496  
FXUS10 KWNH 071710  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1210 PM EST WED MAR 07 2018  
 
VALID MAR 07/1200 UTC THRU MAR 11/0000 UTC  
 
...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR  
INGEST...  
 
12Z NAM/GFS EVALUATION WITH PRELIMINARY PREFERENCES  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
 
 
 
...SERIES OF SHORTWAVES IN THE VICINITY OF THE NORTHEAST U.S.  
CLOSED LOW...   
..COASTAL LOW IN THE VICINITY OF NEW ENGLAND THROUGH FRIDAY
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
THERE HAS BEEN CONVERGENCE WITH THE SURFACE LOW COMPARED TO  
YESTERDAY'S MODELS, WITH THE GFS MOVING EAST AND THE ECMWF MOVING  
WEST. SOME MINOR FLIP FLOPPING HAS BEEN OBSERVED AND CAN CONTINUE  
TO BE EXPECTED THROUGH THURSDAY NIGHT AS THE SURFACE LOW TRACKS UP  
THE NEW ENGLAND COAST TONIGHT AND TOMORROW BEFORE WRAPPING BACK  
INLAND TO THE WEST BY FRIDAY MORNING. HOWEVER, THE LATEST  
DETERMINISTIC MODELS AND ENSEMBLE SCATTER LOW PLOTS SHOW GOOD  
AGREEMENT, ENOUGH TO SUPPORT A GENERAL MODEL BLEND FOR THE  
PREFERENCE.  
 
   
..SECONDARY SURFACE LOW NEAR NOVA SCOTIA FRIDAY NIGHT
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 12Z GFS / 00Z ECMWF BLEND  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
A FAIR DEGREE OF SPREAD REMAINS IN THE ENSEMBLE WITH ANOTHER  
SURFACE LOW FORECAST TO DEVELOP IN THE WESTERN ATLANTIC EARLY  
FRIDAY MORNING, ABOUT 500 MI SOUTH OF NOVA SCOTIA, WITH THE LOW  
EXPECTED TO TRACK NORTH BEFORE TRACKING WEST TOWARDS NEW BRUNSWICK  
ON SATURDAY. WHILE THE ENSEMBLE SPREAD REMAINS, THE 00Z CMC IS A  
DETERMINISTIC OUTLIER WITH ITS LOW POSITION ON SATURDAY, WELL TO  
THE EAST OF THE 12Z NAM, 12Z GFS, 00Z ECMWF AND 00Z UKMET. AMONG  
THE NON-00Z CMC GUIDANCE, THE 12Z NAM AND 00Z UKMET ARE WEST OF  
THE LATEST ENSEMBLE SPREAD WHICH HAS NOT SHOWN MOVEMENT TOWARD THE  
WEST AS SEEN IN THE 12Z NAM AND 00Z UKMET, SO A 12Z GFS AND 00Z  
ECMWF BLEND IS PREFERRED AT THIS TIME.  
 
...PAIR OF SHORTWAVES CONVERGING ON THE NORTHWEST  
THURSDAY/THURSDAY NIGHT...   
..COLD FRONT(S) IMPACTING THE WESTERN U.S. THROUGH FRIDAY
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: NON 00Z CMC BLEND  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
THE FIRST IN A PAIR OF SHORTWAVE TO IMPACT THE U.S. IS EXPECTED TO  
WEAKEN AS IT REACHES THE PACIFIC NORTHWEST ON THURSDAY. THERE IS  
GOOD AGREEMENT WITH THIS IDEA IN THE LATEST MODEL GUIDANCE WITH A  
GENERAL MODEL BLEND PREFERRED. THE NEXT SHORTWAVE IS FORECAST TO  
DROP SOUTH DOWN THROUGH BRITISH COLUMBIA FRIDAY MORNING. ONLY THE  
00Z CMC DIFFERS SIGNIFICANTLY HERE WITH ITS HANDLING OF VORTICITY  
WITHIN THE BASE OF THE SHORTWAVE TROUGH OVER WASHINGTON, WHICH  
LAGS THE NORTHERN PORTION OF THE TROUGH ADVANCING THROUGH  
SOUTH-CENTRAL CANADA. A NON 00Z CMC BLEND IS PREFERRED WITH THIS  
SYSTEM, AND FURTHER IMPACTS FROM THIS FEATURE ARE DISCUSSED IN THE  
BELOW SECTION.  
 
...SHORTWAVE TROUGH MOVING INTO THE CENTRAL/NORTHERN PLAINS AT  
SOME POINT ON SATURDAY...  
...POSSIBLE SURFACE LOW DEVELOPING OVER THE SOUTHERN PLAINS LATE  
SATURDAY...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 12Z GFS / 00Z ECMWF BLEND  
OR 06Z GEFS MEAN/00Z ECMWF MEAN BLEND  
CONFIDENCE: BELOW AVERAGE  
 
AS THE SHORTWAVE ABOVE ADVANCES SOUTHEASTWARD FROM THE  
NORTHWESTERN U.S. FRIDAY NIGHT INTO SATURDAY, MODEL DIFFERENCES  
BECOME LARGE. THE 12Z GFS HAS BEEN CONSISTENT WITH A STRONGER  
SHORTWAVE OVER THE OK/AR BORDER BY SATURDAY NIGHT. TAKING A LOOK  
AT SPAGHETTI HEIGHTS FROM THE ENSEMBLES, SPREAD OVER THE PAST 4  
12/00Z CYCLES HAS ONLY NARROWLY REDUCED, BUT THE 00Z UKMET AND 12Z  
GFS APPEAR TOWARD THE STRONGER SIDE OF THE ENSEMBLE SPREAD, WHILE  
THE 00Z ECMWF AND 00Z CMC ARE FLATTER OVER THE SOUTH VALID 00Z/11.  
 
VALID 00Z/11, ENSEMBLE SCATTER LOW PLOTS SHOW A BROAD ENVELOPE OF  
LOWS EXTENDING FROM WEST-CENTRAL TEXAS INTO CENTRAL ARKANSAS, WITH  
A MID-POINT NEAR THE EASTERN TEXAS/OKLAHOMA BORDER. IT IS HERE  
THAT THE PREFERENCE LIES, WHICH IS BETWEEN THE STRONGER 12Z GFS  
AND WEAKER 00Z ECMWF. THE THINKING IS THAT VERIFICATION WILL BE  
SOMEWHERE IN THE MIDDLE, WHICH IS SUPPORTED RELATIVELY WELL BY THE  
06Z GEFS MEAN AND 00Z ECMWF MEAN.  
 
...WEAK SHORTWAVE APPROACHING CALIFORNIA FROM THE WEST SATURDAY  
EVENING...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 12Z NAM, 12Z GFS, 00Z ECMWF BLEND  
CONFIDENCE: AVERAGE  
 
THE PAST 3 ENSEMBLE SPAGHETTI PLOTS HAVE MEANDERED ABOUT A COMMON  
TIMING WITH THIS FEATURE AS A FLAT SHORTWAVE TROUGH APPROACHES THE  
WEST COAST SATURDAY EVENING. THE 00Z ECMWF MEAN APPEARS TO FIT THE  
MIDDLE OF THE TRENDS AND LATEST MEMBERS WELL, WHICH IS BEST  
SUPPORTED BY A BLEND OF THE 12Z GFS, 12Z NAM AND 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  
 
OTTO  
 

 
 
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