691  
FXUS10 KWNH 301636  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1136 AM EST THU NOV 30 2017  
 
VALID NOV 30/1200 UTC THRU DEC 04/0000 UTC  
 
...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR  
INGEST...  
 
12Z MODEL EVALUATION WITH PRELIMINARY PREFERENCES AND CONFIDENCE  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
 
   
..NORTHERN STREAM TROUGH OVER THE GREAT LAKES TODAY
 
EXITING THE  
NORTHEASTERN U.S. LATER ON FRIDAY...   
..TRAILING COLD FRONT SWEEPING ACROSS MOST OF EASTERN CONUS
 
   
..AMPLIFICATION AND WEAK CYCLOGENESIS OFFSHORE OVER ATLANTIC
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: BLEND OF 12Z NAM / 12Z GFS / 00Z ECMWF  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
THE 00Z CMC CONTINUED TO INITIALIZE WITH HIGHER HEIGHTS THAN OTHER  
MODELS NEAR THE MID ATLANTIC COAST AND EXTENDING OFFSHORE OVER THE  
NORTHWEST ATLANTIC. THIS WAS PARTICULARLY EVIDENT BETWEEN 25-40N  
AND 55-75W. THIS DOES NOT NECESSARILY INCLUDE THE PRIMARY TROUGH  
OVER THE GREAT LAKES...BUT DOES AFFECT ITS DEPICTION OF A LOWER  
AMPLITUDE SECONDARY SHORTWAVE NEAR COASTAL NC/VA AND THE  
DOWNSTREAM RIDGE OVER THE ATLANTIC. GIVEN IT IS AN OUTLIER...IT  
WAS EXCLUDED FROM THE PREFERENCE FOR THIS SYSTEM.  
OTHERWISE...THROUGH THE FIRST 24 HOURS OF THE FORECAST...THERE IS  
NOT AN APPRECIABLE DIFFERENCE WITH HOW THE MODELS HANDLE THE WAVES  
ALOFT OR THE PROGRESSION OF THE COLD FRONT IN THE LOW LEVELS.  
 
ONCE THE TROUGH PROGRESSES OFFSHORE...THE 00Z UKMET ALSO BECOMES  
AN OUTLIER WITH A SECONDARY VORT MAX ON THE BACK SIDE OF THE  
TROUGH BECOMING MORE DOMINANT...WHICH LEADS TO A SLOWER  
PROGRESSION OF THE TROUGH RELATIVE TO OTHER MODELS...AND MORE OF A  
POSITIVE TILT. EVENTUALLY ITS SURFACE LOW POSITIONS OVER THE  
ATLANTIC END UP WEST OF THE OTHER MODELS. GREATER DIFFERENCES  
BEGIN TO EMERGE LATER IN THE WEEKEND...BUT BY THIS POINT THE  
TROUGH IS WELL OFFSHORE (OVER 2000 KM) AND ITS IMPACTS ON SENSIBLE  
WEATHER IN THE CONUS SHOULD BE NEGLIGIBLE. THEREFORE...A MODEL  
BLEND THAT EXCLUDES THE CMC AND UKMET IS PREFERRED.  
 
...ELONGATED SOUTHERN STREAM TROUGH AND DEVELOPING CLOSED LOW NEAR  
NORTHERN BAJA CALIFORNIA BY FRIDAY...   
..EJECTING AS AN OPEN WAVE INTO SOUTHERN PLAINS THIS WEEKEND
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 00Z ECMWF  
CONFIDENCE: SLIGHTLY BELOW AVERAGE  
 
DIFFERENCES CONTINUE TO EXIST WITH THIS SYSTEM...PRIMARILY RELATED  
TO TIMING AS THE CLOSED LOW QUICKLY DEVOLVES INTO AN OPEN WAVE AND  
GETS EJECTED ENE INTO THE SOUTHERN PLAINS BY THE SUBTROPICAL JET.  
THERE ARE A RANGE OF POSSIBILITIES WITH THE 12Z NAM AND 12Z GFS ON  
THE FAST END OF MODEL SPREAD (ALTHOUGH THE GFS HAS TRENDED SLOWER  
THAN ITS PREVIOUS TWO RUNS)...AND THE 00Z UKMET CONSIDERABLY  
SLOWER. IN FACT...THE NAM/GFS AND UKMET TROUGHS ARE ALMOST  
COMPLETELY OUT OF PHASE BY 03/00Z. THE 00Z CMC IS ALSO ON THE  
SLOWER END...BUT NOT AS MUCH AS THE UKMET. THE 00Z ECMWF AND 00Z  
ECMWF ENSEMBLE MEAN SIT IN THE MIDDLE OF THE DISTRIBUTION.  
ALTHOUGH THE ECMWF ENSEMBLE MEAN HAS BEEN FAIRLY CONSISTENT...IT  
HAS ALSO TRENDED FASTER OVER THE PAST SEVERAL MODEL CYCLES. WITH  
THE 12Z GFS...IT DOES SEEM THAT THERE ARE THE BEGINNINGS OF  
CONVERGENCE OF TIMING ON THE MODELS. THE 00Z ECMWF ENSEMBLE MEAN  
AND 12Z GFS ARE ACTUALLY FAIRLY CLOSE...WITH THE 00Z OPERATIONAL  
ECMWF SITTING IN THE MIDDLE. GIVEN THAT MODELS GENERALLY AGREE ON  
THE AMPLITUDE OF THE TROUGH...THE PREFERENCE IS TO AVOID A  
BROAD-BASED BLEND WHICH WOULD WASH OUT THE TROUGH MORE THAN IS  
REALISTIC. GIVEN THAT THE ECMWF ENSEMBLE MEAN AND GFS APPEAR TO BE  
GRADUALLY CONVERGING TOWARD THE 00Z OPERATIONAL ECMWF  
POSITION...THE PREFERENCE IS TO LEAN TOWARD THE ECMWF.  
 
   
..LOW AMPLITUDE SHORTWAVE OVER THE PACIFIC NORTHWEST
 
PUSHING  
ALONG THE NORTHERN TIER OF STATES THROUGH SUNDAY...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: BLEND OF THE 00Z ECMWF AND 00Z CMC  
CONFIDENCE: AVERAGE  
 
DIFFERENCES IN THE MASS FIELDS ASSOCIATED WITH THIS WAVE ARE  
FAIRLY MINOR THROUGH FRIDAY...BUT GROW OVER THE WEEKEND AS TROUGH  
AMPLIFICATION OCCURS. THE 00Z UKMET SHOWS A SLOWER/STRONGER WAVE  
WITH GREATER AMPLIFICATION THAN IS INDICATED BY OTHER MODELS.  
THERE IS NOT MUCH SUPPORT FOR THIS IDEA IN THE ENSEMBLES...AS THE  
UKMET IS SLOWER AND MORE AMPLIFIED THAN JUST ABOUT EVERY  
GEFS...CMC...AND ECMWF ENSEMBLE MEMBER OVER TH NORTHEAST BY  
04/00Z. THE 12Z NAM AND 12Z GFS TRENDED TOWARD A FASTER WAVE WITH  
LESS AMPLIFICATION. THEY WERE IN BETTER AGREEMENT ON THE 00Z  
CYCLE...SO IT REMAINS TO BE SEEN WHETHER THIS IS A TREND PICKED UP  
BY THE ENTIRETY OF THE 12Z MODEL CYCLE...OR IS BASED ON MODEL  
DIFFERENCES. FOR NOW...WILL TREND CLOSER TO THE 00Z ECMWF AND 00Z  
CMC WHICH REPRESENT A CONSENSUS SOLUTION FROM THE PREVIOUS MODEL  
CYCLE...AND WILL AWAIT THE REMAINDER OF THE 12Z RUNS BEFORE  
JUMPING ON THE NEW TREND.  
 
   
..GULF OF ALASKA TROUGH DIGGING INTO PACIFIC NORTHWEST BY SAT
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: NON-GFS BLEND  
CONFIDENCE: AVERAGE  
 
THE 12Z GFS HAS TRENDED TOWARD LOWER HEIGHTS WITH THIS TROUGH AS  
IT ARRIVES ALONG THE PACIFIC COAST...AND THIS IS A NOTABLE CHANGE  
FROM ITS 00Z RUN WHICH WAS IN BETTER AGREEMENT WITH OTHER MODELS.  
THE 12Z NAM REMAINS CLOSER TO MODEL CONSENSUS...AND ALL REMAIN  
LESS AMPLIFIED THAN THE NEW GFS. THE OPERATIONAL GFS ALSO HAS  
LOWER HEIGHTS THAN ITS ENSEMBLE MEAN (06Z GEFS)...SO IT APPEARS TO  
BE A BIT OF AN OUTLIER AT THIS POINT. GIVEN THAT THIS IS A  
RELATIVELY NEW TREND ON THE MODEL...PREFER TO LEAN IN THE  
DIRECTION OF THE BETTER CONSENSUS FOR NOW. WHILE THERE ARE DETAIL  
DIFFERENCES ON THE OTHER AVAILABLE MODELS...THEY DO GENERALLY  
AGREE ON THE OVERALL TROUGH AMPLITUDE AND ARE FAIRLY CLOSE IN  
TIMING. THEREFORE...A NON-GFS BLEND IS PREFERRED.  
 
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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