118  
FXUS10 KWNH 091835  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
134 PM EST FRI FEB 09 2018  
 
VALID FEB 09/1200 UTC THRU FEB 13/0000 UTC  
 
   
..SEE NOUS42 KWNO (ADMNFD) FOR STATUS OF THE UPPER AIR INGEST
 
 
   
..12Z MODEL EVALUATION WITH FINAL PREFERENCES/CONFIDENCE
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
 
   
..SHORTWAVE MOVING THROUGH THE CENTRAL ROCKIES ON SATURDAY
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: BLEND OF 12Z ECMWF, CMC, UKMET  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
---18Z UPDATE---  
AS THE REMAINDER OF THE 12Z RUNS HAVE COME IN, THE CMC AND UKMET  
HAVE MOVED INTO MUCH BETTER ALIGNMENT WITH THE ECMWF, AND ALL ARE  
CLOSE TO THE ECMWF ENSEMBLE MEAN. THE GFS REMAINS FASTER, ALBEIT  
LESS AMPLIFIED, WHILE THE NAM IS THE FASTEST AND MOST AMPLIFIED.  
GIVEN THE TRENDS OF THE CMC AND UKMET, THE PREFERENCE WILL SHIFT  
TO A BLEND OF THOSE MODELS AND THE ECMWF, REMOVING THE 12Z GFS.  
 
---PREV. DISCUSSION---  
MODELS ARE IN DECENT AGREEMENT WITH THE SHORTWAVE THAT WILL BE  
PUSHING THROUGH THE INTERMOUNTAIN WEST AND CENTRAL ROCKIES ON  
SATURDAY, BEFORE EJECTING INTO THE PLAINS AND CONTRIBUTING TO THE  
EVENTUAL WEAK CYCLOGENESIS ALONG THE FRONT IN THE EASTERN CONUS.  
THE 12Z NAM SHOWS A MUCH MORE AMPLIFIED WAVE WITH CONSIDERABLY  
FASTER TIMING; THIS IS NOT SUPPORTED BY ANY OTHER MODELS AND ALSO  
EVENTUALLY CONTRIBUTES TO DIFFERENCES IN THE EASTERN CONUS  
(DISCUSSED BELOW). OTHERWISE, THERE IS A TYPICAL SPREAD WITH LESS  
AMPLIFIED MODELS (12Z GFS, 00Z CMC) A LITTLE FASTER WITH THE  
TIMING, AND MORE AMPLIFIED MODELS (00Z ECMWF, UKMET) A LITTLE  
SLOWER. BOTH THE GFS AND ECMWF HAVE DECENT SUPPORT FROM THEIR  
ENSEMBLES, SO AT THIS POINT THE PREFERENCE IS FOR A GENERAL BLEND  
THAT EXCLUDES THE NAM.  
 
...EXTENSIVE FRONT STRETCHING FROM THE SOUTHERN US NORTHEASTWARD  
INTO NEW ENGLAND AND BROAD SOUTHWESTERLY FLOW ALOFT...   
..WEAK SURFACE LOW DEVELOPMENT ALONG FRONT ON SUNDAY
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 60% 12Z ECMWF, 40% GEFS/EC ENS MEAN  
CONFIDENCE: SLIGHTLY BELOW AVERAGE  
 
---18Z UPDATE---  
THE 12Z ECMWF HAS LARGELY MAINTAINED CONTINUITY WITH ITS PREVIOUS  
RUN IN ITS MASS FIELDS. THE UKMET (AND NOW CMC) CONTINUES TO SHOW  
A TRACK OF THE SURFACE LOW FURTHER INLAND OVER THE NORTHEAST. THE  
PREFERENCE REMAINS FOR THE SCENARIO SHOWN BY THE ECMWF, WHICH IS  
ALSO CLOSE TO THE 12Z GEFS MEAN AND 00Z ECMWF ENS MEAN. WILL  
SUBSTITUTE THE ENSEMBLE MEANS FOR THE CMC FOR THE FINAL PREFERENCE.  
 
---PREV. DISCUSSION---  
AS MENTIONED IN THE PREVIOUS SECTION (CENTRAL ROCKIES SHORTWAVE),  
THE 12Z NAM HAS A MUCH STRONGER WAVE EJECTING MORE QUICKLY, AND  
THIS EVENTUALLY LEADS TO A STRONGER, FURTHER-NORTHWEST SURFACE LOW  
IN THE NORTHEAST ON SUNDAY AND SUNDAY NIGHT. THE PREFERENCE  
REMAINS TO EXCLUDE THE NAM FOR THIS SYSTEM AS WELL. THE 00Z UKMET  
ALSO HAS A STRONGER SURFACE LOW FURTHER TO THE NORTHWEST, BUT IT  
ARRIVES AT THAT VIA DIFFERENT MEANS THAN THE NAM. FROM A  
CONCEPTUAL MODEL STANDPOINT, WITH A BROAD ELONGATED LOW-LEVEL  
BAROCLINIC ZONE AND A LACK OF DISTINCT, STRONG WAVE ALOFT ARRIVING  
TO FOCUS CYCLOGENESIS, THE EXPECTATION WOULD BE FOR A WEAKER  
SURFACE LOW THAT WOULD TEND TO MOVE QUICKLY ALONG THE FRONT  
(RATHER THAN CURLING BACK TO THE NORTHWEST). THEREFORE, THE WEAKER  
SCENARIOS OFFERED BY THE GFS, ECMWF AND CMC SEEM MORE REASONABLE.  
 
THE 12Z GFS, HOWEVER, DEVELOPS A LEAD SURFACE LOW WELL AHEAD OF  
OTHER MODELS ALONG THE FRONT. IT EVENTUALLY DEVELOPS A SECONDARY  
LOW MORE IN LINE WITH THE CONSENSUS TIMING. THE LEAD SURFACE LOW  
APPEARS TO BE ASSOCIATED WITH AN EJECTING WAVE CURRENTLY OVER  
NORTHERN MEXICO, AND PERHAPS SOME CONVECTION OVER THE TENNESSEE  
VALLEY. GIVEN A LACK OF SUPPORT FROM THE OTHER MODELS AND THE GEFS  
MEAN, THE PREFERENCE IS TO LEAN MORE IN THE DIRECTION OF THE CMC  
AND ECMWF, WITH A GREATER WEIGHT PLACED ON THE ECMWF.  
 
...SHORTWAVE UNDERCUTTING NORTHWARD EXTENSION OF RIDGE IN THE GULF  
OF ALASKA AND PUSHING INTO PACIFIC NORTHWEST BY SUNDAY...  
...CONTINUED DIGGING AND AMPLIFICATION OF THE TROUGH ON THE WEST  
COAST INTO MONDAY AND TUESDAY...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: BLEND OF 12Z GFS, ECMWF, CMC  
CONFIDENCE: AVERAGE  
 
---18Z UPDATE---  
THE UKMET CONTINUES TO HAVE THE FURTHEST SOUTH APPROACH OF THE  
SHORTWAVE ALONG THE COAST OF THE PACIFIC NW, AND IS EVEN FURTHER  
SOUTH OF ITS 00Z RUN. THE ECMWF AND CMC TRENDED SOUTH VERY  
SLIGHTLY, BUT THE CONSENSUS BETWEEN THOSE TWO MODELS AND THE GFS  
REMAINS REASONABLY GOOD. THEREFORE, THERE IS NO CHANGE TO THE  
PRELIMINARY PREFERENCE.  
 
---PREV. DISCUSSION---  
MODEL SPREAD STILL EXISTS WITH THE LATITUDE THAT THE SHORTWAVE  
WILL APPROACH THE PACIFIC NW. THE 00Z UKMET HAS THE MOST SOUTHERLY  
SOLUTION, BRINGING THE TROUGH INTO THE COAST AROUND 46-47N. THE  
12Z GFS AND 00Z ECMWF/CMC HAVE A DECENT CONSENSUS AND LIE IN THE  
MIDDLE CLOSER TO VANCOUVER ISLAND AND 49N. THE 12Z NAM AND 00Z  
ECMWF ENSEMBLE MEAN ARE FURTHEST NORTH AROUND 51N. THE 00Z UKMET  
SEEMS LESS LIKELY AS IT PUSHES THE WAVE EAST WELL TO THE SOUTH OF  
THE APEX OF THE RIDGE, WHICH WOULD BE ATYPICAL FOR THESE CASES. IT  
ALSO IS UNSUPPORTED BY OTHER MODELS. THE PREFERENCE IN THIS CASE  
IS TO LEAN TOWARD A BLEND OF THE GFS, ECMWF AND CMC, WHICH HAVE  
THE BEST CONSENSUS WITH THE APPROACH OF THE WAVE TO THE PACIFIC  
NORTHWEST; THEY EVENTUALLY SHOW GREATER DIFFERENCES AS THE  
POSITIVE TILT TROUGH BUILDS DOWN THE WEST COAST, BUT ARE  
REASONABLY SIMILAR.  
 
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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