032  
FXUS10 KWNH 171831  
PMDHMD  
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
131 PM EST TUE DEC 17 2019  
 
VALID DEC 17/1200 UTC THRU DEC 21/0000 UTC  
 
...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR  
INGEST...  
 
12Z EVALUATION WITH PREFERENCE AND FORECAST CONFIDENCE  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
   
..EASTERN CONUS
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
EXCEPTION: 12Z NAM/ECMWF/UKMET WITH CANADIAN COASTAL LOW AFTER  
36HRS  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
19Z UPDATE: THE CMC CONTINUES TO BE SLIGHTLY DEEPER/COLDER/MORE  
CONCENTRIC...SO IT IS SLOWER WITH THE CANADIAN MARITIME SURFACE  
LOW DEVELOPMENT...SO LESS FAVORABLE THERE. SO WILL FAVOR A BLEND  
OF THE UKMET/NAM AND ECMWF OVER THE GFS/CMC.  
 
---PRIOR DISCUSSION---  
THE MODELS CONTINUE TO BE STRONGLY ALIGNED ACROSS WITH THE SURFACE  
WAVE AND HIGHLY ELONGATED VORTICITY STRIPE AND WARM CONVEYOR BELT  
REMAINING ALONG THE EAST COAST FOR THE REMAINDER OF THE DAY. THE  
VERY STRONG/COMPACT COLD AIR DESCENDING ACROSS ONTARIO INTO THE  
GREAT LAKES TOMORROW AND THROUGH NEW ENGLAND THURS IS ALSO FAIRLY  
WELL AGREED UPON. THE 12Z GFS REMAINS A TAD WEAKER AND THEREFORE  
A BIT FASTER WITH THE ASSOCIATED SHORTWAVE...BUT NOT SUBSTANTIALLY  
SO FOR A LATE DAY 2 INTO DAY 3 SPREAD IN THE CONUS. AS THE  
SURFACE CYCLONE DEVELOPS ALONG THE SE CANADIAN MARITIMES, THE GFS  
BEING FAST IS NORTH WITH THE DEVELOPING WAVE; WHILE THE SLIGHTLY  
COLDER SOLUTIONS OF THE CMC,UKMET AND 12Z NAM ARE SLIGHTLY SLOWER.  
THE 12Z GFS IS CORRECTING RELATIVE TO THE 00Z/06Z BUT STILL  
SHOULD BE REMOVED IF FORECASTING FOR CANADIAN MARITIME REGION.  
FOR THE CONUS AND US WATERS, HOWEVER, WILL CONTINUE SUPPORTING A  
GENERAL MODEL BLEND FOR THESE SYSTEMS AT ABOVE AVERAGE CONFIDENCE.  
 
...TROUGH OFF WEST COAST SPLITS THROUGH TO NORTH-CENTRAL AND  
SOUTHERN PLAINS...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
EXCEPTIONS: NON-NAM IN NORTHERN STREAM ON LATE DAY 3 IN NORTHERN  
PLAINS  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
19Z UPDATE: UKMET TRENDED SLOWER TO MATCH THE GFS, PLACING THE NAM  
WELL FASTER/STRONGER THAN THE REMAINING GUIDANCE ACROSS THE  
NORTHERN PLAINS ON DAY 3. AS FOR THE SOUTHERN STREAM, THE 12Z  
ECMWF TRENDED A BIT BROADER WITH THE REMAINING SHORTWAVE ENERGY  
WHICH IN TURN REDUCED THE SOUTHWARD DIG ACROSS NM INTO TX...AND  
NOW MATCHES THE LATITUDE OF THE REMAINING GUIDANCE ACROSS THE RED  
RIVER BY DAY 3...SO INCREASED CONFIDENCE IN A GENERAL MODEL BLEND  
IN THE SOUTHERN STREAM, THOUGH TYPICAL TIMING DIFFERENCES EXIST  
(GFS/NAM-FAST; EC/CMC-SLOW).  
 
---PRIOR DISCUSSION---  
GOES-WV SUITE DEPICT AN ELONGATED TROF ALONG/JUST WEST OF 130W AND  
APPEARS TO BE SHARPENING INTO A MORE CONSOLIDATED BASE. THIS  
WOULD FAVOR THE EVOLUTION OF THE UKMET/CMC AND ECMWF (ECENS/CMCE  
MEANS) COMPARED TO THE 12Z NAM/GFS THAT STILL HAVE STRONGER ENERGY  
(AND SURFACE REFLECTION) SLIGHTLY NORTH, BUT MOSTLY IN THE LOWER  
TROPOSPHERIC LEVELS/SURFACE HOWEVER, AS THE WAVE ENTERS THE  
WESTERN CONUS AND STRONG PACIFIC JET ADDS SPEED ENERGY/VIGOR TO  
THE TROUGHING TOWARD THE PACIFIC NW BY LATE WED INTO THURS. THE  
GUIDANCE STARTS TO ALIGN MUCH BETTER BUT STARTS SPLITTING THE  
FLOW. THE NORTHERN STREAM ENERGY EVOLVES INTO A MEDIUM WAVELENGTH  
SHORTWAVE WITH SOME NEGATIVE TILT ORIENTATION MOVING INTO THE  
NORTHERN PLAINS BY FRIDAY. THE 12Z NAM SHOWS SOME LATE-TERM  
OVER-AMPLIFICATION BIAS THAT TYPICALLY DOES NOT MANIFEST, THOUGH  
IS ALONG THE FASTER FRINGE OF THE GUIDANCE SUITE AS WELL. IT IS  
ACCOMPANIED BY THE UKMET, BUT THE 12Z GFS WHICH WAS PARTNERED  
SLOWED A BIT AND TRENDED TOWARD THE GEFS MEAN/TIMING. THIS  
GENERALLY MATCHES THE CMC/ECMWF/ECENS MEAN...SO WOULD FAVOR A  
TREND A NON-NAM SOLUTION IN THE NORTHERN STREAM.  
 
IN THE SOUTHERN STREAM, THE REMAINING SHORTWAVE ENERGY REMAINS  
CONSOLIDATED THROUGH SHRINKS IN SIZE AS IT PROGRESSES THROUGH THE  
FOUR CORNERS REGION ON THURSDAY. THE GUIDANCE CONTINUES TO  
TIGHTEN TOWARD A COMMON SOLUTION, ANCHORED BY CONSISTENT  
GEFS/ECENS MEANS. THE OPERATIONAL ECMWF SHOWS SOME NEGATIVE BIAS  
OF VERY SMALL BUT COMPACT VORTICITY CENTER THAT EVENTUALLY DIGS  
SOUTH, SLOWER ACROSS TX (AWAY FROM THE ECENS MEAN). THE 12Z GFS  
SLOWED SUBSTANTIALLY, BUT REMAINS NORTH ALONG THE RED RIVER VALLEY  
AT THE END OF DAY 3...MATCHING THE UKMET/NAM AND CMC ALONG WITH  
THE MEANS. AS SUCH A GENERAL MODEL BLEND WILL SUFFICE ACROSS THE  
SOUTHERN PORTION OF THE TROF. MANY SMALLER INTERNAL  
SHORTWAVE/VORTICITY CENTERS IN A HIGHLY ELONGATED/SHEARED  
ENVIRONMENT MOVING THROUGH COMPLEX TERRAIN IS A FAIRLY UNCERTAIN  
SMALL SCALE FORECAST TO LOCK DOWN BUT OVERALL CONFIDENCE IS  
SLIGHTLY ABOVE AVERAGE THROUGH THE LENGTH OF THE TROF, GIVEN THE  
AGREEMENT OVERALL IN THE MASS FIELDS.  
 
   
..PERSISTENT ONSHORE FLOW INTO THE PACIFIC NORTHWEST...
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
19Z UPDATE: LITTLE CHANGE HERE WITH THE AR; THOUGH THE  
ECMWF/UKMET/CMC TRENDED A BIT WETTER OVERALL (NARROWING THE PLUME  
SLIGHTLY). OTHERWISE, THE GENERAL MODEL BLEND PREFERENCE REMAINS  
SOLID AT SLIGHTLY ABOVE AVERAGE CONFIDENCE.  
 
---PRIOR DISCUSSION---  
IN THE WAKE OF THE ELONGATED TROF ENTERING THE PACIFIC COAST  
WED... A STRONG CLOSED LOW WILL PARK OVER THE GULF OF ALASKA AND  
DIRECT STRONG/PERSISTENT ONSHORE MOIST FLOW INTO THE PACIFIC  
NORTHWEST AS A STRONG ATMOSPHERIC RIVER EVENT. TIMING OF EMBEDDED  
SHORTWAVES ARE TOUGH TO PRECISELY LOCK DOWN, BUT THE LARGE SCALE  
MASS FIELDS ARE FAIRLY STRONGLY ALIGNED. THERE ARE SOME TYPICAL  
TIMING DIFFERENCES BETWEEN THE GFS/NAM/UKMET AND THE ECMWF/CMC  
MAINLY WITH RESPECT TO THE ARRIVAL OF THE FIRST SURGE OF MOISTURE,  
MAKING SHORTER-TERM (LESS THAN 6HR) QPF FORECASTS A BIT DIFFICULT  
BUT LONGER DURATION 12-24HR TOTALS ARE LIKELY TO RESULT IN  
SIGNIFICANT RAINFALL. WITH THE 12Z GFS SLOWING DOWN SIGNIFICANTLY  
WITH THE INITIAL SHORTWAVE/NOSE OF THE JET...THE TIMING IS BETTER  
OVERALL AND THE GFS COULD BE INCORPORATED BACK INTO THE  
PREFERENCE. THIS RESULTS IN A GENERAL MODEL BLEND PREFERENCE FOR  
THE MASS FIELDS...PLEASE SEE WPC QPF FORECAST GRIDS FOR  
PREFERENCES IN TIMING...BUT CONFIDENCE IS INCREASED TO SLIGHTLY  
ABOVE AVERAGE GIVEN SYNOPTIC BETTER AGREEMENT.  
 
MODEL TRENDS AT WWW.WPC.NCEP.NOAA.GOV/HTML/MODEL2.HTML  
500 MB FORECASTS AT WWW.WPC.NCEP.NOAA.GOV/H5PREF/H5PREF.SHTML  
 
GALLINA  
 

 
 
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