802  
FXUS10 KWNH 181701  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
100 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 NAM/GFS MODEL EVALUATION ALONG WITH PRELIMINARY 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: 00Z ECMWF/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 DAM HEIGHT  
LINE THROUGH SATURDAY. CURRENTLY...A BLEND OF THE 00Z ECMWF/UKMET  
APPEAR TO BE BEST FITTED TO THE CENTER OF THE ENSEMBLE MEMBERS  
WITH THE 12Z NAM AND 00Z CMC MUCH FLATTER/WEAKER THAN IDEAL.  
ACROSS THE CENTER OF THE COUNTRY...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 00Z ECMWF 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 00Z CMC BLEND  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
THE 00Z CMC WAS MUCH MORE AMPLIFIED WITH THE SHORTWAVE APPROACHING  
BRITISH COLUMBIA COMPARED TO THE REMAINING DETERMINISTIC/ENSEMBLE  
GUIDANCE AND GIVEN A LACK OF SUPPORT FOR THE 12Z CMC...AND GOOD  
AGREEMENT OTHERWISE...A NON 00Z CMC BLEND IS PREFERRED.  
 
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