802  
FXUS10 KWNH 191724  
PMDHMD  
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1223 PM EST TUE DEC 19 2017  
 
VALID DEC 19/1200 UTC THRU DEC 23/0000 UTC  
 
...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR  
INGEST...  
 
12Z MODEL EVALUATION INCLUDING PREFERENCES AND FORECAST CONFIDENCE  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
 
...CLOSED LOW CURRENTLY EMERGING INTO SOUTHERN PLAINS TODAY  
EVENTUALLY WEAKING THROUGH THE TN VALLEY WED AND EAST COAST  
THURS...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
THE 12Z NAM REMAINS A BIT STRONGER/COMPACT WITH THE REMAINING  
INNER CORE OF THE WAVE AS IT TRANSLATES ACROSS THE TN VALLEY  
HOWEVER HAS SHIFTED FROM BEING A SLOW MEMBER OF THE GUIDANCE SUITE  
TO PACE ON THE LEADING EDGE OF THE CLUSTER WITH THIS RUN. OVERALL  
THE TIMING IS SMALL ENOUGH WITHIN THE ENSEMBLE SUITE TO KEEP IT  
WITHIN THE PREFERENCE THOUGH PERHAPS WITH A SLIGHTLY LOWER  
WEIGHTING. THE 12Z GFS SHOWED LITTLE VARIATION TO ACCEPT IT ALONG  
WITH THE 00Z ECMWF AS CORE MEMBERS/HIGHER WEIGHTING IN A GENERAL  
MODEL BLEND. CONFIDENCE REMAINS SLIGHTLY ABOVE AVERAGE GIVEN THE  
MODEST DIFFERENCES IN DEPTH/TIMING REMAINING IN THE SUITE.  
 
...BROAD/LOW AMPLITUDE MID-UPPER LEVEL TROUGH CROSSING THE GREAT  
LAKES/NORTHEAST TODAY /WED...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: ABOVE AVERAGE  
 
OVERALL MODEL AGREEMENT CONTINUES TO TIGHTEN WITH ONLY VERY SMALL  
INNER CORE DIFFERENCES NOTED (NAM DEEPER/CMC SLOWER). THESE  
DIFFERENCES DO NOT APPEAR TO HAVE MUCH IMPACT ON THE SENSISBLE  
WEATHER/MASS FIELDS SUGGESTING AN ABOVE AVERAGE CONFIDENCE IN A  
GENERAL MODEL BLEND.  
 
...UPPER TROUGH AXIS CROSSING THE PACIFIC NORTHWEST THIS EVENING  
REACHING THE FOUR CORNERS ON THU...AND ELONGATING FROM WESTERN  
GREAT LAKES TO NM/OLD MEXICO BY FRI WITH...SURFACE CYCLONE/TROF  
CROSSING THE CENTRAL PLAINS ON THU...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 12Z GFS AND 00Z ECMWF/UKMET BLEND  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
ENSEMBLE AND CURRENT DETERMINISTIC GUIDANCE IS QUITE AGREEABLE  
WITH THE EVOLUTION OF THE DEEP LOW COMING ASHORE TODAY AND EVEN  
THROUGH THE EVOLUTION OF THE ACCELERATING NORTHERN STREAM/SEVERING  
THE SOUTHERN STREAM CLOSED LOW ACROSS THE FOUR CORNERS THROUGH  
EARLY THURSDAY; THOUGH THE 12Z NAM/GFS ARE A BIT QUICKER IN THE  
NORTHERN STREAM WHICH IS IN LINE WITH TYPICAL BIAS. IT IS AFTER  
ABOUT 12/18Z ON THURSDAY AS THE CLOSED LOW BEGINS TO SHEAR/UNFOLD  
INTO A BROAD POSITIVE TILT TROF SOUTHEAST OF THE FOUR CORNER THAT  
MODEL SPREAD BECOMES MORE EVIDENT. THE 12Z NAM HAVING THE FASTER  
NORTHERN STREAM ALSO DRAGS THE SOUTHERN STREAM A BIT FASTER THAN  
PRIOR CYCLES AND IS CLEARLY ON THE LEADING EDGE OF THE ESEMBLE  
SUITE. THE 00Z CMC IS ON THE OTHER SIDE OF THE SPECTRUM WITH A  
TYPICAL SLOW TO UNFOLD/COMPACT WAVE THROUGH NM BY FRI. THE 00Z  
ECMWF/ECENS MEAN/UKMET ALL GENERALLY FAVOR THIS SLOWER STRONGER  
SOUTHERN STREAM COMPARED TO EARLIER RUNS OF THE GFS... HOWEVER THE  
12Z GFS RUN HAS SLOWED SIGNIFICANTLY TO DEPART FROM THE NAM AND  
MATCH THE ECMWF/UKMET IN TIMING AS WELL AS GENERAL BREADTH OF THE  
WAVE. THIS PROVIDES INCREASED CONFIDENCE IN A 12Z GFS AND 00Z  
ECMWF/UKMET BLEND...UP TO SLIGHTLY ABOVE AVERAGE.  
 
...UPPER TROUGH AXIS MOVING INTO THE PACIFIC NORTHWEST FROM THE  
NORTH ON THU AND STRETCHING ACROSS NORTHERN ROCKIES/NORTHERN  
PLAINS BY FRI...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 12Z GFS AND 00Z ECMWF BLEND  
CONFIDENCE: AVERAGE  
 
BY THURSDAY... A BROAD POSITIVE TILT TROF REACHES SE AK/SW YUKON  
TER. BREAKING DOWN THE LARGER SCALE RIDGE. THE TRAILING EDGE AND  
ASSOCIATED JET STREAK DROPS DUE SOUTH ALONG THE COASTAL RANGE IN  
BC REACHING THE US TOWARD EARLY FRI. THE LEADING EDGE OF THE  
HEIGHT FALLS WILL CARVE OUT/BROADEN THE ARCTIC TROF ACROSS  
SASKATCHEWAN AND DESEND TOWARD THE NORTHERN US PLAINS BY 00Z SAT.  
THE 00Z UKMET IS MOST DISAGREEABLE WITH THE REMAINING GUIDANCE  
GIVEN THE TROF IS MUCH LESS PHASED IN TIMING/ORIENTATION ACROSS SW  
CANADA...WITH THE LEAD PORTION OF THE TROF OUTPACING AND BECOMING  
MORE COMPACT AND NEARLY SEPARATING WITH THE TRAILING PORITON OF  
THE TROF WHICH AS IS EXPECTED MUCH SLOWER THAN OTHER GUIDANCE.  
THE 00Z CMC WHILE MATCHING THE ECMWF QUITE WELL HAS SOME  
INTERACTION ISSUES WITH THE OVERDEVELOPED DOWNSTREAM CLOSED WAVE  
TOWARD THE END OF FRI...WHILE IT MAY BE USEFUL UP UNTIL MIDDAY  
FRIDAY...THE ECMWF/ECENS MEAN ARE GOOD REPRESENTATIVES IN THE  
REMAINING GUIDANCE SUITE TO ACCOUNT FOR IT IN ANY BLEND. THE 12Z  
NAM IS ALSO DECENTLY TIMED INITIALLY BUT SHOWS A STRONGER  
NORTHERLY JET OVER THE NW AND DRIVES THE HEIGHT FALLS WELL SOUTH  
OF THE GFS/ECMWF BY 00Z SAT. ALL CONSIDERED A 12Z GFS/00Z ECMWF  
BLEND BEST REPRESENT THE ENSEMBLES WHILE PRESENTING A MORE  
METEROLOGICALLY SOUND SOLUTION (IN TIMING/ORIENTAITON WITH OTHER  
FEATURES). CONFIDENCE IS AVERAGE IN THIS BLEND.  
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
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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