439  
FXUS10 KWNH 211842  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
142 PM EST WED FEB 21 2018  
 
VALID FEB 21/1200 UTC THRU FEB 25/0000 UTC  
 
...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR  
INGEST...  
   
..12Z NAM/GFS EVALUATION INCLUDING FINAL PREFERENCES
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
***NOTE THAT DATA FLOW ISSUES AT WPC/NCWCP CONTINUE TO AFFECT THE  
GUIDANCE AVAILABLE TO BE USED AS INPUT INTO THIS DISCUSSION. THE  
PREFERENCES REFERENCED IN THIS DISCUSSION ARE BETWEEN THE 12Z NAM  
AND 12Z GFS WITH NO ACCESS TO THE ECMWF/UKMET BEYOND THE 00Z/20  
CYCLE AND ONLY LIMITED ACCESS TO THE 12Z/21 CMC.***  
 
...FRONTAL BOUNDARY CURRENT STRETCHING FROM THE NORTHEAST TO  
TEXAS...  
...WEAK SURFACE WAVE TRACKING THROUGH THE UPPER MS VALLEY AND OHIO  
VALLEY ON FRIDAY...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 70% 12Z GFS, 30% 12Z NAM BLEND  
CONFIDENCE: AVERAGE  
 
THE 12Z NAM AND GFS ARE RATHER SIMILAR TO ONE ANOTHER WITH THE  
EVOLUTION OF THE FRONT, BUT THE 12A NAM IS A BIT STRONGER THAN THE  
12Z GFS. THE 12Z NAM IS A LITTLE STRONGER WITH THE SURFACE WAVE  
CROSSING THE UPPER MS VALLEY BUT OTHERWISE NO MAJOR DIFFERENCES  
WERE NOTED.  
 
...SHORTWAVE CROSSING THE FOUR CORNERS AND EJECTING INTO THE  
PLAINS SATURDAY MORNING...  
...SURFACE CYCLOGENESIS AND RELATED COLD/WARM FRONTS THROUGH THE  
PLAINS ON SATURDAY...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 70% 12Z GFS, 30% 12Z NAM BLEND  
CONFIDENCE: BELOW AVERAGE  
 
SIMILAR TO THE ABOVE REFERENCED SYSTEM, THE 12Z NAM AND GFS ARE  
SIMILAR TO ONE ANOTHER WITH THE EVOLUTION OF THE FRONT, BUT THE  
12Z NAM IS A NOTICEABLY STRONGER THAN THE 12Z GFS AT 500 MB FOR  
SATURDAY. THE 12Z NAM IS ALSO LITTLE NORTHWARD DISPLACED WITH THE  
WARM FRONT EXTENDING ACROSS THE OHIO VALLEY ON SATURDAY. SURFACE  
LOW PLACEMENT NEAR IOWA/MISSOURI BY 00Z/25 AND COLD FRONTAL TIMING  
THROUGH THE MS VALLEY INTO TEXAS WAS SIMILAR BETWEEN THE NAM/GFS  
STRONGER WITH THE SURFACE WAVE CROSSING THE UPPER MS VALLEY. MORE  
WEIGHT TO THE 12Z GFS IS RECOMMENDED OVERALL WITH THIS SYSTEM, BUT  
BOTH THE 12Z NAM/GFS REMAIN STRONGER WITH THE SURFACE LOW AND 500  
MB SHORTWAVE COMPARED TO OLDER RUNS OF THE ECMWF/UKMET/CMC.  
 
19Z UPDATE: THE 12Z CMC TRENDED NORTH, IN THE DIRECTION OF THE THE  
12Z NAM/GFS, BUT REMAINS WEAKER REGARDING THE SURFACE LOW NEAR  
MISSOURI VALID 00Z/25. THE 500 MB WAVE IS STRONGER COMPARED TO ITS  
PREVIOUS CYCLE.  
 
...FAST MOVING SHORTWAVE REACHING THE PACIFIC NORTHWEST ON  
FRIDAY...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 12Z NAM  
CONFIDENCE: AVERAGE  
 
PREVIOUS GUIDANCE ALONG WITH THE 12Z NAM AND 12Z GFS SHOW THE 12Z  
GFS AS FASTER WITH THE PROGRESSION OF THE SHORTWAVE INTO THE  
NORTHWEST ON FRIDAY. TRENDS IN THE NAM AND GFS ALSO SHOW A FLATTER  
WAVE COMPARED TO YESTERDAY'S RUNS, BUT STILL WITH MORE AMPLITUDE  
THAN THE PREVIOUSLY FLATTER 00Z/20 ECMWF. GIVEN AN APPARENT  
CONSENSUS FOR A SLOWER SHORTWAVE THAN SEEN IN THE 12Z GFS, THE  
PREFERENCE WILL BE GIVEN TO THE 12Z NAM.  
 
19Z UPDATE: THE 12Z CMC APPEARS TO SUPPORT THE SLOWER TIMING OF  
THE 12Z NAM COMPARED TO THE 12Z GFS VALID 00Z/25 ACROSS THE  
NORTHWEST.  
 
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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