736  
FXUS10 KWNH 281856  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
256 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 FINAL PREFERENCES AND CONFIDENCE  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
 
   
..TROPICAL DEPRESSION EIGHTEEN
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
NHC BEST PROXY: 12Z ECMWF AND 12Z NAM  
PLEASE REFER TO NHC FORECASTS  
 
THE 15Z ADVISORY FROM THE NATIONAL HURRICANE CENTER SHOWS TROPICAL  
DEPRESSION EIGHTEEN MOVING NEAR THE FLORIDA STRAITS LATER TODAY  
AND TONIGHT...AND THEN ACCELERATING QUICKLY NORTHEAST. THE TRACK  
DEPICTED BY THE NHC FORECAST IS GENERALLY BEST APPROXIMATED BY THE  
12Z NAM AND 12Z ECMWF. THE NHC FORECAST DOES SHOW THE STORM  
BECOMING EXTRATROPICAL AND MERGING WITH THE FRONTAL SYSTEM BETWEEN  
36 AND 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 ECMWF / 12Z GFS  
SLIGHT WEIGHT TOWARD THE 12Z ECMWF  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
TIMING DIFFERENCES WITH THE KICKER SHORTWAVE AS IT DIGS INTO THE  
BASE OF THE TROUGH HAVE NARROWED FROM THE 00Z MODEL CYCLE. THE  
ECMWF AND GFS PREVIOUSLY SHOWED A FASTER PROGRESSION OF THE WAVE.  
THE 12Z GFS REMAINS THE FASTEST...WHEREAS THE 12Z ECMWF TRENDED A  
BIT SLOWER (ALTHOUGH STILL FASTER THAN THE OTHER GLOBAL MODELS).  
MEANWHILE...THE 12Z CMC AND 12Z UKMET TRENDED A BIT FASTER WHILE  
THE 12Z NAM REMAINS THE SLOWEST. THE RESULT HAS BEEN A NARROWING  
OF MODEL SPREAD WITH THE TIMING OF THE WAVE...AND AS A RESULT THE  
12Z MODEL SOLUTIONS ARE QUITE SIMILAR WITH THE ENTIRE CYCLOGENESIS  
PROCESS AND STRENGTH/TRACK OF THE RESULTING SURFACE LOW.  
 
ENSEMBLE SENSITIVITY ANALYSIS OF THE CYCLONE FORECAST AT 30/00Z  
SHOWS MOST OF THE REMAINING VARIABILITY IS DUE TO INTENSITY  
DIFFERENCES WITH THE LOW...AND THEN WITH TIMING DIFFERENCES. THE  
INTENSITY OF THE LOW APPEARS TO BE SOMEWHAT SENSITIVE TO LOWER  
SURFACE PRESSURES DEVELOPING IN THE FAR SOUTHEASTERN GULF OF  
MEXICO AND SOUTH FLORIDA IN THE FIRST 12-24 HOURS OF THE FORECAST.  
THERE ARE SOME SIGNS OF THIS HAPPENING...WITH A FAIRLY ROBUST AREA  
OF CONVECTION OBSERVED BY SATELLITES AND RADAR AND PERHAPS A  
SECONDARY LOW. KEY WEST RADAR VELOCITY DID SHOW A MID-LEVEL  
CIRCULATION AS OF 18Z. THEREFORE...THE PREFERENCE IS WEIGHTED  
TOWARD MODEL SOLUTIONS THAT SHOW A STRONGER FRONTAL WAVE THAT  
GROWS INTO THE EVENTUAL CYCLONE...OR EVEN A SECONDARY SURFACE LOW  
CLOSER TO THE EAST COAST PRIOR TO THE RAPID CYCLOGENESIS. THE  
DEEPER SOLUTIONS ARE THE 12Z GFS AND 12Z ECMWF...WITH THE ECMWF  
SHOWING A MORE OBVIOUS STRONGER FRONTAL WAVE / SECONDARY LOW NORTH  
OF T.D. 18.  
 
THE TIMING DIFFERENCES OF THE LOW IN THE NORTHEAST APPEAR TO BE  
SENSITIVE TO THE SPEED OF THE DIGGING WAVE AS IT ROUNDS THE BASE  
OF THE TROUGH...WITH MODELS DEPICTING A SLOWER SHORTWAVE ALSO  
SLOWER WITH THE APPROACH OF THE LOW IN THE NORTHEAST. THE  
PREFERENCE HERE IS ALSO TOWARD THE FASTER SOLUTIONS. THIS  
MAINTAINS CONSISTENCY WITH MODEL PREFERENCES OVER THE PAST COUPLE  
CYCLES...AND ALSO HAS SUPPORT FROM THE ECMWF AND GEFS ENSEMBLES.  
 
...SHORTWAVE DIGGING INTO THE NORTHERN PLAINS BY SUNDAY AND  
PHASING WITH EAST COAST CYCLONE INTO MONDAY AND TUESDAY...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: NON-NAM BLEND  
CONFIDENCE: ABOVE AVERAGE  
 
THE 12Z NAM SHOWS DIFFERENCES IN THE BASE OF THE TROUGH...WITH A  
MUCH STRONGER SECONDARY VORT MAX...AND IT MAINTAINS THIS FEATURE  
AND SLIGHTLY MORE AMPLIFIED WAVE ALL THE WAY INTO THE CENTRAL  
PLAINS. THIS IS DIFFERENT FROM THE OVERALL STRUCTURE SHOWN BY THE  
GLOBAL MODELS...SO A NON-NAM BLEND IS PREFERRED. OTHERWISE...THERE  
ARE SOME SLIGHT DIFFERENCES BETWEEN THE REMAINING GLOBAL  
MODELS...BUT THEY CONTINUE TO SHOW GOOD OVERALL AGREEMENT WITH  
MANY OF THE MASS FIELDS.  
 
...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.  
 
WWW.WPC.NCEP.NOAA.GOV/HTML/MODEL2.SHTML  
WWW.WPC.NCEP.NOAA.GOV/H5PREF/H5PREF.SHTML  
 
LAMERS  
 

 
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab HPC Page Main Text Page