779  
FXUS10 KWNH 211651  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1150 AM EST TUE NOV 21 2017  
 
VALID NOV 21/1200 UTC THRU NOV 25/0000 UTC  
 
...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR  
INGEST...  
 
12Z NAM/GFS EVALUATION WITH PRELIMINARY PREFERENCES  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
NAM/GFS INITIALIZATION ERRORS DO NOT SEEM TO DEGRADE THEIR SHORT  
RANGE FORECASTS.  
 
   
..UPPER TROUGH AXIS CURRENTLY OVER THE GREAT LAKES
 
MOVING TO  
THE NORTHWEST ON WED...  
...SURFACE LOWS TRACKING ALONG AND OFFSHORE OF THE NEW ENGLAND  
COAST ON WED...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 00Z ECMWF / 12Z GFS / 00Z UKMET BLEND  
CONFIDENCE: ABOVE AVERAGE  
 
ENSEMBLE SCATTER LOW PLOT TRENDS HAVE BEEN TO SHIFT THE GULF OF  
MAINE LOW ALONG AT 18Z/22 - 00Z/23 TOWARD THE SOUTH WITH THE  
LATEST MODEL CONSENSUS APPEARING IN REASONABLE AGREEMENT WITH THIS  
FIRST LOW.  
 
A SECOND LOW SHOWS A BIT MORE SPREAD...FORECAST TO TRACK EAST OF  
THE 40N/70W BENCHMARK LATE WED. THE MORE PROGRESSIVE UPPER TROUGH  
SHOWN IN THE 12Z NAM AND 00Z CMC DO NOT FIT WELL TO THE LATEST  
ENSEMBLE SPAGHETTI HEIGHT CHARTS AND THE 00Z ECMWF/12Z GFS SHOW  
TOWARD THE DEEPER SIDE. GIVEN TRENDS AND THE LATEST ENSEMBLE  
CLUSTERING...A BLEND OF THE SLIGHTLY MORE PROGRESSIVE 00Z UKMET  
WITH THE MORE WOUND UP 12Z GFS AND 00Z ECMWF APPEARS BEST AT THIS  
TIME.  
 
...UPPER TROUGH AXIS CROSSING THE PACIFIC NORTHWEST ON THU AND  
REACHING THE N-CNTRL U.S. BY LATE FRI...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 12Z GFS...00Z ECMWF...00Z CMC BLEND  
CONFIDENCE: AVERAGE  
 
DIFFERENCES WITH THIS SYSTEM ARISE FROM HOW THE MODELS HANDLE  
INTERACTIONS WITH SHORTWAVE ENERGY TRACKING NORTH OF THE MAIN  
TROUGH AXIS. ENSEMBLES SHOW THE 00Z UKMET AND 12Z NAM AS NEAR  
OUTLIERS WITH THE DEPTH/PLACEMENT OF THE TROUGH AXIS BY FRI  
AFTERNOON...WITH A BLEND OF THE 12Z GFS...00Z ECMWF...00Z CMC  
CLOSEST TO THE ENSEMBLE CONSENSUS.  
 
...BROAD MID-LEVEL TROUGH AXIS CLIPPING THE GREAT LAKES AND  
NORTHEAST THU/FRI...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: ABOVE AVERAGE  
 
THE TWO SYSTEMS EMBEDDED WITHIN THIS TROUGH SHOW GOOD AGREEMENT  
OVER THE NEXT FEW DAYS. A COMPROMISE OF THE 00Z GFS, 00Z NAM, 00Z  
ECMWF, 00Z UKMET, AND 00Z CANADIAN IS FAVORED WITH ABOVE AVERAGE  
CONFIDENCE.  
 
   
..SLOW MOVING MID-UPPER LEVEL TROUGH ALONG THE GULF COAST
 
   
..SURFACE LOWS IN THE EASTERN GULF OF MEXICO/NEAR FLORIDA
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 12Z GFS...00Z ECMWF BLEND  
CONFIDENCE: SLIGHTLY BELOW AVERAGE  
 
STARTING THURSDAY MORNING...THE NAM STARTS DIVERGING FROM THE  
OTHER GUIDANCE IN BECOMING DEEPER/SLOWER ALOFT WITH THIS SYSTEM.  
ENSEMBLE SPREAD IS FAIRLY LARGE WHICH LOWERS CONFIDENCE A BIT  
LOWER THAN NORMAL...BUT THE LATEST ENSEMBLE SCATTER LOW PLOTS SHOW  
THE 00Z UKMET/CMC ON THE WESTERN SIDE OF THE SPREAD WITH THE  
SURFACE LOW AS IT NEARS THE SOUTHEAST COAST ON FRI. THE 00Z ECMWF  
AND 12Z GFS APPEAR LIKE THE MOST REASONABLE BLEND FOR THIS SYSTEM  
AT THIS TIME. FOR QPF CONSIDERATIONS...SEE OUR QUANTITATIVE  
PRECIPITATION DISCUSSION (QPFPFD).  
 
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WWW.WPC.NCEP.NOAA.GOV/H5PREF/H5PREF.SHTML  
 
OTTO  
 

 
 
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