359  
FXUS10 KWNH 201838  
PMDHMD  
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
137 PM EST WED DEC 20 2017  
 
VALID DEC 20/1200 UTC THRU DEC 24/0000 UTC  
 
...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR  
INGEST...  
 
12Z MODEL EVALUATION INCLUDING PREFERENCES AND FORECAST CONFIDENCE  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
 
...FILLING WAVE ENTERING TN VALLEY TODAY AND EXITING EAST COAST  
THURS...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: NON-CMC BLEND  
CONFIDENCE: ABOVE AVERAGE  
 
GOES-16 AIRMASS RGB SHOWS THE STEADY FILLING OF THE UPPER LOW  
ACROSS THE LOWER MS VALLEY MOVING INTO TN. GUIDANCE CONTINUES TO  
SHOW GRADUAL WEAKENING AS IT TRANSLATES ACROSS THE SOUTH INTO THE  
CAROLINAS BY TOMORROW MORNING BEFORE EXITING. THE 12Z NAM AND GFS  
BOTH SLOWED AND SHOWED A SLIGHTLY DEEPER INFLECTION IN LINE WITH  
CURRENT SET UP/PLACEMENT AND STRENGTH OF THE INTERNAL S/W THIS  
PUTS IT MORE INLINE WITH THE 00Z ECMWF/UKMET. THE 00Z CMC WHILE  
NOT POORLY PLACED IS A BIT WEAKER OVERALL AND IS QUICKER TO  
DEVOLVE IT. GIVEN THE STRONGER ALIGNMENT A NON-CMC BLEND SEEMS  
PRUDENT FOR A VERY HIGH CONFIDENCE BLEND. INCLUSION OF THE CMC IS  
NOT LIKELY A TERRIBLE PROSPECT...BUT CONFIDENCE IS HIGHER WITHOUT  
IT.  
 
19Z UPDATE: LITTLE CHANGE WITH ONLY TIGHTER AGREEMENT EXCLUDING  
THE CMC WHICH REMAINS SLIGHTLY WEAKER/FASTER TO FILL. AS SUCH  
WILL KEEP INITIAL PREFERENCE/CONFIDENCE.  
 
...AMPLIFYING UPPER TROUGH AXIS ACROSS NORTHWEST US THIS EVENING  
REACHING THE FOUR CORNERS ON THUR...AND ELONGATING FROM WESTERN  
GREAT LAKES TO NM/OLD MEXICO BY FRI WITH...SURFACE CYCLONE/TROF  
CROSSING THE LOWER MS/TN VALLEYS FRI/SAT...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 12Z GFS/ECMWF BLEND  
CONFIDENCE: AVERAGE  
 
GOES-16 AIRMASS RGB SHOWS A BROAD PINK ZONE ASSOCIATED WITH THE  
DEEPENING TROF ACROSS THE NORTHWEST WITH A SLIGHT SPLIT IN STREAMS  
NOTED JUST NORTH OF THE US/CANADIAN BORDER. THIS SPLIT WILL  
BECOME MORE PROMINENT WITH TIME THOUGH BOTH REMAIN FAIRLY WELL  
PHASED UNTIL LATE THURS WHEN THE OVERALL TROF BEGINS STRETCH  
POSITIVELY...PLEASE SEE MORE DETAILS AFTER THIS TIME IN SECTION  
BELOW. THE FOCUS SHIFTS TO THE INNER CORE OF THE DEVELOPING UPPER  
LOW AS IT DESCENDS TOWARD THE FOUR CORNERS. THE ENSEMBLES  
CONTINUE TO SHOW A SLIGHTLY SLOWING OF THE TREND AND GREATER  
BREADTH TO THE WAVE IN THE SOUTHWEST INTO FRIDAY WITH A FEW  
LINGERING CLOSED/MORE SYMMETRIC SOLUTIONS BEST REPRESENTED BY THE  
00Z CMC THAT CLOSE OFF IN NORTHERN CHIHUAHUA/WESTERN TX PANHANDLE.  
GIVEN SETUP WITH DOWNSTREAM RIDGING ACROSS GULF AND FAIRLY STRONG  
SWLY FLOW OUT OF EASTERN PACIFIC AND DEEPENING COLD POOL/TROF  
FURTHER NORTH...THINKING A MORE PROGRESSIVE SOLUTION SEEMS LIKELY.  
THIS IS PRESENTED BY THE 00Z ECMWF AND UKMET. THE 12Z NAM IS  
ALSO PROGRESSIVE BUT ALSO SHOWS A MORE CONSOLIDATED POTENTIALLY  
VORTICITY STRETCHING/TIGHTENING ON THE LEE OF THE SACRAMENTO  
MOUNTAINS BEFORE DEVOLVING INTO THE LARGER SCALE SYSTEM THROUGH  
THE LOWER MS VALLEY. WHILE POSSIBLE THIS SEEMS A BIT TOO SPECIFIC  
FOR THIS FAR OUT IN TIME AND IT SHOWS SOME TYPICAL KNOWN NEGATIVE  
BIAS OF THE NAM BY DAY 3 TO NOT FAVOR IT. THE 12Z GFS SHIFTED  
BACK A BIT FASTER THAN THE 06Z RUN EMERGING FROM THE SOUTHERN  
ROCKIES INTO THE PLAINS...THIS IS A SHADE FASTER THAN THE ECMWF  
BUT NOT AS FAST AS THE UKMET. WHILE THE 06Z GEFS AND 00Z ECENS  
MEANS BOTH LAG THEIR OPERATIONAL SOLUTIONS THE OVERALL SETUP  
FAVORS SLIGHT FASTER EMERGENCE AS PRESENTED BY THE 12Z GFS/00Z  
ECMWF SOLUTIONS. AS SUCH WILL FAVOR THEM AT AVERAGE CONFIDENCE  
GIVEN THE MODEST SPREAD.  
 
19Z UPDATE: LITTLE CHANGE WITH THE 12Z UKMET STILL FASTER EMERGING  
OUT INTO THE SOUTHERN PLAINS AND THROUGH THE TN VALLEY. THE 12Z  
ECMWF SHOWED A MORE SUBSTANTIAL SHIFT...MAINLY IN WEAKER OVERALL  
UPPER LOW...MORE IN LINE WITH THE 12Z GFS. IN FACT IT IS A BIT  
FASTER IN EJECTING THE WAVE THAN THE GFS. THE HEAVY RAINFALL  
IMPACTS ONLY SEEM TO FOCUS TIGHTER WITH THE AXIS OF MOISTURE FLUX  
THROUGH THE WESTERN GULF AS WELL AS STRONG FETCH OFF THE EASTERN  
PACIFIC ACROSS THE MX PLATEAU. THIS TIGHTNESS/CONTINUITY WITH THE  
RECENT RUNS OF THE DETERMINISTIC GFS/ECMWF SUPPORT GROWING  
CONFIDENCE EVEN THOUGH THE SPREAD REMAINS FAIRLY LARGE (CMC/ECENS  
MEMBERS SOME 12Z GEFS MEMBERS STILL SLOW MORE CLOSED/SYMMETRIC).  
 
   
..NORTHERN STREAM PORTION OF TROF AXIS
 
SURFACE WAVE FROM CO/KS  
THURS TO NEW ENGLAND LATE FRI/EARLY SAT...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: NON-NAM BLEND  
CONFIDENCE: AVERAGE  
 
AS MENTIONED IN SECTION ABOVE...THE NORTHERN STREAM ACCELERATES  
AND DECOUPLES FROM THE MAIN CORE OF THE UPPER LOW LATE THURS INTO  
FRIDAY CROSSING INTO THE GREAT LAKES. HOWEVER...THE HEIGHT FALLS  
SPUR A WEAK SURFACE WAVE ALONG THE INVERTED SFC TROF THAT SLOWLY  
WEAKENS INTO THE GREAT LAKES/NEW ENGLAND BY FRIDAY. THE 12Z NAM  
SHOWS A BIT GREATER AMPLIFICATION/CONNECTION TO THE SOUTHERN  
STREAM AND ALLOWS FOR A DEEPER WAVE CROSSING THE MID-MS VALLEY  
INTO OHIO VALLEY AS WELL AS THE COASTAL REDEVELOPMENT IN THE GULF  
OF ME. THE 00Z CMC SEEMS A BIT SLOW AND A BIT NORTH OF THE OTHER  
GUIDANCE BUT NOT TERRIBLY SO. THE 12Z GFS CONTINUES TO BE SLIGHTLY  
FASTER THAN THE 00Z ECMWF BUT OVERALL THE ORIENTATION AND  
MAGNITUDES OF SENSIBLE WEATHER SEEM MODEST ENOUGH TO SUPPORT A  
NON-NAM BLEND HOWEVER...DIFFERENCES IN LOWER LEVEL THERMAL  
PROFILES ALLOW FOR SOME REDUCTION IN CONFIDENCE IN THIS BLEND YET  
IS STILL AVERAGE OVERALL.  
 
19Z UPDATE: 12Z UKMET/CMC AND ECMWF SHOW STRONG SIMILARITIES TO  
THEIR PRIOR RUNS AND WHILE THE 12Z UKMET IS A BIT DEEPER/FURTHER  
NORTH WITH THE SURFACE WAVE THROUGH NEW ENGLAND ON SAT... WILL  
CONTINUE WITH INITIAL PREFERENCE THOUGH WEIGHT TOWARD THE  
GFS/ECMWF/CMC WHERE THERE IS GREATER CONGRUENCE.  
 
...LEAD S/W IN UPPER TROF AXIS BECOMING CARVING OUT MIDDLE OF  
LARGE SCALE ARCTIC TROF OVER NORTHERN TIER FRI/GREAT LAKES SAT...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
GOES-W WV DEPICTS HIGHLY ELONGATED/POSITIVE TILT TROF IN THE  
ENTERING S YUKON TERRITORY WHICH BREAKS DOWN THE RIDGE ALLOWING  
FOR THE USHERING SOUTH OF COLD ARCTIC AIR. THE LEAD PORTION OF  
THE WAVE BREAKS FROM THE TRAILING (SEE SECTION BELOW) AND ALONG  
WITH ARCTIC JET STREAK NORTH OF THE BROOKS RANGE BEGINS TO DROP  
SOUTH AND AMPLIFY ACROSS THE CANADIAN PRAIRIES FRIDAY SPURRING A  
SURFACE WAVE AND TRAILING ARCTIC FRONT INTO THE NORTHERN TIER FRI.  
THE OVERALL EVOLUTION AND TIMING HAVE A SIMILAR LOOK WITHIN THE  
DETERMINISTIC/ENSEMBLE SUITE. THE SMALL INTERNAL S/W VORT CENTERS  
LEAD TO THE MINOR DIFFERENCES IN TIMING/SHAPE OF MASS FIELDS  
THROUGH SOUTHERN CANADA BUT OVERALL A GENERAL MODEL BLEND SHOULD  
SMOOTH THIS OUT TO SOMETHING QUITE REASONABLE. AS SUCH CONFIDENCE  
IS ALSO SLIGHTLY ABOVE AVERAGE GIVEN THIS AGREEMENT WITH ONLY  
SMALL DETAILS TO WASH OUT WITH TIME.  
 
19Z UPDATE: THE 12Z ECMWF SHOWED A SLIGHTLY FASTER SOLUTION  
FURTHER TIGHTENING THE TIMING/AMPLITUDE OF THE WAVE. LIKEWISE THE  
UKMET/CMC REMAINED SIMILAR ENOUGH TO KEEP INITIAL  
PREFERENCE/CONFIDENCE.  
 
...TRAILING S/W IN UPPER TROF AXIS ENTERING WA/ID/MT LATE FRIDAY  
AND SWINGING THROUGH THE BASE OF LARGE SCALE TROF INTO CENTRAL  
GREAT PLAINS LATE SAT...LEE SURFACE LOW IN NE NM...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 12Z ECMWF/UKMET BLEND  
CONFIDENCE: SLIGHTLY BELOW AVERAGE  
 
THE TRAILING SHORTWAVE ENERGY OF THE ELONGATED TROF WILL HANG  
ALONG THE COASTAL RANGE OF BC AND WITH UPSTREAM AMPLIFICATION OF  
THE RIDGE ALLOWS FOR SOME INCREASED AMPLIFICATION NEAR VANCOUVER  
ISLAND BY 00Z SAT. MODEL SPREAD INCREASES AFTER THIS POINT MAINLY  
IN TIMING BUT ALSO WITH RESPECT TO MOISTURE FEED OFF THE PACIFIC.  
THE 12Z NAM/GFS BOTH DELAY INTERACTION WITH RETURN SWLY FLOW UNDER  
THE OMEGA BLOCK INTO OR/N CA COMPARED TO THE UKMET/ECMWF/CMC.  
WITH LESS INTEGRATION...BOTH ACCELERATE FASTER ALONG THE SW  
PORTION OF THE DEVELOPING LARGE SCALE ARCTIC TROF. GIVEN SLIGHTLY  
BETTER SATELLITE ASSIMILATION IN THE EASTERN PACIFIC WILL FAVOR  
THE ECMWF SOLUTION WHICH IS SUPPORTED IN CONTINUITY/TRENDS AS WELL  
AS WITH THE UKMET/CMC. AS SUCH WILL FAVOR A 00Z ECMWF/UKMET AND  
CMC BLEND FOR THIS FEATURE. CONFIDENCE IS SLIGHTLY BELOW AVERAGE  
DUE TO THE SPREAD AND UNCERTAINTY WITH THE RETURN FLOW.  
 
19Z UPDATE: THE 12Z UKMET/ECMWF BOTH TRENDED A BIT WEAKER WITH THE  
AMPLIFICATION ACROSS SW BC ON SAT AND WITH SLIGHTLY FASTER  
DOWNSTREAM SOLUTIONS...ALSO PICKED UP A BIT OF SPEED CROSSING INTO  
GREAT BASIN/SW WY/UINTA REGION BY 00Z SUN. THE CMC LAGS A  
BIT...ENOUGH SO TO SUGGEST A BETTER BLEND BETWEEN THE 12Z  
UKMET/ECMWF AS PREFERENCE. CONFIDENCE REMAINS SLIGHTLY BELOW  
AVERAGE.  
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
MODEL TRENDS AT WWW.WPC.NCEP.NOAA.GOV/HTML/MODEL2.SHTML  
500 MB FORECASTS AT WWW.WPC.NCEP.NOAA.GOV/H5PREF/H5PREF.SHTML  
 
GALLINA  
 

 
 
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