGRESSION ALLOWING THE BULK OF  
NORTHERN STREAM ENERGY TO SLIP BY. THE 00Z GFS LIKE RUNS BEFORE  
IT CONTINUE TO SHOW THE GREATEST SHEARING EFFECTS WITH THE  
GREATEST DRAW NORTHWARD EVENTUALLY LEADING TO AN ELONGATED TROF  
FROM THE FLORIDA STRAITS THROUGH THE EAST COAST OF FL/GA BY  
SUN...LIKELY DUE TO A SLIGHTLY FASTER TRANSITION OF THE NEGATIVE  
TILT TROF AFFECTING THE EASTERN US BY SUN (PLEASE SEE SECTION  
BELOW). THE 00Z NAM IS A BIT DEEPER THAN THE 12Z ECMWF BUT THIS  
APPEARS TO BE MORE RELATED TO THE GRID SCALE THAN OTHER REASONS  
AND BOTH HAVE A SIMILAR EVOLUTION IN LIFTING OUT OF THE NW  
CARIBBEAN AS WELL AS ITS INTERACTION/SHEARING WITH THE NORTHERN  
STREAM TROF. A BLEND OF THE 00Z NAM/GFS AND 12Z ECMWF APPEAR TO  
BE MOST CONSISTENT WITH THE ENSEMBLES ESPECIALLY THROUGH SAT/EARLY  
SUNDAY...SHADING AWAY FROM THE GFS MIDDAY SUNDAY SEEMS PRUDENT TO  
MELD WITH PREFERENCES ELSEWHERE.  
 
...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: 12Z ECMWF/00Z NAM BLEND  
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 00Z GFS LIKE THE 18Z RUN BEFORE IT CONTINUE TO ENTER THE BASE  
OF THE TROF TIGHT TO THE EXITING SHORTWAVE IN THE GREAT  
LAKES...THIS DOES NOT ALLOW FOR A SOUTHWARD DIG...AND THEREFORE  
ACCELERATES AROUND THE BASE OF THE TROF FURTHER NORTH/FASTER WITH  
LESS OF A NEGATIVE TILT. CONSEQUENTLY THIS DRAWS A MUCH WEAKER  
AND MUCH FURTHER EAST CYCLONE UP THE COAST TOWARD NEW ENGLAND BY  
LATE SUN/EARLY MON. THIS EVOLUTION CONTINUES TO BREAK FROM PRIOR  
GUIDANCE AND TREND AWAY FROM THE DEEPER MORE NEGATIVE TILT  
SOLUTIONS PRESENTED ELSEWHERE. THE LONE "SUPPORTER" OF THE GFS  
IS THE 12Z CMC THOUGH SHOWS A DEEPER/WOUND UP SOLUTION...THE  
OVERALL ORIENTATION OF THE TROF DOES NOT SUPPORT A BOMB CYCLONE AS  
THE ORIENTATION IS MORE NEUTRAL. THIS ALLOWS FOR THE CMC TO BE  
UNCHARACTERISTICALLY FAST.  
 
THE FLIP SIDE TO THIS IS THE 12Z UKMET SHOWING THE GREATEST  
NEGATIVE TILT THOUGH ALSO A VERY NARROW TROF THROUGH THE OHIO  
VALLEY AND LIKELY A BIT GREATER INTERACTION WITH THE TROPICAL  
SYSTEM (WHICH IS MUCH TOO AMPLIFIED SEE SECTION ABOVE). AS SUCH  
THE UKMET IS NOT CONSIDERED. THE 12Z ECENS MEAN HAS BEEN QUITE  
STEADFAST AND REALISTIC WITH ONLY MODEST SPREAD MAINLY IN TIMING  
OF THE TROF TO HAVE SOME SOLID CONFIDENCE IN IT. THE PROBLEM IS  
THE 12Z ECMWF OPERATIONAL RUN IS VERY STRONG AND MUCH FURTHER WEST  
SHOWING SOME OF THE NEGATIVE BIAS THAT HAS BEEN MANIFESTING OVER  
THE LAST YEAR OR TWO COMPARED TO ITS MEAN... THE 00Z NAM IS A BIT  
FURTHER EAST AND FASTER AND WHILE WOUND UP DUE TO A SOLID NEGATIVE  
TILT TROF ORIENTATION AND BAROCLINIC FORCING AIDED BY TROPICAL  
MOISTURE/LATENT HEAT RELEASE...WOULD COMPENSATE SOME OF THE  
NEGATIVE ATTRIBUTES OF THE 12Z ECMWF TO COME TO A BLEND IN CLOSE  
PROXIMITY TO THE 12Z ECENS MEAN. AS SUCH THIS WILL BE WPC  
PREFERENCE THOUGH SPREAD/IMPORTANCE OF SMALL TIMING DIFFERENCES TO  
KEY PIECES OF ENERGY/MOISTURE... CONFIDENCE IS ONLY AVERAGE IN  
THIS BLEND GIVEN LIKELY RUN TO RUN VARIATIONS TO CONTINUE OVER THE  
NEXT DAY OR SO.  
 
...SOUTHERN AK SHORTWAVE ROUNDING THE TOP OF THE RIDGE AND DIGGING  
INTO THE NORTHERN PLAINS BY SUNDAY...   
..ASSOCIATED SURFACE LOW AND COLD FRONT
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 2/3RDS 12Z ECMWF & 1/3 BLEND OF 00Z NAM/12Z UKMET  
CONFIDENCE: AVERAGE  
 
THE 00Z NAM TRENDED FAVORABLY TOWARD A SLIGHTLY FASTER CONSENSUS  
TRACK...HOWEVER REMAINS A BIT DEEPER AND WOUND UP THROUGH THE  
SOUTHERN CANADIAN PRAIRIES BY LATE SUN/EARLY MONDAY...ALSO A BIT  
NORTH OF THE ENSEMBLE MEANS. HOWEVER THIS IS A MUCH MORE  
FAVORABLE SET UP COMPARED TO THE ENSEMBLES THAN THE  
UNCHARACTERISTICALLY FAST 12Z CMC THAT PRESSES LEAD HEIGHT FALLS  
AT LEAST 6HRS FASTER INTO LP OF MI BY 12Z MONDAY. THE 12Z UKMET  
SEEMS TO FAVOR THE 00Z NAM EVOLUTION HOWEVER ALSO SHIFTS LEAD  
HEIGHT FALLS INTO THE GREAT LAKES A BIT QUICKER THAN THE  
NAM...MORE IN LINE TIMING WITH GFS/ECMWF. THE GFS/GEFS HAVE BEEN  
ON THE SOUTHERN EDGE OF THE ENSEMBLE SUITE WITH RESPECT TO THE  
SURFACE WAVE HOWEVER...SHOW A QUITE SIMILAR EVOLUTION WITH THE  
ECMWF/ECENS MEAN. THE 00Z GFS LIKELY DUE TO DOWNSTREAM FASTER  
TREND...ALSO SHIFTED SLIGHTLY FASTER WHICH MANIFESTS BY DIGGING  
SOUTH A BIT MORE AND THEREFORE FURTHER EXTENDS ITS ENSEMBLE TREND  
ON THE SOUTHERN SIDE OF THE GUIDANCE THOUGH STILL NOT AS FAST AS  
THE CMC. AS SUCH WILL HEAVILY WEIGHT THE PREFERENCE 2/3RDS ECMWF  
AND 1/3 BLEND OF THE 12Z UKMET/00Z NAM. CONFIDENCE IS AVERAGE.  
 
...BREAKDOWN OF WESTERN UPPER RIDGE AND MID-UPPER LEVEL LOW  
APPROACHING CALIFORNIA COAST BY SUNDAY...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: ABOVE AVERAGE  
 
THE 00Z NAM SHIFTED A BIT DEEPER/MORE CONCENTRIC AND THEREFORE A  
BIT FURTHER SOUTH THAN THE PRIOR RUN. THIS MATCHES WITH THE PRIOR  
LONE "OUTLIER" IN THE 12Z UKMET. THE 00Z GFS REMAINED QUITE  
SIMILAR A BIT WEAKER AND FURTHER NORTH NEARER THE OLDER RUNS AND  
THE 12Z ECMWF/CMC. HOWEVER...THE DISTANCE BETWEEN THE 12Z  
UKMET/00Z NAM AND THE 12Z ECMWF/CMC/00Z GFS IS NOT GREAT AND GIVEN  
THE OVERALL EVOLUTION IS QUITE SIMILAR WITH SIMILAR MASS/SENSIBLE  
WX FIELDS WILL CONTINUE TO SUPPORT A GENERAL MODEL BLEND AT ABOVE  
AVERAGE CONFIDENCE. =  
 
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