465  
FXUS10 KWNH 181631  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1130 AM EST SUN FEB 18 2018  
 
VALID FEB 18/1200 UTC THRU FEB 22/0000 UTC  
   
..SEE NOUS42 KWNO (ADMNFD) FOR STATUS OF THE UPPER AIR INGEST
 
 
   
..12Z MODEL EVALUATION WITH PRELIMINARY PREFERENCES/CONFIDENCE
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
 
...AMPLIFIED WESTERN US TROUGH GRADUALLY BUILDING INTO THE CENTRAL  
US WITH SEVERAL SHORTWAVES ROUNDING THE TROUGH...  
...ACTIVE FRONT ON LEADING EDGE OF THE TROUGH AND LEE SURFACE  
CYCLONE, WITH ADDITIONAL WEAK CYCLOGENESIS IN THE GREAT LAKES...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: BLEND OF 00Z ECMWF, CMC, UKMET  
CONFIDENCE: AVERAGE  
 
ON THE SYNOPTIC SCALE, MODELS ARE GENERALLY IN GOOD AGREEMENT ON  
THE OVERALL POSITION AND STRUCTURE OF THE LARGE TROUGH THAT WILL  
BE SITUATED OVER THE WESTERN HALF OF THE CONTIGUOUS UNITED STATES,  
PARTICULARLY THROUGH TUESDAY. THEREFORE, THEY ARE ALL GENERALLY  
SHOWING THE SAME BASIC SCENARIO WITH SLIGHTLY DIFFERENT PLACEMENT  
OF SENSIBLE WEATHER IMPACTS -- EMANATING FROM SMALL DIFFERENCES IN  
TIMING OF SHORTWAVES OR MESOSCALE DETAILS.  
 
THE 12Z NAM IS VERY SIMILAR TO THE OTHER AVAILABLE MODELS IN ITS  
STRENGTH OF BOTH THE TROUGH, AND THE LARGE MID-UPPER LEVEL  
ANTICYCLONE POSITIONED JUST OFF THE SOUTHEAST ATLANTIC COAST. IT  
DOES SHOW THE TROUGH SHIFTED SLIGHTLY OUT OF PHASE WITH THE OTHER  
MODELS, WITH THE TROUGH AXIS BY 21/00Z ABOUT 250KM TO THE WEST.  
THIS ALSO LEADS TO SLIGHTLY HIGHER HEIGHTS ACROSS THE CENTRAL U.S.  
(WHERE MUCH OF THE PRECIPITATION IMPACTS WILL BE). THIS BIAS DOES  
APPEAR TO AFFECT ITS PROGRESSION OF THE AXIS OF HEAVIEST QPF, AND  
THUS THE PREFERENCE IS TO NOT INCORPORATE THE NAM FOR THIS SYSTEM.  
 
LOOKING AT THE GLOBAL MODELS, ONE MORE NOTABLE DIFFERENCE IS  
BETWEEN THE 12Z GFS AND THE 00Z RUNS OF THE OTHER GLOBAL MODELS  
(CMC, UKMET, ECMWF). THE GFS IS SLOWER IN EJECTING A SHORTWAVE AND  
ASSOCIATED SHEARED LOBE OF VORTICITY TO THE NORTHEAST ACROSS THE  
PLAINS ON TUESDAY, AND THIS RESULTS IN A SURFACE LOW THAT IS ABOUT  
6 HOURS SLOWER THAN THE OTHER MODELS. THIS AFFECTS THE EASTWARD  
PROGRESSION OF THE NORTHERN EXTENT OF THE SURFACE FRONT, AND THUS  
MAY AFFECT PRECIPITATION PATTERNS AND OTHER SENSIBLE WEATHER  
ELEMENTS. GIVEN A LACK OF SUBSTANTIAL SUPPORT AMONG GEFS/ECMWF  
ENSEMBLE CLUSTERS FOR THIS SLOWER SCENARIO, THE PREFERENCE IS TO  
LEAN TOWARD THE CMC, UKMET, AND ECMWF.  
 
...SHORTWAVE DROPPING SOUTH ALONG WEST COAST OF NORTH AMERICA FROM  
MONDAY NIGHT INTO WEDNESDAY...   
..ASSOCIATED WEAK SURFACE LOW JUST OFFSHORE
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: BLEND OF 00Z ECMWF, CMC, UKMET  
CONFIDENCE: AVERAGE  
 
A LOW-AMPLITUDE SHORTWAVE DROPPING SOUTH ALONG THE WEST COAST IS  
FORECAST TO AMPLIFY SLIGHTLY AS IT PASSES BY THE COASTS OF OREGON  
AND CALIFORNIA. THE 12Z GFS (AND SECONDARILY THE 12Z NAM) IS THE  
MOST AMPLIFIED WITH THE TROUGH, AND THUS PRODUCES THE STRONGEST  
SURFACE LOW JUST OFFSHORE. THIS ALSO TENDS TO FOCUS THE  
PRECIPITATION MORE OFFSHORE RELATIVE TO THE OTHER GLOBAL MODELS,  
WHICH SHOW SOME LIGHT PRECIPITATION EXTENDING INLAND. THE  
PREFERENCE IS TO LEAN TOWARD THE LOWER AMPLITUDE SOLUTIONS WHICH  
AT THE MOMENT HAVE BETTER ENSEMBLE SUPPORT. HOWEVER, WITH A  
SOUTHWARD DIGGING WAVE, A FUTURE TREND TOWARD THE GFS SOLUTION  
COULD NOT BE RULED OUT.  
 
MODEL TRENDS AT WWW.WPC.NCEP.NOAA.GOV/HTML/MODEL2.SHTML  
500 MB FORECASTS AT WWW.WPC.NCEP.NOAA.GOV/H5PREF/H5PREF.SHTML  
 
LAMERS  
 

 
 
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