660  
FXUS10 KWNH 181841  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
140 PM 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 MODEL EVALUATION...WITH FINAL 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 12Z ECMWF/UKMET ARE A BIT STRONGER WITH THE 500 MB  
CORE OF THE MID-LEVEL LOW TUE NIGHT WHILE THE 12Z GFS IS SLIGHTLY  
WEAKER. THE 12Z ECMWF/UKMET/CMC ARE ALSO A BIT FARTHER SOUTH WITH  
THE MAIN VORTICITY MAX CROSSING THE MS VALLEY AND TOWARD THE EAST  
COAST. OVERALL...A GENERAL MODEL BLEND IS RECOMMENDED TO IRON OUT  
THESE MINOR SMALLER SCALE DIFFERENCES.  
 
NO SIGNIFICANT DIFFERENCES WERE NOTED WITH THE 12Z ECMWF/UKMET/CMC  
COMPARED TO THEIR 00Z CYCLES REGARDING THIS SYSTEM.  
 
...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.  
 
NO SIGNIFICANT DIFFERENCES WERE NOTED WITH THE 12Z ECMWF/UKMET/CMC  
COMPARED TO THEIR 00Z CYCLES REGARDING THIS SYSTEM.  
 
...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 12Z 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 12Z 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/GFS/ECMWF BY 00Z/22 WITH THE  
12Z UKMET/CMC FARTHER BACK TO THE WEST. NO SIGNIFICANT DIFFERENCES  
WERE NOTED WITH THE 12Z ECMWF/UKMET/CMC COMPARED TO THEIR 00Z  
CYCLES REGARDING THIS SYSTEM.  
 
...UPPER TROUGH AXIS NEARING THE PACIFIC NORTHWEST FROM THE NORTH  
ON THU...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: NON 12Z UKMET BLEND  
CONFIDENCE: AVERAGE  
 
THE ENSEMBLE MEANS SUPPORT A TIMING CLOSER TO THAT OF THE  
REASONABLY AGREEABLE NON 12Z UKMET GUIDANCE WITH THE UKMET A BIT  
FASTER THAN THE MEANS VALID 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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