492  
FXUS10 KWNH 121827  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
127 PM EST FRI JAN 12 2018  
 
VALID JAN 12/1200 UTC THRU JAN 16/0000 UTC  
 
...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR  
INGEST...  
 
12Z MODEL EVALUATION INCLUDING PREFERENCES AND FORECAST CONFIDENCE  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
 
   
..NEGATIVE TILT SHORTWAVE EJECTING UP THE EAST COAST
 
 
...ASSOCIATED SURFACE LOW DEVELOPING IN THE CAROLINAS AND SHIFTING  
UP THE EAST COAST ALONG EXTENSIVE COLD FRONT...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: BLEND OF 12Z GFS AND 12Z ECMWF  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
---18Z UPDATE---  
NO CHANGE TO THE PRELIMINARY PREFERENCE. THE UKMET AND CMC  
CONTINUE TO SHOW DIFFERENCES FROM THE GFS/ECMWF CONSENSUS AS  
DESCRIBED IN THE PRELIMINARY DISCUSSION.  
 
---PREV. DISCUSSION---  
OVERALL THERE IS NOT SUBSTANTIAL DIFFERENCE BETWEEN THE MODELS  
MASS FIELDS ALOFT WITH THIS EJECTING TROUGH AS IT PUSHES UP THE  
EAST COAST. HOWEVER...THERE ARE SOME DIFFERENCES IN TIMING AND  
TRACK OF THE SURFACE LOW. THE 00Z CMC IS MUCH SLOWER AS THE LOW  
ARRIVES IN THE NORTHEAST...WHICH MAY RELATE TO THE HIGHER HEIGHT  
FIELDS ALOFT ALONG THE EAST COAST. THE MODEL IS RESTRICTING THE  
EASTWARD EXTENT OF THE TROUGH MORE THAN OTHER MODELS...WHICH MAY  
ACCOUNT FOR SOME OF THE TIMING DIFFERENCES.  
 
THE 00Z UKMET CONTINUES TO TAKE A MORE INLAND TRACK WITH THE  
SURFACE LOW...WHICH WAS THE CASE YESTERDAY ON THE 12Z RUN AS WELL.  
THE 00Z RUN JUMPS THE LOW AROUND A BIT...WITH THE SURFACE LOW WEST  
OF THE BLUE RIDGE MOUNTAINS THIS AFTERNOON...SHIFTING TO JUST EAST  
BY THIS EVENING IN VIRGINIA...AND THEN BACK TO THE WEST IN  
SOUTH-CENTRAL PA WITH THE LOWEST MSLP AS WELL. IN ADDITION TO THE  
SOMEWHAT ERRATIC TRACK...IT IS ONE OF THE FURTHEST INLAND MODELS  
THROUGH MUCH OF THE NORTHEAST...BUT THE EXPECTATION IS THAT THE  
LOW WILL BECOME MORE FOCUSED ALONG THE COASTAL FRONT WHERE  
LOW-LEVEL CONVERGENCE AND THETAE GRADIENT ARE MOST LIKELY TO BE  
MAXIMIZED.  
 
THE 12Z NAM IS NOT TOO DIFFERENT OVER THE NEXT 12 HOURS (THROUGH  
13/06Z) BUT THEN BEGINS TO ACCELERATE THE SURFACE LOW FASTER TO  
THE NORTHEAST...WHICH WAS ITS TREND SINCE YESTERDAY'S MODEL RUNS.  
THE PREFERENCE IN THIS CASE IS TO REMAIN CLOSE TO THE 12Z GFS AND  
00Z ECMWF...WHICH TRACK VERY CLOSE TO THEIR ENSEMBLE MEANS.  
 
...LOW AMPLITUDE SHORTWAVE QUICKLY DIGGING FROM THE NRN ROCKIES  
SATURDAY TO THE LOWER MISS RVR VLY SUNDAY...  
...ROUNDING THE BROAD EASTERN NOAM TROUGH AND AIDING CYCLOGENESIS  
MONDAY OFFSHORE THE EAST COAST ALONG BAROCLINIC ZONE...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: BLEND OF 12Z GFS / 12Z ECMWF / 12Z CMC  
CONFIDENCE: AVERAGE  
 
---18Z UPDATE---  
NO SIGNIFICANT CHANGES ON THE 12Z MODEL CYCLE RELATIVE TO THE 00Z  
MODEL CYCLE...
SO THE PRELIMINARY PREFERENCE IS MAINTAINED.  
 
---PREV. DISCUSSION---  
THE 12Z NAM INITIALLY DIGS THIS SHORTWAVE FURTHER EAST THAN THE  
OTHER MODELS...AND AS SUCH IT REACHES THE BASE OF THE TROUGH  
FASTER AND KICKS A LITTLE FURTHER OUT INTO THE ATLANTIC. THIS IS  
UNSUPPORTED BY OTHER MODELS WHICH ARE GENERALLY IN GOOD AGREEMENT  
THROUGH SUNDAY MORNING (14/12Z). OTHERWISE...THE 00Z UKMET DIGS  
THE SHORTWAVE FURTHER SOUTH AND GENERALLY HAS LOWER HEIGHTS ALOFT  
ACROSS MUCH OF THE SOUTHEAST BY LATE SUNDAY. THIS MAY BE RELATED  
TO ITS MORE SOUTHERN POSITION OF THE UPPER LEVEL LOW OF THE GREAT  
LAKES AS WELL (WHICH IS ALSO NOT A PREFERRED SOLUTION AT THIS  
TIME). THEREFORE...THE PREFERENCE IS INSTEAD FOR A BLEND OF THE  
GFS...ECMWF...AND CMC WHICH ARE ALL IN FAIRLY GOOD AGREEMENT.  
 
...SHORTWAVE DIVING SOUTH FROM THE CANADIAN ARCTIC TO THE NORTHERN  
PLAINS BY LATE SUNDAY...   
..CLOSED LOW DEVELOPING ALOFT BY MONDAY OVER WRN GREAT LAKES
 
   
..ACCOMPANYING SURFACE LOW AND FRONT PUSHING INTO NRN CONUS
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: NON-UKMET BLEND  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
---18Z UPDATE---  
THE 12Z UKMET CONTINUES TO BE MUCH FURTHER SOUTH THAN THE OTHER  
MODELS WITH THE UPPER LEVEL LOW AS IT PUSHES INTO THE GREAT LAKES.  
THEREFORE...A NON-UKMET BLEND CONTINUES TO BE PREFERRED.  
 
---PREV. DISCUSSION---  
REGARDING THE EVENTUAL POSITION OF THE UPPER LEVEL LOW OVER THE  
GREAT LAKES...THE 00Z UKMET IS OUT OF ALIGNMENT WITH THE OTHER  
MODELS BY APPROXIMATELY 150-200MI (FURTHER SOUTH). ALTHOUGH THIS  
CANNOT BE RULED OUT AS A POSSIBILITY...IT DOES NOT CURRENTLY HAVE  
ANY SUPPORT FROM OTHER OPERATIONAL MODELS AND IS FURTHER SOUTH  
THAN ALL BUT ONLY ONE OR TWO GEFS/CMC/ECMWF ENSEMBLE MEMBERS.  
BECAUSE OF THAT IT IS EXCLUDED FROM THE PREFERENCE.  
OTHERWISE...MODEL DIFFERENCES ARE MINOR ENOUGH OVERALL  
(PARTICULARLY FOR A 60+ HOUR FORECAST AS THE TROUGH ARRIVES IN THE  
NORTHERN US) THAT A NON-UKMET GENERAL BLEND IS PREFERRED.  
 
   
..TROUGH GRADUALLY APPROACHING PACIFIC NORTHWEST BY MONDAY
 
 
...DEVELOPING BROAD SURFACE LOW OVER THE EASTERN PACIFIC BEGINNING  
TO IMPACT THE COAST BY LATE MONDAY...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: SLIGHTLY BELOW AVERAGE  
 
---18Z UPDATE---  
MODELS CONTINUE TO SHOW QUITE A BIT OF VARIABILITY ON THE EXACT  
POSITION OF THE TROUGH AND ASSOCIATED SURFACE LOW. THESE SOLUTIONS  
ALL STILL SEEM TO BE WITHIN THE ENSEMBLE SPREAD SO A GENERAL BLEND  
IS PREFERRED AT THIS TIME.  
 
---PREV. DISCUSSION---  
AS THE TROUGH BUILDS EAST TOWARD THE PACIFIC NORTHWEST  
COASTLINE...THERE IS BROAD AGREEMENT ON THE OVERALL TRENDS BUT  
DETAIL DIFFERENCES DO PRODUCE SOME DIFFERENT RESULTS. THE 12Z GFS  
BUILDS THE TROUGH EAST THE FASTEST AND AS A RESULT HAS A SURFACE  
LOW POSITION AT A MORE EASTERLY LONGITUDE BY MONDAY MORNING THAN  
ANY OF THE OTHER OPERATIONAL MODELS...OR THE GEFS AND ECMWF  
ENSEMBLE MEANS. AMONG THE OTHER MODELS THERE IS SOME LATITUDINAL  
VARIATION IN THE POSITION OF THE TROUGH AND ASSOCIATED SURFACE  
LOW. HOWEVER ALL THE OPERATIONAL MODELS ARE WELL WITHIN THE SPREAD  
ENVELOPE DEFINED BY MULTI-MODEL ENSEMBLE MEMBERS. THEREFORE... A  
GENERAL MODEL BLEND IS PREFERRED AT THIS TIME TO ACCOUNT FOR THE  
REASONABLE POSSIBILITIES AFFORDED BY THE DIFFERENT OPERATIONAL  
MODELS.  
 
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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