918  
FXUS10 KWNH 021826  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
126 PM EST SAT DEC 02 2017  
 
VALID DEC 02/1200 UTC THRU DEC 06/0000 UTC  
 
...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR  
INGEST...  
 
12Z MODEL EVALUATION WITH FINAL PREFERENCES AND CONFIDENCE  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
 
...POSITIVELY-TILTED TROUGH DIGGING INTO PACIFIC NORTHWEST AND  
REACHING THE PLAINS BY MONDAY...  
...INCREASING NEGATIVE TILT WITH ACCOMPANYING RAPID CYCLOGENESIS  
OVER THE WESTERN GREAT LAKES...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: THRU 36HR (04/06Z) -- GENERAL MODEL BLEND  
AFTER 36HR -- BLEND OF 12Z GFS / 00Z ECMWF ENS MEAN  
CONFIDENCE: AVERAGE  
 
A TROUGH WILL DIG INTO THE PACIFIC NORTHWEST COAST LATER TODAY AND  
THROUGH THE INTERMOUNTAIN WEST OVER THE REMAINDER OF THE WEEKEND.  
MODEL AGREEMENT IS QUITE GOOD DURING THIS FORECAST  
PERIOD...ROUGHLY THROUGH 04/06Z OR ABOUT 36 HOURS FROM NOW. THE  
00Z UKMET IS A LITTLE FASTER THAN THE OTHER GUIDANCE WITH  
ADVANCING THE TROUGH AND ALSO SLIGHTLY LESS AMPLIFIED IN THE BASE  
OF THE TROUGH. HOWEVER...THE UKMET IS STILL WELL WITHIN THE  
ENSEMBLE SPREAD ENVELOPE FROM BOTH THE GEFS AND ECMWF.  
THEREFORE...A GENERAL MODEL BLEND IS PREFERRED THROUGH 36  
HOURS...AFTER WHICH POINT DIFFERENCES BEGIN TO GET LARGER.  
 
OVER THE PAST COUPLE MODEL CYCLES...THERE HAS DEFINITELY BEEN A  
TREND TOWARD A DEEPER CYCLONE AND RAPID CYCLOGENESIS OVER THE  
WESTERN GREAT LAKES REGION. THE 01/00Z ECMWF AND UKMET RUNS HAD  
BEEN ADVERTISING THIS YESTERDAY...BUT ON THE 01/12Z CYCLE THERE  
WAS A TREND BACK TOWARD A WEAKER AND FASTER SOLUTION. AS OF THIS  
MORNING...ALL OF THE AVAILABLE OPERATIONAL MODELS DEPICT A  
SUB-990MB LOW BY MONDAY NIGHT (05/06Z)...WITH A FEW QUITE A BIT  
DEEPER THAN THAT BENCHMARK. ALTHOUGH THE MODELED LOW POSITIONS AND  
INTENSITIES HAVE CONVERGED...THERE ARE STILL IMPORTANT DETAIL  
DIFFERENCES.  
 
THE 00Z ECMWF IS ABOUT 6 HOURS SLOWER THAN OTHER MODELS WITH THE  
SHORTWAVE AS IT KICKS THROUGH THE PLAINS AND BECOMES INCREASINGLY  
NEGATIVE TILT. THIS EVENTUALLY LEADS TO A DEEP CYCLONE LIKE THE  
OTHER MODELS...BUT THE LOW PROGRESSION IS SLOWER AND THE POSITION  
RELATIVE TO OTHER MODELS IS TO THE SOUTHWEST. THIS WAS ON THE  
SOUTHWESTERN PERIPHERY OF THE RANGE OF SOLUTIONS OFFERED BY  
ENSEMBLES...SO A FASTER SOLUTION THAN THE OPERATIONAL ECMWF IS  
PREFERRED. THE 00Z UKMET CONTINUES TO BE THE FASTEST...SO IT WAS  
ALSO NOT PREFERRED (AND ITS SURFACE LOW POSITION WAS ON THE  
WESTERN PERIPHERY OF THE RANGE OF SOLUTIONS).  
 
FOR NOW THE PREFERENCE IS TO LEAN TOWARD A BLEND OF THE 12Z GFS  
AND 00Z ECMWF ENSEMBLE MEAN. THE 06Z GFS...LIKE THE 00Z  
UKMET...HAD BEEN ON THE STRONG/WEST END OF ENSEMBLE SPREAD WITH  
THE SURFACE LOW...BUT THE 12Z GFS HAS TRENDED SLIGHTLY WEAKER AND  
FURTHER EAST. IT IS STILL WEST OF THE ECMWF AND GEFS ENSEMBLE  
MEANS...BUT THE ENSEMBLE MEMBERS ALSO HAVE A GREATER DISTRIBUTION  
SLIGHTLY WEST OF THE MEAN POSITIONS. THIS PLACES THE 12Z GFS  
FAIRLY CLOSE TO SOME OF THE TIGHTEST ENSEMBLE CLUSTERING. PREFER  
TO BLEND THIS WITH THE ECMWF ENSEMBLE AS THE GFS STILL SKEWS  
SLIGHTLY ON THE COLD END OF MODELS WITH THE TEMPERATURE PROFILES  
ACROSS THE REGION...AND THE ECMWF ENSEMBLE MEAN DOES NOT DIFFER  
SUBSTANTIALLY IN TIMING/PLACEMENT OF THE LOW. THE ECMWF ENSEMBLE  
MEAN ALSO DEPICTS A SLIGHTLY SLOWER PROGRESSION OF THE  
SHORTWAVE...CLOSER TO THE 00Z CMC AND 12Z NAM...BUT NOT AS SLOW AS  
THE OPERATIONAL ECMWF.  
 
18Z UPDATE: NO CHANGE TO THE PRELIMINARY PREFERENCE. THE 12Z ECMWF  
AND 12Z UKMET HAVE BOTH TRENDED CLOSER TO THE CONSENSUS SOLUTION  
ROUGHLY REPRESENTED BY THE 12Z GFS AND 00Z ECMWF ENSEMBLE MEAN AS  
OUTLINED EARLIER. THE UKMET IS STILL SLIGHTLY FURTHER WEST THAN  
THE GFS...BUT HAS A VERY SIMILAR TIMING AND INTENSITY OF THE  
SURFACE LOW. THE ECMWF WAS THE SLOW OUTLIER EARLIER...AND  
CONTINUES TO BE ON THE SLOWER END OF MODELS (ALONG WITH THE NAM)  
BUT HAS TRENDED FASTER RELATIVE TO THE PREVIOUS MODEL CYCLE. THE  
12Z CMC CONTINUES TO BE WEAKER WITH THE SHORTWAVE AND  
WEAKER/FASTER WITH THE SURFACE LOW COMPARED TO THE OTHER MODELS.  
GIVEN THAT THE TRENDS HAVE LARGELY BEEN IN THE DIRECTION OF THE  
EARLIER PREFERENCE...WILL MAINTAIN THAT NOW IN THE FINAL  
PREFERENCE.  
 
   
..CLOSED LOW OVER NORTHERN BAJA CALIFORNIA
 
 
...DEAMPLIFYING AND ACCELERATING ENE WITH THE SUBTROPICAL JET INTO  
TEXAS BY SUNDAY AFTERNOON/EVENING...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: ABOVE AVERAGE  
 
MODELS SHOW A GREAT DEGREE OF SIMILARITY WITH THE MASS FIELDS FOR  
THIS SYSTEM. THEREFORE...A GENERAL MODEL BLEND IS PREFERRED.  
 
18Z UPDATE: NO CHANGE TO THE PRELIMINARY PREFERENCE.  
 
...LOW AMPLITUDE SHORTWAVES OVER THE CENTRAL U.S. IN ZONAL FLOW  
QUICKLY PROGRESSING EAST THIS WEEKEND...   
..POSSIBLE WEAK SURFACE WAVE DEVELOPING NEAR COASTAL NC
 
   
..AMPLIFYING TROUGH OFF THE NORTHEAST ON SUNDAY INTO MONDAY
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
WITH THE SOUTHERN STREAM SHORTWAVE NEAR EASTERN NC...THERE IS  
LIMITED MODEL SPREAD AND A GENERAL MODEL BLEND IS PREFERRED. WITH  
THE WAVE OVER THE GREAT LAKES...MODELS SHOW A GREAT DEGREE OF  
SIMILARITY THROUGH 03/18Z (24 HOUR FORECAST) WHEN THE TROUGH AXIS  
REACHES THE ATLANTIC COAST. AFTER THAT POINT...THE ECMWF AND ECMWF  
ENSEMBLE HAVE A SLOWER PROGRESSION OF THE TROUGH THAN THE OTHER  
MODELS...AND GREATER DIFFERENCES EMERGE OFFSHORE AS AMPLIFICATION  
CONTINUES. HOWEVER...WHEN THE TROUGH IS WITHIN 1000 KM OF THE  
ATLANTIC COAST...THE MODELS ARE ALL WELL WITHIN THE ENVELOPE OF  
ENSEMBLE MEMBERS...SO A GENERAL MODEL BLEND IS PREFERRED FOR THIS  
SYSTEM AS WELL.  
 
18Z UPDATE: NO CHANGE TO THE PRELIMINARY PREFERENCE.  
 
MODEL TRENDS AT WWW.WPC.NCEP.NOAA.GOV/HTML/MODEL2.SHTML  
500 MB FORECASTS AT WWW.WPC.NCEP.NOAA.GOV/H5PREF/H5PREF.SHTML  
 
LAMERS  
 

 
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab HPC Page Main Text Page