273  
FXUS10 KWNH 211857  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
156 PM 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 MODEL EVALUATION WITH FINAL 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: 12Z ECMWF / 12Z GFS / 12Z 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 12Z 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 12Z CMC DO NOT FIT WELL TO THE LATEST  
ENSEMBLE SPAGHETTI HEIGHT CHARTS AND THE 12Z ECMWF/GFS AND NOW  
THE 12Z UKMET SHOW TOWARD THE DEEPER SIDE. GIVEN TRENDS AND THE  
LATEST ENSEMBLE CLUSTERING...A BLEND OF THE 12Z GFS/ECMWF/UKMET  
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/ECMWF/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 NAM AS A NEAR OUTLIER WITH THE  
DEPTH/PLACEMENT OF THE TROUGH AXIS BY FRI AFTERNOON AND THE 12Z  
CMC AS MUCH FASTER THROUGH FRI. A BLEND OF THE 12Z GFS...12Z  
ECMWF...12Z UKMET IS CLOSEST TO THE ENSEMBLE CONSENSUS THROUGH  
00Z/25.  
 
...BROAD MID-LEVEL TROUGH AXIS CLIPPING THE GREAT LAKES AND  
NORTHEAST THU/FRI...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: ABOVE AVERAGE  
 
THE MODELS SHOW SIMILARLY WITH THIS SYSTEM.  
 
   
..SLOW MOVING MID-UPPER LEVEL TROUGH ALONG THE GULF COAST
 
   
..SURFACE LOWS IN THE EASTERN GULF OF MEXICO/NEAR FLORIDA
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: NON 12Z NAM 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. 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 BUT THE 12Z UKMET/CMC  
TRENDED AWAY FROM THE COAST INTO THE BETTER ENSEMBLE LOW  
CLUSTERING. THE 12Z ECMWF TRENDED SLOWER LIKE THE 12Z NAM BUT IT  
IS NOT AS DEEP. THEREFORE...A NON 12Z NAM BLEND APPEARS REASONABLE  
FOR THIS SYSTEM BUT DETAIL DIFFERENCES REMAIN LARGER. FOR QPF  
CONSIDERATIONS...SEE OUR QUANTITATIVE PRECIPITATION DISCUSSION  
(QPFPFD).  
 
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OTTO  
 

 
 
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