589  
FXUS10 KWNH 201848  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
247 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 MODEL EVALUATION ALONG WITH FINAL 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  
12Z ECMWF/UKMET/CMC 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 12Z ECMWF/UKMET/CMC BEYOND 00Z/22.  
 
...SOUTHERN STREAM LOWER MS VALLEY CLOSED LOW DEVELOPMENT BY MON  
ALONG WITH SURFACE LOW/COLD FRONT...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: NEAR THE 12Z 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 AND 12Z UKMET TRENDED FASTER FROM  
THEIR 00Z RUNS BUT LIE TOWARD THE FASTER EDGE OF THE LATEST  
ENSEMBLE SPREAD LOOKING AT THE 500 MB 570 DAM HEIGHTS. THE 12Z CMC  
SLOWED SIGNIFICANTLY FROM ITS PREVIOUSLY FAST OUTLIER SOLUTION AND  
NOW LIES NEAR THE 00Z/12Z ECMWF. ONLY VERY MINOR ADJUSTMENTS WERE  
SEEN IN THE 12Z ECMWF COMPARED TO ITS 00Z RUN.  
 
THERE WILL BE 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 12Z NAM IS  
THE SLOWEST OF THE 12Z DETERMINISTIC GUIDANCE. GIVEN THE FLIP  
FLOPPING OBSERVED...THE CLOSEST MODEL TO THE MIDDLE OF THE LATEST  
ENSEMBLE GUIDANCE IS THE 12Z AND 00Z ECMWF...BUT A NUDGE SLOWER  
TOWARD THE 00Z UKMET IS PREFERRED GIVEN THE 00Z ECMWF MEAN IS  
SLOWER THAN THE 00Z/12Z 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: 12Z ECMWF/UKMET/CMC BLEND  
CONFIDENCE: AVERAGE  
 
DESPITE TRENDING WEAKER...THE 12Z UKMET STILL STANDS A BIT 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 12Z  
ECMWF AND FASTER 12Z UKMET IS BEST. THE 12Z CMC IS IN THE MIDDLE  
TO SLOWER SIDE OF THE ECMWF/UKMET. 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 12Z ECMWF/UKMET/CMC.  
 
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OTTO  
 

 
 
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