767  
FXUS10 KWNH 221634  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1118 AM EST FRI DEC 22 2017  
 
VALID DEC 22/1200 UTC THRU DEC 26/0000 UTC  
 
...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR  
INGEST...  
 
12Z MODEL EVALUATION INCLUDING PREFERENCES AND FORECAST CONFIDENCE  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
 
... TROF CENTERED OVER NEW MEXICO EMERGING INTO SOUTHERN PLAINS  
EARLY SAT SPURRING SFC WAVE THAT CROSSES LOWER MS TODAY/EARLY SAT  
AND THRU NEW ENGLAND/MID-ATLANTIC SUN...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: NON-CMC BLEND  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
MODELS CONTINUE TO TREND TOWARD A COMMON SOLUTION WITH SOME  
REMAINING TIMING DIFFERENCES THE THE EVOLUTION/SHAPE/TRACK APPEARS  
TO BE SOLID INCLUDING ACROSS THE MID-ATLANTIC/NEW ENGLAND ON SAT  
INTO SUN WHEN NORTHERN STREAM ENHANCEMENT HELPS TO DEEP A COASTAL  
SURFACE REFLECTION TIGHT TO DOWNEAST MAINE. THE 12Z NAM HAS  
SHIFTED SLIGHTLY FASTER WITH THIS ITERATION TO PLACE IT TIGHTLY  
WITH THE TIMING OF THE 00Z ECMWF/UKMET THOUGH IS MAY SHOW A BIT  
STRONGER REFLECTION THIS MAY BE DUE MORE TO THE HIGHER RESOLUTION  
THAN ACTUAL MODEL DIFFERENCES. THE GFS SHOWS SOME TYPICAL FAST  
BIAS ESPECIALLY ACROSS NEW ENGLAND WITH THE MID-UPPER LEVEL  
PATTERN BUT DOES NOT REFLECT POORLY ON THE TRACK OR TIMING OF MAIN  
METEOROLOGICAL FEATURES IN THE LOWER ATMOSPHERE TO REMOVE IT FROM  
THE PREFERENCE AT THIS TIME. THE 00Z CMC WHILE CONTINUING TO  
CATCH UP...REMAINS SLOWEST ESPECIALLY ACROSS THE LOWER MS AND TN  
VALLEYS AND CROSSING INTO VA/NC...THOUGH WITH BETTER NORTHERN  
STREAM ENHANCEMENT TIMING MAY BE SALVAGEABLE FOR NEW ENGLAND/SE  
CANADA CONCERNS BUILDING CONFIDENCE/BLEND THERE. AS A WHOLE  
THOUGH WILL SUPPORT A NON-CMC BLEND AT SLIGHTLY ABOVE AVERAGE  
CONFIDENCE.  
 
...S/W CROSSING CANADIAN PRAIRIES TODAY WITH SURFACE WAVE/COLD  
FRONT THEN SLIDING THROUGH GREAT LAKES BY LATE SAT AMPLIFYING  
THROUGH SE CANADA LATE SUN  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: NON-CMC BLEND  
CONFIDENCE: ABOVE AVERAGE  
 
MODELS SHOW STRONG SIMILARITY TIMING AND DEPTH TO THE WAVE AS A  
WHOLE WHICH IS REFLECTED AT LOWER ATMOSPHERIC LEVELS/SFC MASS  
FIELDS. A SMALL NIT-PICK MAY BE ASSOCIATED WITH THE 00Z CMC WHERE  
IT IS A BIT QUICK TO PRESS IN A STRONGER HIGH EARLIER ACROSS E  
ONTARIO/WESTERN QUEBEC SUNDAY EXITING SHORTWAVE INTO THE LABRADOR  
SEA TOO QUICK. AS SUCH WILL SUPPORT A NON-CMC BLEND HERE MAINLY  
AFTER 00Z SUNDAY. CONFIDENCE IS ABOVE AVERAGE IN THE REMAINING  
BLEND.  
 
...TRAILING PORTION OF TROF IN NW CANADA ENTERS US ROCKIES FRI/SAT  
AND CARVES OUT BASE OF VAST ARCTIC VORTEX ACROSS GREAT LAKES/OHIO  
VALLEY SUN INTO MON AND MID-ATLANTIC MON SPARKING RAPID DEVELOPING  
COASTAL SURFACE WAVE...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: NON-UKMET BLEND  
CONFIDENCE: AVERAGE  
 
THE TAIL END S/W CAN BE SEEN WELL IN GOES-WV MOSAIC CURRENTLY  
NEARING VANCOUVER ISLAND STRETCHING TOWARD EXT SE BC NEARING US  
BORDER...WITH A STRENGTHENING JET OVERTOPPING THE RIDGE OVER  
CALIFORNIA...MODELS ACCELERATE THE WAVE ALONG THE SW PERIPHERY OF  
THE UPSCALE UPPER LOW OVER CENTRAL CANADA. MODEL TIMING SEEMS  
FAIRLY GOOD THROUGH THIS TIME PERIOD THOUGH THE NAM/GFS DEPICT A  
STRONGER (150KT) JET LIKELY DUE TO SLIGHTLY FASTER ARCTIC AIR  
INTRUSION ACROSS THE NORTHERN PLAINS WITH RESPECT TO THE DELAYED  
ECMWF. THESE LEAD TO COMPOUNDING DIFFERENCES WITH RESPECT TO  
AMPLIFICATION ACROSS THE OHIO VALLEY LATE SUN INTO MONDAY IN THE  
SHARPNESS OF THE TROF AND MORE IMPORTANTLY IN THE DOWNSTREAM  
SHORTWAVE RIDGING ACROSS THE NORTHEAST. THE 00Z UKMET IS MOST OUT  
OF PHASE BEING A BIT SLOW CROSS THE OHIO VALLEY AND WITH GREATER  
TIME UNDER THE DOWNSTREAM RIDGE DEVELOPS THE COASTAL LOW MORE  
ASHORE ACROSS MAINE THAN OFFSHORE IN THE GULF OF MAINE. THERE ARE  
DIFFERENCES NOTED BETWEEN THE 00Z ECMWF/ECENS MEMBERS AND THE  
GFS/GEFS WITHIN THE ENSEMBLE SFC CLUSTER ANALYSIS. THE ECMWF  
FAVORS A BIMODAL SURFACE DEVELOPMENT...KEEPING THE WAVE ALONG THE  
FRONT SEPARATE FROM THE DEVELOPING LOW IN THE GULF OF MAINE;  
OPPOSED BY THE GFS THAT TAKES THE FRONTAL WAVE AND DRAWS IT LEFT  
OF TRACK TOWARD THE APPROACHING HEIGHT FALLS. THIS SUBTLE  
DIFFERENCE IS LIKELY TO PLAY A ROLE IN DIFFERENCE IN QPF MAGNITUDE  
AND AXIS BUT AT THIS POINT BEING DEPENDENT ON MANY SMALLER SCALE  
FACTORS WILL HEDGE TOWARD A COMMON SOLUTION...NEAR A NON-UKMET  
BLEND. GIVEN THIS SMALL SCALE INFLUENCES/VARIATION BUT ALSO A  
GROWING CONSENSUS IN THE TIMING/LARGE SCALE EVOLUTION CONFIDENCE  
IN THIS BLEND IS CONSIDERED AVERAGE.  
 
...ARCTIC COLD FRONT/SURFACE WAVE ACROSS NORTHERN TIER LATE SUN  
SLIDING THROUGH GREAT LAKES MON...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 12Z NAM/GFS AND 00Z UKMET  
CONFIDENCE: SLIGHTLY BELOW AVERAGE  
 
A SUBTLE S/W AT THE LEADING EDGE OF THE SECONDARY AND MORE  
IMPRESSIVE COLD PUNCH WILL SPUR WEAK SFC AMPLIFICATION ACROSS THE  
GREAT LAKE BY MIDDAY SUNDAY INTO MONDAY. THE PRECURSORY  
CONDITIONS DO NOT FAVOR INCLUSION OF THE CMC GIVEN IT IS A BIT  
FAST WITH WITH FRONT/DOWNSTREAM BUILDING HIGH PRESSURE ACROSS  
ONTARIO. ON THE OTHER SIDE...THE ECMWF IS VERY SLOW PULLING THE  
COLDEST AIR SOUTHWARD AND IN ITS DELAY AND ORIENTATION TO THE  
GREAT LAKES SPARKS A DEEP LOW OVER LK SUPERIOR THAT SEEMS A BIT  
SPURIOUS AT THIS TIME...WHILE THE 12Z GFS MAY BE EVEN A BIT TOO  
FAST AND PRESSING THE SURFACE WAVE TOWARD GEORGIAN BAY MAY  
COUNTERACT SOME OF THE SLOW/NORTH LESS DESIRABLE NATURE OF THE 00Z  
UKMET. THE 12Z NAM LOOKS LIKE A BEST CASE COMPROMISE AND LIKELY  
WILL BE WEIGHTED A BIT HIGHER IN THE BLEND FOR THIS SYSTEM. THE  
SPREAD IN TIMING OF THE COLD AIR AND PLACEMENT OF THE SURFACE WAVE  
LEADS TO SLIGHTLY BELOW AVERAGE CONFIDENCE IN THIS WEAK SYSTEM.  
 
...PACIFIC SHORTWAVE ENERGY TRYING TO SLIDE UNDER CANADIAN WEST  
COAST OMEGA BLOCK LATE SUN INTO PACIFIC NW/CALIFORNIA...SHEARS  
THROUGH NW FLOW AND SPARKS LEE CYCLONE IN SE CO BY LATE MON...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 12Z GFS AND 00Z CMC/ECMWF BLEND  
CONFIDENCE: SLIGHTLY BELOW AVERAGE  
 
YESTERDAY THERE WAS A LARGE MODEL SPREAD WITH THE 21/00Z ECMWF RUN  
VERY SLOW AND VERY STRONG WHILE THE NAM/GFS WERE FAST IN BRINGING  
THE SOUTHEASTWARD EXTENSION OF THE GULF OF AK TROF INTO THE  
PACIFIC NW SUN INTO MONDAY. BY 12Z THE GUIDANCE WAS COMING INTO  
BETTER AGREEMENT... HOWEVER THE 22/00Z ECMWF HAS SHIFTED EVEN  
FASTER WITH THE NCEP CAMP IN GUIDANCE SLOWING GENERATING A VERY  
UNCOMMON TIMING ORIENTATION: ECMWF/CMC/GFS/UKMET/NAM. THE 00Z  
UKMET HAS SIGNIFICANTLY WEAKENED THE EASTWARD EXTENSION OF THIS  
TROF SLOWING THE PROGRESS ASHORE AND WAITING FOR A WEAK VORTEX  
ROLL UP ALONG THE SHEAR AXIS TO SLIDE SOUTHEAST BY MONDAY...THIS  
SEEMS VERY ANTITHETICAL TO THE OVERALL CONTINUITY/TREND AND  
EXPECTATION. THE 12Z NAM IS MUCH TOO SLOW WITH THE TROF COMPARED  
TO THE ENSEMBLES INCLUDING LAST FEW DAYS. THE 12Z GFS HAS SHIFTED  
A BIT QUICKER THAN THE 06Z RUN BUT STILL SHOWS AN OVERALL DELAY  
COMPARED TO YESTERDAY AND THE 00Z RUN; HOWEVER THIS IS THE  
GREATEST LONG TERM CONTINUITY THAT IS AVAILABLE AND BUILDS SOME  
CONFIDENCE MOVING FORWARD. WHILE THE ECMWF SEEMS A BIT TOO QUICK A  
BLEND WITH WITH THE SLOWER GFS/CMC SEEMS A GOOD COMPROMISE  
ESPECIALLY GIVEN THE DOWNSTREAM EVOLUTION/LEE CYCLONE DEVELOPMENT  
LATE MON/00Z TUES. CONFIDENCE REMAINS SLIGHTLY BELOW AVERAGE  
GIVEN ALL THE RUN TO RUN VARIATION.  
 
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