114  
FXUS10 KWNH 261840  
PMDHMD  
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
139 PM EST THU DEC 26 2019  
 
VALID DEC 26/1200 UTC THRU DEC 30/0000 UTC  
 
...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR  
INGEST...  
 
12Z MODEL EVALUATION WITH PREFERENCES/CONFIDENCE INTERVALS  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
 
SYSTEM MOVING FROM THE WEST INTO THE MIDWEST FRI-SUN  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND HEDGING TOWARD ECMWF/ECENS  
CONFIDENCE: AVERAGE  
 
19Z UPDATE: WHILE THERE REMAIN SMALLER, BUT IMPORTANT INTERNAL  
TIMING DIFFERENCE (E.G. NAM/GFS A BIT FASTER, THEREFORE A BIT  
FURTHER NORTHEAST WITH THE WARM CONVEYOR INTO THE WESTERN GREAT  
LAKES), THERE IS CONTINUED GROWING CONSENSUS IN THE MASS FIELDS  
FOR THE DEVELOPMENT ACROSS THE CENTRAL PLAINS. SO WPC IS STILL  
FAVORING THE ECMWF/ECENS/UKMET WITHIN A GENERAL MODEL BLEND.  
AVERAGE CONFIDENCE REMAINS DUE TO THE REMAINING UNCERTAINTY OF THE  
INTERNAL TIMING/INTERACTION OF THESE FEATURES.  
 
---PRIOR DISCUSSION---  
WELL DEFINED CLOSED LOW ENTERING SOUTHERN CALIFORNIA, WILL  
CONTINUE TO TRACK EASTWARD THROUGH FRIDAY, AS THE NORTHERN STREAM  
SHORTWAVE ENERGY ALONG THE EASTERN SIDE OF SHARPENING/ANTICYCLONIC  
MERIDIONAL JET FROM BC TO THE GREAT BASIN CONTINUES TO SHARPEN AND  
DRAW THE REMAINING CLOSED WAVE ENERGY NORTH INTO THE HIGH PLAINS  
BY SAT. THE HIGH-AMPLITUDE TROF WILL BEGIN TO HAVE INTERNAL  
BINARY ACTION INTO A LARGE CLOSED LOW BY 29.00Z. THERE REMAINS  
SMALL TIMING DIFFERENCES THAT LEAD TO LONGITUDINAL DIFFERENCES IN  
THE PLACEMENT/EVOLUTION OF THE CLOSED LOW THROUGH THE CENTRAL  
PLAINS, WITH THE SLIGHTLY FASTER (BROADLY WEAKER) GFS/GEFS ALONG  
WITH THE UKMET/12Z NAM FURTHER EAST WHILE THE ECMWF/ECENS MEAN AND  
THE CMC ARE SLIGHTLY SLOWER WITH THE CENTER AND THEREFORE WEST  
RELATIVE TO THE PACKING. STILL, A BLEND BETWEEN THE TWO CAMPS IS  
STILL QUITE VIABLE UNTIL THE STRONG NORTHERN STREAM SHORTWAVE  
ROTATES THROUGH THE BASE OF THE CLOSED LOW AND GOES NEGATIVE TILT  
THROUGH THE MIDDLE TO LOWER MS RIVER VALLEY INTO SUNDAY. SO THE  
STRONGER/MORE CONCENTRIC SOLUTIONS OF THE ECMWF/CMC ARE SLIGHTLY  
FASTER KICKING THE WAVE THROUGH THE BASE WHILE THE GFS/NAM/UKMET  
SHOW GREATER ELONGATION THROUGH THE NEGATIVE TILT/PRESSING THE  
FRONTAL ZONE/WARM CONVEYOR THROUGH THE TN VALLEY FASTER.  
CONTINUITY GOES TO THE ECMWF THOUGH IT REMAINS WEST OF THE ECENS  
SOLUTIONS...SO A GENERAL MODEL BLEND IS PREFERRED BUT HEDGING  
TOWARD THE ECMWF/CMC PARTICULARLY ON SUNDAY, BUT THE CHANGES OF  
THE 12Z GFS/NAM ALLOW FOR GREATER WEIGHTING THERE AS WELL IN THE  
TN VALLEY. CONFIDENCE REMAINS AVERAGE GIVEN SOME OF THE IMPORTANT  
FEATURES REMAIN OFF THE DENSE OBSERVATION NETWORK AND TIMING OF  
THE INTERACTIONS OF THE INTERNAL SHORTWAVE FEATURES WILL ALLOW FOR  
CONTINUED RUN TO RUN VARIATIONS THAT ARE IMPACTFUL.  
 
SYSTEM CLIPPING THE GREAT LAKES INTO THE NORTHEAST THU-SAT  
SOUTHEAST US, MOISTURE FETCH OFF THE SOUTHEAST ATLANTIC  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
19Z UPDATE: BETTER OVERALL AGREEMENT NOW EXISTS, THOUGH THE  
TRAILING NORTHERN STREAM SHORTWAVE STILL SHOWS TIMING DIFFERENCES  
THROUGH SOUTHEAST CANADA...THOUGH WITH THE ODD PAIRING OF THE  
CMC/GFS (FAST) AND THE UKMET/NAM/ECMWF SLOWER. STILL THE IMPACTS  
ACROSS THE NORTHEAST REMAIN SOLID. FURTHER SOUTH, THE TROPICAL  
MOISTURE PLUME CONTINUES TO TREND FURTHER WEST IN THE ECMWF TO GET  
CLOSER TO THE GFS. STILL A GENERAL BLEND OF THE ORIENTATION AND  
QPF MAGNITUDE STILL APPEAR APPROPRIATE.  
 
---PRIOR DISCUSSION---  
COMPACT SHORTWAVE LEAVING N MN WILL CONTINUE TO PROGRESS THROUGH  
SOUTHERN CANADA WITH THE TRAILING ENERGY CLIPPING THROUGH THE  
GREAT LAKES INTO NEW ENGLAND FURTHER ENFORCED BY A PROGRESSIVE  
ARCTIC STREAM SHORTWAVE THAT CLIPS N MAINE BY LATE SAT. THEIR  
INFLUENCES ARE WEAK, BUT THE TRAILING WARM FRONT AND BROAD  
SOUTHWESTERLY FLOW ALOFT (ALONG WITH RETURN MOISTURE FEED OUT OF  
THE BAHAMAS/ANTILLES ISLAND CHAIN) WILL PROVIDE SOME BROAD ASCENT  
ACROSS THE AREA TO PRODUCED MIXED WINTRY PRECIP THAT HAS SOME  
IMPACTS PARTICULARLY THROUGH TERRAIN INTO THE NORTHEAST... WHILE  
KEEPING STRONG ATLANTIC FETCH INTO THE SOUTHEAST. MASS FIELDS ARE  
SOLID ENOUGH TO SUPPORT A GENERAL MODEL BLEND AT THIS POINT. THE  
UNCERTAINTY IS IN THE ACTIVITY ALONG THE BAHAMIAN ISLAND FETCH AND  
STRENGTH OF CONFLUENT/CONVERGENCE EITHER INTO SE FL OR ANGLING  
AROUND THE WESTERN PERIPHERY OF THE STRONG BUT RETROGRADING  
SUBTROPICAL RIDGE IN THE SARGASSO SEA.  
 
SYSTEM APPROACHING PACIFIC NORTHWEST SUNDAY MORNING  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 12Z NAM/UKMET/ECMWF BLEND  
CONFIDENCE: AVERAGE  
 
19Z UPDATE: THE 12Z CMC BROKE FROM THE COMPACT CLUSTER SUPPORTING  
FASTER/GREATER STRETCHING SOUTHWARD AND IN THIS WEAKNESS, STARTS  
TO CLOSE OFF A SMALL LOW ON THE NOSE OF THE JET/DIGGING TROF.  
THIS SEEMS WELL OUT OF FORM, AND THEREFORE IS REJECTED FROM THE  
BLEND. THE ECMWF/UKMET BOTH TRENDED TOWARD A BIT STRONGER/BROADER  
SHORTWAVE AND IN DOING SO, SPORT GREATER ELONGATION NORTH TO SOUTH  
SIMILAR TO THE NAM BUT NOT TO THE EXTENT OF THE 12Z GFS. SO  
WHILE, THERE WAS STRONGER AGREEMENT WITH THE 00Z SUITE, THE RUN TO  
RUN VARIATION NOTED HERE, SHOWS THE AVERAGE DEGREE OF CONFIDENCE  
IN A 12Z NAM/ECMWF/UKMET BLEND.  
 
---PRIOR DISCUSSION---  
THE SHARP/AMPLIFYING RIDGE OVER THE WEST COAST, WILL QUICKLY BE  
SQUEEZED BY THE APPROACH OF THE NEXT STRONG SYSTEM ENTERING THE  
GULF OF ALASKA. WHILE THE LEADING SHORTWAVE/HEIGHT-FALLS ARE  
EXPECTED TO DRIVE THROUGH VANCOUVER ISLAND, THE BLOCKING  
FLOW,ENHANCED BY THE COASTAL RANGES, WILL ALLOW FOR RAPID  
CYCLOGENESIS FOR THE TRAILING KICKER SHORTWAVE BY LATE SAT INTO  
EARLY SUNDAY, JUST WEST OF THE JUAN DE FUCA STRAIT EVENTUALLY  
DEFLECTING SOUTHWARD. ENSEMBLES, PARTICULARLY THE ECENS SHOW A  
FAIRLY LARGE SPREAD IN THE STRENGTH OF THE RIDGING/BLOCKING IN  
ADVANCE OF THE CYCLONE DEVELOPMENT SUGGESTING HIGHER UNCERTAINTY  
AND LOWER PREDICTABILITY. HOWEVER, THE 12Z GFS/NAM AND 00Z  
NON-NCEP DETERMINISTIC GUIDANCE IS FAIRLY WELL PACKED OVERALL.  
THE NON-NCEP SUITE SUGGESTS THE ENERGY OF THE CYCLONE IS A BIT  
MORE COMPACT AND SMALLER IN SCALE BY THE END OF THE SHORT-RANGE  
PERIOD, FOLLOWED BY THE 12Z NAM AND THEN THE GFS, WHICH IS  
STRONGER, AND BEING A TAD FASTER (WITH ITS KNOWN BIAS), IS  
STARTING TO ELONGATE/STRETCH SOUTHWARD ALONG THE N CA COAST. GEFS  
SOLUTIONS WOULD SUGGEST IT BEING FURTHER EAST AND CLOSER TO THE  
NAM/ECMWF/UKMET/CMC...SO THINKING A NON-GFS SOLUTION SEEMS  
APPROPRIATE WHICH FEEDS WELL INTO THE MEDIUM RANGE FORECAST  
PREFERENCES (SEE WPC PMDEPD FOR MORE DETAILS). GIVEN THE LARGE  
VARIANCE IN THE ENSEMBLE SUITE, CONFIDENCE IS ONLY AVERAGE AND  
WITH THE SYSTEM STILL WELL OUT IN THE PACIFIC, RUN TO RUN  
VARIATION IS EXPECTED.  
 
MODEL TRENDS AT WWW.WPC.NCEP.NOAA.GOV/HTML/MODEL2.HTML  
500 MB FORECASTS AT WWW.WPC.NCEP.NOAA.GOV/H5PREF/H5PREF.SHTML  
 
GALLINA  
 

 
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab HPC Page Main Text Page