107  
FXUS10 KWNH 071855  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
155 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 MODEL EVALUATION WITH FINAL 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 OVERALL AND THE ECMWF  
GENERALLY 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 / 12Z 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. THE 12Z  
CMC ADJUSTED WEST TOWARD THE REMAINING MODEL CLUSTERING, BUT STILL  
LIES FARTHER NORTH AND EAST SATURDAY EVENING. AMONG THE NON-CMC  
GUIDANCE, THE 12Z NAM AND 12Z UKMET ARE SIMILAR TO ONE ANOTHER  
LYING WEST OF THE LATEST ENSEMBLE SPREAD WHICH HAS NOT SHOWN  
MOVEMENT TOWARD THE WEST AS SEEN IN THE 12Z NAM/UKMET, SO A 12Z  
GFS AND 12Z ECMWF BLEND IS PREFERRED AT THIS TIME. THE 12Z ECMWF  
DID NOT DEVIATE MUCH FROM ITS PREVIOUS CYCLE.  
 
...PAIR OF SHORTWAVES CONVERGING ON THE NORTHWEST  
THURSDAY/THURSDAY NIGHT...   
..COLD FRONT(S) IMPACTING THE WESTERN U.S. THROUGH FRIDAY
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: NON 12Z 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 12/00Z 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 12Z  
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/ECMWF/CMC BLEND  
OR 12Z GEFS MEAN/00Z ECMWF MEAN BLEND  
CONFIDENCE: BELOW AVERAGE  
 
AS THE SHORTWAVE ABOVE ADVANCES SOUTHEASTWARD FROM THE  
NORTHWESTERN U.S. FRIDAY NIGHT INTO SATURDAY (REFERENCED ABOVE),  
MODEL DIFFERENCES BECOME LARGE. THE 12Z GFS HAS BEEN CONSISTENT  
WITH A STRONGER SHORTWAVE OVER THE OK/AR BORDER BY SATURDAY NIGHT  
AND SOME TRENDS HAVE MOVED IN THIS DIRECTION FROM THE 12Z ECMWF,  
UKMET AND CMC. TAKING A LOOK AT SPAGHETTI HEIGHTS FROM THE  
ENSEMBLES, SPREAD OVER THE PAST 4 12/00Z CYCLES HAS ONLY NARROWLY  
REDUCED, BUT THE 12Z UKMET AND 12Z GFS APPEAR TOWARD THE STRONGER  
SIDE OF THE ENSEMBLE SPREAD, WHILE THE 12Z ECMWF/NAM AND ARE  
FLATTER OVER THE SOUTH VALID 00Z/11. THE 12Z CMC SPLITS THE  
DIFFERENCE TO SOME DEGREE WITH THIS FEATURE.  
 
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 12Z ECMWF ALONG WITH THE 12Z CMC. THE THINKING IS THAT  
VERIFICATION WILL BE SOMEWHERE IN THE MIDDLE, WHICH IS SUPPORTED  
RELATIVELY WELL BY THE 12Z GEFS MEAN AND 00Z ECMWF MEAN.  
 
...WEAK SHORTWAVE APPROACHING CALIFORNIA FROM THE WEST SATURDAY  
EVENING...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: NON-12Z UKMET 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 12Z MODELS HAVE COME INTO BETTER  
AGREEMENT ON THE TIMING OF THE WAVE, BUT DIFFER ON THE AMPLITUDE.  
FOR EXAMPLE, THE 12Z UKMET IS WEAKER WITH THIS FEATURE BUT THE  
REMAINING DETERMINISTIC/ENSEMBLE CONSENSUS SHOWS BETTER SUPPORT  
FOR A NON 12Z UKMET BLEND. 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/GFS/ECWMF AND 12Z CMC.  
 
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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