381   
FXUS10 KWNH 250755  
PMDHMD  
  
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
354 AM EDT WED SEP 25 2019  
  
VALID SEP 25/0000 UTC THRU SEP 28/1200 UTC  
  
...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR  
INGEST...  
  
00Z FINAL MODEL EVALUATION WITH PREFERENCES AND FORECAST CONFIDENCE  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
   
..NORTHWEST  
  
~~~~~~~~~~~~~~~~~~  
PREFERENCE: 00Z GFS/ECMWF/GEFS AND 12Z ECENS  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
  
07Z UPDATE:  
THROUGH 60 HOURS A GENERAL CONSENSUS REMAINS SUPPORTED BY LITTLE  
SPREAD, BUT AGAIN INTO D3 DISCREPANCIES ARISE, SOME OF WHICH ARE  
SIGNIFICANT. THE 00Z CMC AND UKMET HAVE BOTH MERGED TOWARDS THE  
PREVIOUS BLEND WITH THE EVOLUTION OF THE LONGWAVE TROUGH DIGGING  
INTO THE WEST. HOWEVER, BOTH OF THESE MODELS ARE FEATURE A LARGER  
BREADTH OF LONGITUDINAL WAVELENGTH LEADING TO LESS AMPLIFICATION  
AND A FLATTER TROUGH TOWARDS THE GREAT BASIN. THIS AGAIN APPEARS  
TO BE DUE TO A SLOWER POLE-WARD PUSH OF THE NORTHERN PACIFIC RIDGE  
LEADING TO A WEAKER 300MB JET STREAK DIVING TOWARDS THE PACNW. THE  
ECMWF IS ACTUALLY FEATURES THE DEEPEST TROUGH TILTED POSITIVELY  
INTO THE NORTHWEST. WHILE THIS IS A SUBTLE OUTLIER, IT REMAINS  
WELL WITHIN THE EXPECTED ENSEMBLE ENVELOPE BY D3 AND REMAINS IN  
THE BLEND.  
  
PREVIOUS DISCUSSION:  
THROUGH 60 HOURS (12Z FRIDAY) A GENERAL MODEL BLEND IS ACCEPTABLE  
WITH LESS WEIGHT ON THE CMC AS IT BEGINS TO DIVERGE FRIDAY  
MORNING. HOWEVER, THE MOST IMPACTFUL WEATHER AND LARGEST SPREAD  
BEGINS ON D3 /FRIDAY AND FRIDAY NIGHT/ AS AN ANOMALOUSLY DEEP  
CLOSED LOW DIGS INTO WASHINGTON/IDAHO/OREGON. THIS FEATURE WILL  
SPAWN SURFACE CYCLOGENESIS AND LEAD TO AN IMPRESSIVE EARLY SEASON  
WINTER STORM ACROSS THE NORTHERN ROCKIES. WHILE ALL GUIDANCE  
OUTSIDE THE CMC SUGGESTS THIS CLOSED LOW, THERE EXISTS  
CONSIDERABLE TIMING AND INTENSITY DIFFERENCES AMONG THE REST OF  
THE SUITE.  
  
EXCLUDING THE CMC WHICH IS VERY MUCH AN OUTLIER, THE UKMET BEGINS  
TO LAG THE REMAINING GLOBAL MODELS EARLY ON D3 WITH ITS SOUTHWARD  
PUSH OF THE CLOSED LOW, LIKELY DUE TO A MUCH WEAKER AND LESS  
AMPLIFIED 300MB JET DIVING EQUATOR-WARD FROM ALASKA AND INTO THE  
NORTH PACIFIC. THE NAM TAKES UP THE OTHER SIDE OF THE ENVELOPE  
BECOMING A FAST SOLUTION WITH THE SOUTHWARD PUSH OF THE CLOSED  
LOW. WHILE THE SPREAD BETWEEN THE FAST (NAM) AND SLOW (UKMET) IS  
NOT EXTREME, THE DIFFERENCES IN TIMING ARE IMPORTANT AS IT CHANGES  
THE PRECIPITATION ONSET FOR SNOWFALL ACROSS THE REGION. THE  
GFS/GEFS/ECMWF/ECENS ARE ALL IN VERY GOOD AGREEMENT WITH BOTH THE  
POSITION AND DEPTH OF THE UPPER LOW AT 12Z SATURDAY, AS WELL AS  
THE 850-700MB THERMAL EVOLUTION, AND FOR THIS REASON ARE PREFERRED  
AT THIS TIME.  
  
   
..REST OF CONUS  
  
~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND THROUGH 48 HOURS. NON-CMC  
THEREAFTER.  
CONFIDENCE: ABOVE AVERAGE  
  
07Z UPDATE:  
THE CMC REMAINS A FLAT OUTLIER WITH THE TROUGH-RIDGE PATTERN  
ACROSS THE CONUS, BUT OTHERWISE THERE EXISTS MINIMAL SPREAD AMONG  
THE GLOBAL SUITE.  
  
PREVIOUS DISCUSSION:  
THROUGH 48 HOURS THE GLOBAL SUITE FEATURES LITTLE SPREAD. THE  
GENERAL SYNOPTIC EVOLUTION IS FOR A CLOSED LOW ACROSS THE  
SOUTHWEST TO SLOWLY LIFT NORTHEAST AND BECOME ABSORBED IN THE  
WESTERLIES AHEAD OF THE AMPLIFYING NORTHWEST TROUGH BY DAY 3. IN  
THE EAST, A RIDGE WILL PERSIST WITH THE GREATEST HEIGHT ANOMALIES  
ACROSS THE SOUTHEAST. THIS FORCES THE SENSIBLE WEATHER AND  
GREATEST FORECAST CHALLENGES ACROSS THE NORTHERN TIER IN FAST FLOW  
PINCHED GENERALLY ABOVE 40N LATITUDE. OUTSIDE OF THE CMC WHICH IS  
VASTLY DIFFERENT IN ITS MID-LEVEL WAVE AMPLITUDE AND EVOLUTION, A  
SHORTWAVE DRIVING A COLD FRONT THROUGH THE MIDDLE PART OF THE  
COUNTRY BY D3 FEATURES LITTLE SPREAD IN QPF, PLACEMENT, OR TIMING.  
THE MINOR EXCEPTION IS THE ECMWF WHICH LAGS A BIT COMPARED TO THE  
CONSENSUS. WHILE IT CAN BE INCLUDED FOR NOW, THERE IS POTENTIAL  
WITH SIGNIFICANT QPF POSSIBLE INTO D3 AND BEYOND THAT THE SLOWER  
SOLUTION COULD POSE MORE OF A FLOODING PROBLEM TOWARDS THE END OF  
THE PERIOD. LOW FORECAST SKILL IN THAT AT THIS TIME RANGE, AND LOW  
CONFIDENCE AS WELL, BUT SHOULD BE MENTIONED FOR POSSIBLE SLIGHTLY  
LESS WEIGHT ON THE ECMWF AT D3.  
  
MODEL TRENDS AT WWW.WPC.NOAA.GOV/HTML/MODEL2.HTML  
500 MB FORECASTS AT WWW.WPC.NOAA.GOV/H5PREF/H5PREF.SHTML  
  
WEISS  
  
  
 
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