401  
FXUS10 KWNH 181647  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1147 AM EST MON DEC 18 2017  
 
VALID DEC 18/1200 UTC THRU DEC 22/0000 UTC  
 
...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR  
INGEST...  
 
12Z NAM/GFS EVALUATION...WITH PRELIMINARY PREFERENCES  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
 
...CLOSED LOW CURRENTLY OVER THE DESERT SOUTHWEST/NW  
MEXICO...EJECTING OUT ACROSS THE SOUTHERN PLAINS ON TUE AND  
WEAKENING ACROSS THE EAST COAST LATE WED...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
ONLY MINOR DIFFERENCES IN TIMING/AMPLITUDE ARE SEEN WITH THIS  
FEATURE. THE 00Z ECMWF IS A BIT STRONGER WITH THE 500 MB CORE OF  
THE MID-LEVEL LOW WHILE THE 12Z GFS IS SLIGHTLY WEAKER AND THE 00Z  
UKMET/CMC A BIT FARTHER SOUTH WITH THE MAIN VORTICITY MAX CROSSING  
THE MS VALLEY. OVERALL...A GENERAL MODEL BLEND IS RECOMMENDED TO  
IRON OUT THESE MINOR SMALLER SCALE DIFFERENCES.  
 
...BROAD/LOW AMPLITUDE MID-UPPER LEVEL TROUGH CROSSING THE GREAT  
LAKES/NORTHEAST TUE/WED...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: ABOVE AVERAGE  
 
THE 12Z NAM IS ON THE DEEPER SIDE WITH THE MID-LEVEL CORE OF THE  
TROUGH EARLY WED MORNING AS THE SYSTEM BEGINS TO EXIT ACROSS THE  
CANADIAN MARITIMES. HOWEVER...DIFFERENCES ACROSS THE NORTHEAST  
U.S. ARE MINOR SO A GENERAL MODEL BLEND IS RECOMMENDED HERE.  
 
...UPPER TROUGH AXIS CROSSING THE PACIFIC NORTHWEST TUE NIGHT AND  
REACHING THE FOUR CORNERS ON THU...   
..SURFACE CYCLONE CROSSING THE CENTRAL PLAINS ON THU
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: BLEND OF THE 00Z ECMWF/UKMET  
CONFIDENCE: SLIGHTLY BELOW AVERAGE  
 
THERE IS A FAIR AMOUNT OF SPREAD IN THE LATEST ENSEMBLE GUIDANCE  
AND WHILE TRENDS SHOW A LESSENING IN THAT SPREAD...DIFFERENCES ARE  
STILL SIGNIFICANT WHICH LOWERS CONFIDENCE.  
 
THE 12Z NAM IS UNIQUE REGARDING THE LATEST DETERMINISTIC MODELS  
WITH MORE OF AN OPEN...POSITIVELY TILTED TROUGH AXIS FROM THE FOUR  
CORNERS INTO THE N-CNTRL U.S. BY 00Z/22 WHEREAS THE REMAINING  
DETERMINISTIC GUIDANCE SHOWS MORE ENERGY HANGING BACK TO THE  
SOUTHWEST. INITIALLY ZONAL FLOW OVER THE CONUS MAY FAVOR A FASTER  
SOLUTION LIKE THE 12Z GFS...AND THE ENSEMBLE SPAGHETTI PLOTS SHOW  
THE GFS AS REASONABLE...BUT THE LATEST ENSEMBLE MEANS ARE CLOSER  
TO A BLEND OF THE 00Z ECMWF/UKMET. GIVEN AGREEMENT TO THE ENSEMBLE  
MEANS AND NO DISCERNIBLE SLOWER/FASTER TREND IN THE GUIDANCE  
CONCERNING PROGRESSION OF THE TROUGH AXIS...STAYING CLOSEST TO THE  
ENSEMBLE MEANS IS CONSIDERED BEST FOR THIS SYSTEM.  
 
REGARDING THE SURFACE LOW...THE LATEST ENSEMBLE SCATTER LOW PLOTS  
SUPPORT A BLEND BETWEEN THE 12Z NAM/12Z GFS AND 00Z ECMWF BY  
00Z/22 WITH THE 00Z UKMET/CMC FARTHER BACK TO THE WEST.  
 
...UPPER TROUGH AXIS NEARING THE PACIFIC NORTHWEST FROM THE NORTH  
ON THU...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 12Z NAM...12Z GFS...00Z ECMWF BLEND  
CONFIDENCE: AVERAGE  
 
THE ENSEMBLE MEANS SUPPORT A TIMING CLOSER TO THAT OF THE 12Z  
NAM/12Z GFS/00Z ECMWF WITH THE 00Z UKMET/CMC FASTER TO BRING THE  
UPPER TROUGH INTO THE PACIFIC NORTHWEST LATE THU.  
 
MODEL TRENDS AT WWW.WPC.NCEP.NOAA.GOV/HTML/MODEL2.SHTML  
500 MB FORECASTS AT WWW.WPC.NCEP.NOAA.GOV/H5PREF/H5PREF.SHTML  
 
OTTO  
 

 
 
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