141  
FXUS10 KWNH 191652  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1152 AM EST MON FEB 19 2018  
 
VALID FEB 19/1200 UTC THRU FEB 23/0000 UTC  
   
..SEE NOUS42 KWNO (ADMNFD) FOR STATUS OF THE UPPER AIR INGEST
 
 
   
..12Z MODEL EVALUATION WITH PRELIMINARY PREFERENCES/CONFIDENCE
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
 
...AMPLIFIED WESTERN US TROUGH WITH PRIMARY SHORTWAVE LIFTING  
NORTHEAST FROM THE FOUR CORNERS TODAY TO ONTARIO BY TUESDAY  
NIGHT...   
..ASSOCIATED WEAK SURFACE LOW TRACKING INTO THE GREAT LAKES
 
 
...TRAILING, SLOW-MOVING FRONT STRETCHING FROM THE GREAT LAKES TO  
THE SOUTHERN PLAINS THROUGH WEDNESDAY...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: NON-NAM BLEND  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
FROM A SYNOPTIC SCALE, THE MODELS ARE IN VERY GOOD AGREEMENT ON  
THE BIG PICTURE, WHICH IS TO BE EXPECTED WITH A RELATIVELY STABLE  
1-WAVE PATTERN OVER THE CONUS -- AN AMPLIFIED TROUGH IN THE WEST  
AND AN AMPLIFIED RIDGE IN THE EAST. THERE ARE STILL SOME SMALL  
DETAIL DIFFERENCES ON THE MESOSCALE, OR WITH TIMING OF INDIVIDUAL  
SHORTWAVES EMBEDDED IN THE BROADER FLOW PATTERN, THAT COULD MAKE  
AN IMPACT ON THE FORECAST.  
 
THE 12Z NAM KICKS THE PRIMARY SHORTWAVE OUT OF THE BASE OF THE  
TROUGH A LITTLE EARLIER THAN THE OTHER MODELS, AND THUS IS ABOUT  
SIX HOURS FASTER WITH THE PROGRESSION OF THE WAVE AND THE  
ASSOCIATED SURFACE LOW. WITH A LACK OF SUPPORT FOR SUCH A SCENARIO  
FROM OTHER MODELS OR ENSEMBLE MEMBERS, THE PREFERENCE IS TO  
EXCLUDE THE NAM AT THIS TIME. THE 12Z GFS, MEANWHILE, CONTINUES TO  
BE ON THE SLOWER END OF THE MODEL SPREAD, WHICH WAS ALSO THE CASE  
YESTERDAY. HOWEVER, THE DIFFERENCE BETWEEN THE GFS AND THE OTHER  
GLOBAL MODELS HAS NARROWED SUFFICIENTLY, THAT IT IS A REASONABLE  
POSSIBILITY. THE 06Z GEFS WAS SLIGHTLY FASTER WITH THE FORWARD  
PROGRESSION OF THE SURFACE LOW, INDICATING THAT THE GFS MAY  
CONVERGE FURTHER TO THE OTHER MODELS OVER THE NEXT COUPLE CYCLES.  
 
OTHERWISE, THE PREFERENCE IS FOR A BROAD-BASED BLEND TO ACCOUNT  
FOR MESOSCALE UNCERTAINTIES. ALONG THE SOUTHERN EXTENT OF THE  
FRONT IN PARTICULAR, CONVECTION MAY BE THE PRIMARY DRIVER OF  
CERTAIN THINGS, SUCH AS HOW QUICKLY THE FRONT ADVANCES TO THE  
SOUTHEAST. A BROADER BLEND WOULD ACCOUNT FOR THE POSSIBILITY OF  
SMALL MASS FIELD VARIATIONS HAVING AN EFFECT ON THE SENSIBLE  
WEATHER IMPACTS.  
 
...SECONDARY SHORTWAVE DROPPING SOUTH ALONG WEST COAST OF NORTH  
AMERICA THROUGH WEDNESDAY, REINFORCING THE LARGER TROUGH OVER THE  
WESTERN CONUS...  
...SHORTWAVE KICKING INTO THE FOUR CORNERS ON THURSDAY WITH FRONT  
POSSIBLY RETURNING NORTH ACROSS THE SOUTH-CENTRAL US...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: AVERAGE  
 
MODELS HAVE COME INTO BETTER AGREEMENT WITH THE SHORTWAVE DIGGING  
ALONG THE WEST COAST ON TUESDAY AND WEDNESDAY. THE GFS HAD  
PREVIOUSLY BEEN MUCH MORE AMPLIFIED WITH THE SHORTWAVE TROUGH  
ALOFT, AND HAD A MUCH STRONGER SURFACE LOW. WHILE THE OTHER MODELS  
HAVE TRENDED SLIGHTLY MORE AMPLIFIED THAN THEY WERE A COUPLE MODEL  
CYCLES AGO, THE GFS HAS TRENDED SUBSTANTIALLY LESS AMPLIFIED WITH  
THE TROUGH ALOFT. THE RESULTING MODEL MASS FIELDS ARE RELATIVELY  
SIMILAR. THE PRIMARY DIFFERENCE APPEARS TO BE TIMING, WITH THE 12Z  
GFS AND NAM SLIGHTLY SLOWER THAN THE 00Z ECMWF, CMC AND UKMET. THE  
GFS AND ECMWF BOTH HAVE SUPPORT FROM THEIR RESPECTIVE ENSEMBLE  
MEANS. THE DIFFERENCES THROUGH THE SHORT TERM PERIOD WOULD BE  
RELATIVELY SUBTLE, BUT A FASTER SOLUTION (CLOSER TO THE ECMWF)  
WOULD PRODUCE HEIGHT FALLS OVER THE SOUTHERN PLAINS SOONER, WITH A  
CORRESPONDING EARLIER LOW-LEVEL MASS FIELD RESPONSE. THUS, THE  
RETURN FLOW AND SURFACE FRONT MAY BEGIN TO PUSH NORTH FROM THE  
NORTHWESTERN GULF A LITTLE SOONER. THAT BEING SAID, THERE IS NOT A  
CLEAR PREFERENCE BETWEEN THE SMALL TIMING DIFFERENCE, SO A GENERAL  
MODEL BLEND IS PREFERRED WITH THIS SYSTEM.  
 
...THIRD SHORTWAVE ALSO DIGGING SOUTH ALONG WEST COAST WEDNESDAY  
NIGHT AND THURSDAY AND FURTHER REINFORCING THE TROUGH...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: BLEND OF 12Z NAM, 06Z GEFS, 00Z ECMWF  
CONFIDENCE: SLIGHTLY BELOW AVERAGE  
 
MORE SIGNIFICANT DIFFERENCES EMERGE WITH THE MID-WEEK SHORTWAVE  
DIGGING INTO THE WEST COAST. THE 12Z GFS DIGS THE PRIMARY  
SHORTWAVE INTO SOUTHERN CALIFORNIA AND HAS THE PRIMARY MID-LEVEL  
VORTICITY MAXIMUM IN THAT AREA BY 23/00Z. THE OTHER MODELS HAVE IT  
FURTHER NORTH, AND THIS IS SUPPORTED BY THE VAST MAJORITY OF THE  
GEFS/ECMWF ENSEMBLE CLUSTERS. THE 06Z GEFS MEAN HAS LOWER HEIGHTS  
NEAR WA/OR, WITH HIGHER HEIGHTS IN THE DESERT SOUTHWEST, SO THE  
GEFS MEAN SEEMS LIKE A REASONABLE ALTERNATIVE TO THE GFS. THE 00Z  
CMC CLOSES OFF AN UPPER LEVEL LOW VERY QUICKLY AND HAS A MUCH MORE  
CONCENTRATED VORTICITY MAXIMUM RELATIVE TO THE OTHER MODELS (WHICH  
HAVE AN ELONGATED/SHEARED APPEARANCE ALONG THE BASE OF THE  
TROUGH). THESE DIFFERENCES IN THE GFS AND CMC SEEM SUBSTANTIAL  
ENOUGH TO NOT BE INCLUDED IN THE PREFERENCE AT THIS TIME.  
THEREFORE, WILL LEAN MORE IN THE DIRECTION OF A BLEND OF THE  
ECMWF, NAM AND GEFS MEAN.  
 
...LARGE MID-UPPER LEVEL ANTICYCLONE JUST OFF THE SOUTHEAST COAST  
WITH EAST COAST RIDGE AXIS...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: BLEND OF 00Z ECMWF, ECMWF ENS MEAN  
CONFIDENCE: AVERAGE  
 
THE 00Z CMC AND UKMET HAVE ONE OF THE STRONGER DEPICTIONS OF THE  
UPPER LEVEL RIDGE IN THE FIRST 24 HOURS OF THE FORECAST, AND THIS  
EVENTUALLY GROWS OVER TIME BY MID-WEEK. THE 00Z UKMET EVENTUALLY  
HAS HIGHER HEIGHTS ALOFT FROM THE TENNESSEE VALLEY INTO THE UPPER  
MIDWEST, AND HAS THE BIGGEST DEVIATION FROM THE MODEL AVERAGE.  
MEANWHILE, THE 12Z GFS IS MUCH FLATTER WITH ITS DEPICTION OF THE  
RIDGE BY WEDNESDAY AND THURSDAY, AND HAS LOWER HEIGHTS OVER MUCH  
OF THE NORTHEAST. THE PREFERENCE IS FOR A DEPICTION IN THE MIDDLE  
OF THESE EXTREMES, AND CLOSEST TO A BLEND OF THE 00Z ECMWF AND 00Z  
ECMWF ENSEMBLE MEAN.  
 
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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