384  
FXUS10 KWNH 181849  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
249 PM EDT WED OCT 18 2017  
 
VALID OCT 18/1200 UTC THRU OCT 22/0000 UTC  
 
...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR  
INGEST...  
 
12Z MODEL EVALUATION ALONG WITH FINAL PREFERENCES  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
NAM/GFS INITIALIZATION ERRORS DO NOT SEEM TO DEGRADE THEIR SHORT  
RANGE FORECASTS.  
 
...FAST MOVING SHORTWAVE TRACKING ACROSS SRN CANADA  
TODAY...CLIPPING NRN NEW ENGLAND THU EVENING...   
..ASSOCIATED COLD FRONT CROSSING THE NORTHEAST BY WEEKS END
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL COMPROMISE  
CONFIDENCE: ABOVE AVERAGE  
 
THE MODELS SHOW SIMILARLY WITH THIS SYSTEM.  
 
...ONGOING ATMOSPHERIC RIVER EVENT INTO THE WEST COAST AHEAD OF  
LARGE UPPER TROUGH MOVING INTO THE WEST THROUGH FRI...   
..COLD FRONT REACHING THE PLAINS FRI NIGHT
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 12Z ECMWF/00Z UKMET BLEND  
CONFIDENCE: AVERAGE  
 
THROUGH 00Z/21...SLOWER/FASTER MODELS FROM PREVIOUS CYCLES HAVE  
CONVERGED ONTO A COMMON SOLUTION WITH THE LATEST  
DETERMINISTIC/ENSEMBLE GUIDANCE SHOWING GOOD AGREEMENT WITH THE  
TIMING/AMPLITUDE OF THE UPPER TROUGH AS IT MOVES THROUGH THE  
INTERMOUNTAIN WEST. DIFFERENCES APPEAR AS THE SHORTWAVE ADVANCES  
BEYOND THE ROCKIES INTO THE NRN PLAINS AND SRN CANADA WITH THE 12Z  
GFS ON THE DEEPER SIDE OF THE GUIDANCE. THE LATEST ENSEMBLES SHOW  
FAIRLY GOOD RUN TO RUN AGREEMENT IN THE LOW PLOTS ALTHOUGH A  
MODERATE AMOUNT OF SPREAD REMAINS IN THE 500 MB 546/564 DAM HEIGHT  
LINES THROUGH SATURDAY. CURRENTLY...A BLEND OF THE 12Z ECMWF/00Z  
UKMET APPEARS TO BE BEST FITTED TO THE CENTER OF THE ENSEMBLE  
MEMBERS WITH THE 12Z NAM MUCH FLATTER/WEAKER THAN IDEAL. THE 12Z  
UKMET/CMC APPEAR A BIT FARTHER SOUTH WITH THE VORTICITY MAX TOWARD  
THE SRN END OF THE TROUGH AXIS MOVING THROUGH THE PLAINS BY LATE  
SATURDAY COMPARED TO THE LATEST AGREEABLE ENSEMBLE MEANS. THE  
TIMING OF THE COLD FRONT IS SIMILAR IN THE LATEST DETERMINISTIC  
GUIDANCE.  
 
   
..SRN STREAM UPPER TROUGH REACHING THE SRN PLAINS ON FRI
 
OUT  
AHEAD OF THE UPPER TROUGH REFERENCED IN THE ABOVE SECTION...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL COMPROMISE  
CONFIDENCE: AVERAGE  
 
THE 12Z CMC IS SLIGHTLY FASTER WHILE THE 12Z NAM IS SLIGHTLY  
SLOWER THAN THE REMAINING DETERMINISTIC GUIDANCE AS THE UPPER  
LEVEL TROUGH REACHES THE PLAINS ON FRI. OVERALL HOWEVER...THE  
DIFFERENCES ARE SMALL ENOUGH TO PREFER A GENERAL MODEL COMPROMISE.  
 
...SHORTWAVE REACHING BRITISH COLUMBIA LATE SAT WITH WARM FRONT  
AND ATMOSPHERIC RIVER APPROACHING THE PACIFIC NORTHWEST LATE  
SAT...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: NON 12Z CMC BLEND  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
THE 12Z CMC ADJUSTED WEAKER FROM ITS 00Z RUN WHICH WAS MUCH MORE  
AMPLIFIED WITH THE SHORTWAVE APPROACHING BRITISH COLUMBIA COMPARED  
TO THE REMAINING DETERMINISTIC/ENSEMBLE GUIDANCE. HOWEVER...THE  
12Z CMC REMAINS A BIT DEEPER AND FASTER WITH THE SHORTWAVE  
COMPARED TO THE REMAINING 12Z GUIDANCE. GIVEN A LACK OF SUPPORT  
FOR THE 12Z CMC...AND GOOD AGREEMENT OTHERWISE...A NON 12Z CMC  
BLEND IS PREFERRED.  
 
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