690  
FXUS10 KWNH 201722  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
122 PM EDT FRI OCT 20 2017  
 
VALID OCT 20/1200 UTC THRU OCT 24/0000 UTC  
 
...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR  
INGEST...  
 
12Z NAM/GFS EVALUATION ALONG WITH PRELIMINARY PREFERENCES  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
 
...SOUTHERN STREAM UPPER TROUGH REACHING THE UPPER MISSISSIPPI  
VALLEY TONIGHT FROM THE SOUTHERN PLAINS...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL COMPROMISE  
CONFIDENCE: ABOVE AVERAGE  
 
THE MODELS SHOW SIMILARLY WITH THIS SYSTEM.  
 
...LONGWAVE TROUGH EVOLUTION INTO THE WESTERN U.S. THROUGH TONIGHT  
AND EJECTING OUT INTO THE PLAINS ON SAT ALONG WITH COLD FRONT...   
..SURFACE LOW DEEPENING OVER SOUTHERN CANADA
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: NON 12Z NAM BLEND THROUGH 00Z/22  
00Z ECMWF/UKMET BLEND 00Z/22 - 00Z/24  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
THE MODELS ARE IN DECENT AGREEMENT THROUGHOUT THE FIRST 36-48  
HOURS BUT SOME DIFFERENCES ARE WORTH NOTING BEYOND THAT TIME. THE  
12Z NAM IS A BIT FASTER THAN THE REMAINING CONSENSUS STARTING  
EARLY IN THE FORECAST AND THE 12Z GFS HAS CONTINUED ITS WEAKENING  
TREND WITH THE MID-LEVEL REFLECTION OF THE TROUGH AXIS ADVANCING  
INTO S-CNTRL CANADA. THE 12Z GFS REMAINS SLIGHTLY STRONGER AND A  
BIT FASTER WITH THE TROUGH WHILE THE BETTER ENSEMBLE CONSENSUS IS  
CLOSER TO A BLEND OF THE 00Z ECMWF/UKMET BEYOND 00Z/22.  
 
...SOUTHERN STREAM LOWER MS VALLEY CLOSED LOW DEVELOPMENT BY MON  
ALONG WITH SURFACE LOW/COLD FRONT...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: NEAR THE 00Z ECMWF  
CONFIDENCE: AVERAGE  
 
BOTH THE ENSEMBLE AND DETERMINISTIC GUIDANCE HAS SHOWN SOME FLIP  
FLOPPING BETWEEN FASTER AND SLOWER BY SUN-MON ACROSS THE LOWER  
MISSISSIPPI VALLEY. THE 12Z GFS HAS TRENDED FASTER FROM ITS 00Z  
RUN BUT IT LIES TOWARD THE FASTER EDGE OF THE LATEST ENSEMBLE  
SPREAD LOOKING AT THE 500 MB 570 DAM HEIGHTS. THERE IS SOME  
INFLUENCE OF THIS FEATURE BY THE END OF THE SHORT RANGE GIVEN AN  
UPSTREAM TROUGH AMPLIFYING ACROSS THE N-CNTRL U.S. BUT UNTIL  
THEN...THE CUT OFF LOW FORECAST BY A MAJORITY OF THE MODELS MAY BE  
SLOW TO PROGRESS TOWARD THE EAST. THE 00Z CMC IS A FAST OUTLIER  
HERE AND THE 12Z NAM IS TOWARD THE SLOWER SIDE OF THE LATEST  
DETERMINISTIC GUIDANCE. GIVEN THE FLIP FLOPPING OBSERVED...THE  
CLOSEST MODEL TO THE MIDDLE OF THE LATEST ENSEMBLE GUIDANCE IS THE  
00Z ECMWF...BUT A NUDGE SLOWER TOWARD THE 00Z UKMET IS PREFERRED  
GIVEN THE 00Z ECMWF MEAN IS SLOWER THAN THE 00Z ECMWF AND THE  
00Z/06Z GEFS MEAN IS SLOWER THAN THE 12Z GFS.  
 
...SHORTWAVE REACHING BRITISH COLUMBIA SAT WITH WARM FRONT  
EXTENDING SOUTH NEAR THE PACIFIC NORTHWEST...   
..ATMOSPHERIC RIVER ARRIVING OVER THE PACIFIC NORTHWEST SAT
 
   
..UPPER TROUGH SWEEPING ACROSS THE NORTHERN PLAINS MON
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 00Z ECMWF/UKMET BLEND  
CONFIDENCE: AVERAGE  
 
THE 00Z UKMET STANDS OUT THE GREATEST WITH MORE RIDGING ACROSS THE  
WEST COAST ON SAT WHICH CAUSES THE ATMOSPHERIC RIVER PLUME TO BE  
AIMED A BIT FARTHER NORTH INTO THE PACIFIC NORTHWEST.  
OTHERWISE...THE MODELS ARE IN GOOD AGREEMENT HERE.  
DOWNSTREAM...THE TROUGH REACHES THE NRN PLAINS WITH SOME TIMING  
AND AMPLITUDE DIFFERENCES. AN EYEBALLING OF THE LATEST ENSEMBLE  
GUIDANCE SUGGESTS A PROGRESSION BETWEEN THE SLOWER 00Z ECMWF AND  
FASTER 00Z UKMET IS BEST. THE 12Z GFS APPEARS A BIT TOO FAST BY  
00Z/24 WITH THE SURFACE LOW TRACKING THROUGH SRN ONTARIO...WITH  
ENSEMBLE SCATTER LOW PLOTS SUPPORTING A POSITION BETWEEN THE 00Z  
ECMWF/UKMET.  
 
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WWW.WPC.NCEP.NOAA.GOV/H5PREF/H5PREF.SHTML  
 
OTTO  
 

 
 
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