052  
FXUS10 KWNH 081837  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
137 PM EST THU FEB 08 2018  
 
VALID FEB 08/1200 UTC THRU FEB 12/0000 UTC  
 
   
..SEE NOUS42 KWNO (ADMNFD) FOR STATUS OF THE UPPER AIR INGEST
 
 
   
..12Z MODEL EVALUATION WITH PRELIMINARY PREFERENCES/CONFIDENCE
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
 
...LOW-AMPLITUDE SHORTWAVE ROTATING AROUND LARGE UPPER LEVEL  
HUDSON BAY LOW, THROUGH THE GREAT LAKES AND NORTHEAST...  
...ASSOCIATED ELONGATED FRONT FROM NORTHERN ROCKIES INTO THE GREAT  
LAKES ALONG NORTHERN STREAM JET CORE...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: NON-CMC BLEND  
CONFIDENCE: ABOVE AVERAGE  
 
---18Z UPDATE---  
NO CHANGE TO THE PRELIMINARY PREFERENCE.  
 
---PREV. DISCUSSION---  
A LOW-AMPLITUDE SHORTWAVE CURRENTLY DIGGING INTO THE CANADIAN  
PRAIRIES WILL CONTINUE TO ROTATE AROUND A LARGE UPPER LEVEL LOW  
NEAR HUDSON BAY. FOR FRIDAY AND SATURDAY, THAT WILL MEAN THE  
SHORTWAVE PUSHING INTO THE GREAT LAKES ON FRIDAY AND GETTING  
GRADUALLY SHEARED OUT, QUICKLY PASSING THE NORTHEAST ON SATURDAY.  
AN ASSOCIATED FRONT WILL BE DRAPED ON THE SOUTH AND SOUTHEAST  
PERIPHERY OF AN ARCTIC HIGH FROM THE NORTHERN ROCKIES INTO THE  
GREAT LAKES.  
 
MODEL DIFFERENCES IN THIS REGION ARE RELATIVELY MINOR IN THE MASS  
FIELDS. THE ONE EXCEPTION WOULD BE THE 00Z CMC WHICH APPEARS TO  
HAVE A HIGH HEIGHT BIAS THROUGH MUCH OF THE CONUS DOMAIN RELATIVE  
TO THE OTHER MODELS. THEREFORE, A GENERAL MODEL BLEND WHICH  
EXCLUDES THE CMC IS THE PREFERENCE AT THIS TIME TO ACCOUNT FOR  
SMALL UNCERTAINTIES RELATED TO THE MINOR AMOUNT OF REMAINING MODEL  
SPREAD.  
 
...DEVELOPING FRONT FROM THE ARKLATEX REGION NORTHEASTWARD TO THE  
NORTHEAST ON SATURDAY AND SUNDAY...   
..EVENTUAL CYCLOGENESIS ALONG THE FRONT IN THE EASTERN US
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: BLEND OF 12Z GFS, 06Z GEFS, 00Z EC ENS  
CONFIDENCE: AVERAGE  
 
---18Z UPDATE---  
IN THE INITIAL PART OF THE FORECAST, MODEL AGREEMENT REMAINS  
FAIRLY GOOD. AS THE SURFACE LOW DEVELOPS IN THE NORTHEAST, THE 12Z  
ECMWF AND UKMET HAVE TRENDED A LITTLE WEAKER AND FURTHER EAST ON  
THIS MODEL CYCLE. GIVEN THE TRENDS IN THESE MODELS, WILL MAINTAIN  
THE PREFERENCE FOR THE 12Z GFS AND THE ENSEMBLE MEANS.  
 
---PREV. DISCUSSION---  
SEVERAL OTHER SHORTWAVES WILL ALSO ROTATE INTO THE CONUS IN THE  
BROAD CYCLONIC FLOW AROUND THE HUDSON BAY LOW. ONE WILL DIG FROM  
THE PACIFIC NORTHWEST INTO THE CENTRAL PLAINS BY SUNDAY, AND  
ANOTHER WILL DIG FROM THE CANADIAN ARCTIC INTO THE WESTERN GREAT  
LAKES IN THE SAME TIME FRAME. BOTH WAVES SHOULD HAVE A RELATIVELY  
LOW AMPLITUDE AND MAINLY CONTRIBUTE TO A GRADUALLY DEVELOPING  
BROAD TROUGH OVER THE CENTRAL US. A LOW-LEVEL FRONT WILL CONTINUE  
TO TAKE SHAPE ALONG THE LEADING EDGE OF THE TROUGH AS AN  
ALREADY-EXISTING BAROCLINIC ZONE SLOWLY RE-ORIENTS TO A MORE SW-NE  
DIRECTION. MODELS DO EVENTUALLY SHOW WEAK SURFACE CYCLOGENESIS  
THIS WEEKEND INTO PORTIONS OF THE NORTHEAST ALONG THIS FRONT, IN  
THE RIGHT REAR QUADRANT OF AN UPPER LEVEL JET STREAK.  
 
INITIAL MODEL DIFFERENCES ARE RATHER SMALL THROUGH THE ENTIRE  
REGION, WITH THE EXCEPTION OF THE 00Z CMC. AS MENTIONED IN THE  
PREVIOUS SECTION, IT APPEARS TO HAVE A HIGH HEIGHT BIAS THROUGH  
MUCH OF THE CONUS DOMAIN RELATIVE TO OTHER MODELS. LATER IN THE  
PERIOD, GREATER DIFFERENCES EMERGE AS THE SURFACE LOW BEGINS TO  
DEVELOP IN THE NORTHEAST. THE 00Z ECMWF AND UKMET HAVE STRONGER  
SURFACE LOWS THAT TRACK FURTHER NORTHWEST AND INLAND OVER PORTIONS  
OF THE NORTHEAST, WHILE THE 12Z GFS HAS A WEAKER SURFACE LOW THAT  
TRACKS FURTHER EAST. GIVEN THE BROAD NATURE OF THE TROUGH AND LACK  
OF ANY FOCUSED REGION OF VERY STRONG PVA AND HEIGHT FALLS ALOFT,  
THE PREFERENCE IS TO LEAN IN THE DIRECTION OF A WEAKER SURFACE  
LOW. THE ECMWF ENSEMBLE MEAN AND GEFS MEAN ARE RELATIVELY CLOSE,  
AND BOTH LIE TO THE EAST OF THE ECMWF/UKMET AS WELL (BUT NOT AS  
FAR EAST AS THE OPERATIONAL GFS). TO ACCOUNT FOR A FEW OF THE MORE  
WESTERLY SOLUTIONS, THE PREFERENCE IS A BLEND OF THE GFS WITH THE  
ENSEMBLE MEANS.  
 
...SHORTWAVE UNDERCUTTING NORTHWARD EXTENSION OF RIDGE IN THE GULF  
OF ALASKA AND PUSHING INTO WESTERN NOAM BY SUNDAY...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 12Z ECMWF  
CONFIDENCE: SLIGHTLY BELOW AVERAGE  
 
---18Z UPDATE---  
THE 12Z ECMWF AND UKMET BOTH TRENDED SOUTH WITH THE SHORTWAVE,  
ALTHOUGH NOT AS FAR SOUTH AS THE NAM AND GFS. THEREFORE, THE  
PREFERENCE IS STILL TO REMAIN CLOSEST TO THE ECMWF, AS IT PROVIDES  
THE BEST CONTINUITY WITH THE PRELIMINARY PREFERENCE WITH THE SAME  
REASONING APPLIED.  
 
---PREV. DISCUSSION---  
MODELS SHOW THE MOST VARIABILITY WITH THIS SYSTEM, AS THERE ARE  
NOTABLE DIFFERENCES WITH HOW THEY HANDLE THE SHORTWAVE AS IT  
UNDERCUTS THE NORTHWARD RIDGE EXTENSION IN THE GULF OF ALASKA.  
THIS IS PRIMARILY RELATED TO WHAT LATITUDE THAT OCCURS, AND HOW  
MUCH DIGGING OCCURS PRIOR TO THE ARRIVAL OF THE WAVE ON THE WEST  
COAST OF NORTH AMERICA. THE 12Z GFS AND 12Z NAM SHOW A MUCH  
STRONGER WAVE DIGGING INTO WASHINGTON AND OREGON, WHILE THE 00Z  
ECMWF, UKMET AND CMC ARE NEARLY 10 DEGREES LATITUDE FURTHER NORTH.  
THE PREFERENCE IS TO LEAN TOWARD THE MODELS THAT HAVE THE WAVE  
FURTHER NORTH. AS THE SHORTWAVE WILL BE PROGRESSING THROUGH A  
RIDGE AXIS, IT SEEMS LESS LIKELY THAT A WAVE WOULD STRENGTHEN OR  
DIG AS THAT PROCESS IS OCCURRING, AND AS THE GFS/NAM SHOW.  
THEREFORE, A FLATTER WAVE THAT ENDS UP FURTHER NORTH SEEMS MORE  
LIKELY. THE 00Z CMC DOES HAVE SOME HIGH HEIGHT BIAS RELATIVE TO  
THE OTHER MODELS THROUGH MUCH OF THE DOMAIN, SO IT IS NOT INCLUDED  
IN THE PREFERENCE. THE 00Z UKMET IS RELATIVELY CLOSE TO THE 00Z  
ECMWF, BUT IS ON THE NORTHERN END OF THE SPREAD. THEREFORE, THE  
PREFERENCE IS TO STICK CLOSEST TO THE 00Z ECMWF.  
 
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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