838  
FXUS10 KWNH 171857  
PMDHMD  
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
157 PM EST FRI NOV 17 2017  
 
VALID NOV 17/1200 UTC THRU NOV 21/0000 UTC  
 
...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR  
INGEST...  
 
12Z MODEL ASSESSMENT INCLUDING PREFERENCES AND CONFIDENCE  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
 
LARGE STORM SYSTEM OVER THE EASTERN U.S. THIS WEEKEND  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: NON-NAM BLEND  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
GOES-WV MOSAIC DEPICTS A LARGE CYCLONE OVER CENTRAL CANADA WITH A  
BROAD POSITIVE TILT TROF EXTENDING BACK TOWARD OFFSHORE CA. THIS  
POSITIVE TILT BASE WILL TRANSLATE FAIRLY QUICKLY ACROSS THE  
WESTERN AND CENTRAL US TODAY INTO SAT SUPPORTING A SURFACE WAVE  
ACROSS THE CENTRAL GREAT PLAINS THAT LIFTS INTO THE SOUTHERN GREAT  
LAKES BY THE SECOND HALF OF SAT AS WELL. AT THIS POINT SPREAD  
BEGINS TO MANIFEST AS THE BASE BEGINS TO AMPLIFY/WIND UP ACROSS  
THE OHIO VALLEY INTO THE EASTERN LAKES/SE CANADA/N NEW ENGLAND.  
THE 12Z NAM HAS TRENDED UNCHARACTERISTICALLY FLATTER THOUGH IS  
ALSO FASTER LIKELY IN THE STRONGER/MORE COMPACT KICKER SHORTWAVE  
DESCENDING OUT OF SOUTHERN CANADA LATE SAT/EARLY SUN. A SECONDARY  
SURGE/SHORTWAVE COMES A DAY LATER AND THE THREE SYSTEMS BUILD INTO  
THE LARGER SCALE VORTEX OVER QUEBEC BY THE END OF THE FORECAST  
PERIOD...BUT THE NAM IS CLEARLY OUTLIER AT THIS POINT. THE 00Z  
UKMET GIVES SOME TIMING SUPPORT TO THE NAM...BUT HOLDS BACK THE  
SURFACE LOW A BIT MORE IN LINE WITH THE MODEST CONSENSUS ANCHORED  
BY THE ECMWF/ECENS MEAN. THE 12Z GFS AND THE 06Z RUN BEFORE IT  
BREAK FROM THE GEFS MEMBER CLUSTER IN FAVORING AN  
UNCHARACTERISTICALLY SLOWER SOLUTION; THE GFS IS A BIT  
STRONGER/COMPACT INITIALLY COMING INTO THE GREAT LAKES ALLOWING  
FOR A EARLIER DEEPENING HAVING A FAVORABLE NEGATIVE TILT  
ORIENTATION WITH THE LARGER SCALE NORTHERN STREAM. UPSTREAM  
KICKER WAVES FOLLOW IN SUBSEQUENT DAYS EXPANDING/BROADENING THE  
VORTEX ACROSS QUEBEC BUT FURTHER SOUTH AND WEST THAN OTHER  
GUIDANCE/GEFS MEAN. THIS IS NOT TOO FAR FROM THE EVOLUTION OF THE  
00Z ECMWF/CMC UNTIL LATE MONDAY AS THE ECMWF/CMC DO NOT MANIFEST  
THE STRONGER KICKER WAVES THAT THEN NAM/GFS RESOLVE...ALLOWING FOR  
THE VORTEX/SURFACE LOW TO PRESS EASTWARD TOWARD LABRADOR BY TUES.  
WHILE THE ECMWF/CMC IS PREFERRED OVER THE GFS...THE VARIATION IS  
NOT LARGE ENOUGH IN THE ENSEMBLE SUITE TO JUSTIFY THE GFS'S  
REMOVAL AT THIS TIME. AS SUCH WILL SUPPORT A ECMWF/CMC AND GFS  
BLEND HEDGING TOWARD THE ECMWF. CONFIDENCE IS STRONG UNTIL SUNDAY  
BUT REDUCES ACROSS NEW ENGLAND...AND IS AVERAGE TO SLIGHTLY ABOVE  
AVERAGE THROUGH THE ENTIRE FORECAST PERIOD.  
 
19Z UPDATE: THE 12Z UKMET TRENDED SLOWER AND IS A GOOD PROXY TO  
THE ECMWF/CMC. SMALL ADJUSTMENTS IN THE CMC MAKE IT A BIT SLOWER  
THOUGH ALSO REMAINS A SLIGHT SOUTH/EAST SOLUTION TO THE MAIN  
CLUSTERING. LITTLE CHANGE IN THE ECMWF PROVIDES INCREASED  
CONFIDENCE IN A NON-NAM BLEND SOLUTION (SLIGHTLY ABOVE AVERAGE  
CONFIDENCE).  
 
NORTHERN PORTION OF TROF APPROACHING PAC NW SUNDAY...TRANSLATES  
INTO NORTHERN TIER WITH DEEP SURFACE WAVE JUST NORTH OF US BORDER  
WITH FRONT ENTERING NORTHERN PLAINS LATE MON  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 12Z GFS/NAM/ECMWF BLEND  
CONFIDENCE: AVERAGE  
 
SHORTWAVE OVER GULF OF AK QUICKLY DEFORMS INTO A SHARP ELONGATED  
TROF BY LATE SAT/SUN... AN EMBEDDED ENHANCED WAVE IN THE TROF  
LIFTS NORTHEAST ALONG THE LEADING TROF INTERFACE SUPPORTING A FAST  
MOVING SURFACE WAVE THAT REACHES THE CENTRAL BC COAST LATE SAT  
NIGHT INTO SUNDAY MORNING...THIS ALONG WITH STRENGTHENING UPSTREAM  
JET STREAK FROM THE NW WILL ALLOW FOR THIS NORTHERN PORTION OF THE  
LARGER SCALE TROF TO BEGIN TO DECOUPLE AND ACCELERATE EASTWARD  
ACROSS SOUTHERN CANADA MONDAY. IT IS THE TIMING/MAGNITUDE SPLIT  
THAT LEAD TO LARGE MODEL SPREAD FURTHER SOUTH INTO THE NORTHWEST  
US...HENCE THE MODEL PREFERENCE SECTIONS WERE SPLIT (SEE SECTION  
BELOW FOR MORE DETAILS).  
 
WITHIN THE ENSEMBLE SUITE...THE 00Z UKMET IS A CLEAR OUTLIER  
HAVING LESS DECOUPLING WITH THE REMAINING BASE OF THE TROF. THIS  
DELAYS THE EASTWARD PROGRESSION THROUGH THE SOUTHERN CANADIAN  
ROCKIES AND ALLOWS FOR A EASY REMOVAL FROM WPC PREFERENCE ATTM.  
THE REMAINING GUIDANCE SHOWS SORT OF TYPICAL BIASES: THE 00Z CMC  
WHILE A BIT QUICKER IS A BIT COLDER AND RAPID TO WINDING THE  
CYCLONE UP/DEEPER SURFACE INFLECTION...IT IS GENERALLY SUPPORTED  
BY THE GFS BUT TYPICAL OF THE GFS IT IS A BIT FAST AND NORTH  
COMPARATIVELY TO THE ENSEMBLE SUITE. THE 12Z NAM AND 00Z ECMWF ON  
THE OTHER HAND SHOW MORE OF A N-S AMPLIFICATION OF THE WAVE  
ALLOWING FOR A SLIGHTLY SOUTHWARD TRACK AND WEAKER SURFACE WAVE.  
OVERALL BY DAY 3...THIS IS A MODEST SPREAD/DIFFERENCE LIKELY TO  
RESULT IN SOME MIDDLE GROUND COMPROMISE OVER THE NEXT FEW RUNS (AS  
THE WAVE ENTERS THE DEEPER DATA NETWORK). AS SUCH A NON-UKMET  
BLEND IS SUPPORTED AT AVERAGE CONFIDENCE.  
 
19Z UPDATE: WHILE THE 12Z CMC SURFACE LOW EVOLUTION/TRACK LOOK  
VERY SIMILAR/ON PAR WITH THE ECMWF/GFS AND NAM... THERE IS A  
NORTHWARD SHIFT IN CLOSING THE UPPER LOW OVER NW CANADA AND AS  
SUCH THE TROF AS A WHOLE SEVERS CONNECTION TO THE SOUTHERN STREAM  
A BIT TOO FAR NORTH LEAD TO IT BECOMING AN OUTLIER IN THE MID TO  
UPPER LEVELS. WHILE PHASING OF THE TWO STREAMS IS REDUCED WITH  
THE 12Z UKMET RUN IT REMAINS MOST CONNECTED AND SLOWER IN THE  
NORTHERN STREAM TO BE CONSIDERED FOR INCLUSION IN THE PREFERENCE.  
THE 12Z ECMWF SHOWS LITTLE CHANGE...AND AS SUCH A 12Z  
NAM/GFS/ECMWF BLEND IS SUGGESTED AT AVERAGE CONFIDENCE.  
 
BASE OF TROUGH APPROACHING THE PAC NW SUNDAY...CONSOLIDATING INTO  
WA/OR MON  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 12Z GFS/ECMWF BLEND  
CONFIDENCE: SLIGHTLY BELOW AVERAGE  
 
4 CYCLE ENSEMBLE SUITE SPAGHETTI ANALYSIS DEPICTS A GROWING LOW  
PREDICTABILITY ENVIRONMENT EVOLVING TOWARD THE END OF THE WEEKEND  
INTO EARLY NEXT WEEK ACROSS THE PACIFIC NORTHWEST INTO THE  
INTERMOUNTAIN WEST...TO THE POINT THAT THE ENSEMBLE MEAN SOLUTIONS  
BECOME NEARLY UNUSABLE GIVEN THE INTERNAL VARIATION IN EVOLUTION.  
THE DETERMINISTIC GUIDANCE IS LITTLE BETTER. A MIDDLE-SCALE SHARP  
TROF OVER THE GULF OF AK WILL DROP SOUTHEASTWARD LATE SUNDAY. AT  
THIS POINT OUTSIDE FACTORS LEAD TO A VERY LARGE VARIETY OF  
OUTCOMES MAINLY IN TIMING OF THE BASE/TAIL END OF THIS TROF. THE  
BASE OF THE TROF BECOMES SEVERED FROM THE NORTHERN STREAM  
WAVE...BUT MORE IMPORTANTLY A STRONG SURGE OF COLD (BUT NOT  
BRUTALLY COLD) AIR AND THE SUBSEQUENT DEEP GULF OF AK LOW  
DEVELOPS. THE UPSTREAM RIDGING BETWEEN THESE SYSTEM ACTUALLY  
GROWS UPSCALE AND OVER WASHES THE LINGERING TAIL END OF THE TROF  
(UNDER STRONG SWLY RIDGE PUMPING FROM THE UPSTREAM DEEP CYCLONE).  
 
YET THE SHORTWAVE ENERGY WILL STILL EXIST IN THIS RIDGE AND HAVE  
LITTLE LARGER STREAM FLOW TO TRANSLATE IT; AND THEREFORE LEAD TO  
THE TREMENDOUS AMOUNT OF VARIATION IN TIMING ENTERING THE  
NORTHWEST AND DESCENDING THE EASTERN SIDE OF THE RIDGE INTO THE  
CENTRAL PLAINS TUES. TO COMPLICATE THE SITUATION THE WAVE STILL  
WILL BE POTENTIALLY VIGOROUS ENOUGH WHEN BREAKING FROM THE  
NORTHERN STREAM TO WIND UP AND BECOME COMPACT AS WELL. THIS  
SCENARIO IS PRESENTED BY THE GFS/CMC/ECMWF AND LESSER SO THE NAM.  
THE 00Z UKMET IS AN OUTLIER EVOLUTIONARILY AS IT REMAINS PHASED  
WITH THE NORTHERN STREAM ENERGY NEVER TRULY SEVERING ITS  
CONNECTION MAKING IT FAST. THE 00Z ECMWF EVOLUTION IS QUITE  
FAVORABLE BUT IS CLEARLY TOO SLOW ALLOWING FOR INCREASED INFLUENCE  
FROM THE UPSTREAM WAVE. THE 00Z CMC APPEARS IDEALLY TIMED WITHIN  
THE MIDDLE OF THE GUIDANCE SUITE BUT DUE TO ORIENTATION OF THE  
TROF AMPLIFYING PRODUCES A VERY DEEP SURFACE WAVE ALONG THE  
FRONTAL ZONE IN LINE WITH THE MODEL'S KNOWN DEEPENING BIAS. THE  
12Z NAM AND GFS BOTH TRENDED FAVORABLY BUT SLOWING...THE GFS  
SIGNIFICANTLY SO. WHILE BOTH DO APPEAR TO SHOW SOME TYPICAL FAST  
BIAS COMPARED TO THE CENTRAL ENSEMBLE MEMBERS BUT LOOK QUITE  
REASONABLE IN THE EVOLUTION/SHAPE AND TRACK OF THE WAVE AS A  
WHOLE. AS SUCH A 12Z GFS/NAM BLEND IS PREFERRED BUT CONFIDENCE IS  
BELOW AVERAGE.  
 
19Z UPDATE: THE 12Z ECMWF CONTINUES TO HAVE A FAVORABLE EVOLUTION  
OF THE TRAILING SOUTHERN PORTION OF THE TROF AND WHILE IT HAS  
SHIFTED FASTER CONTINUES TO LAG THE GFS/NAM...HOWEVER GIVEN THE  
NAM/GFS WERE FASTER THAN THE TIMING PREFERENCE... THIS SOLUTION IS  
MUCH MORE FAVORABLE OVERALL. THE 12Z CMC HAVING NORTHERN STREAM  
ISSUES (I.E. SEVERING THE TROF WELL NORTH OF THE CANADIAN  
BOARDER)...THE STRONGER/AMPLIFIED SOLUTION PRESENTED IN THE 00Z  
RUN GREW STRONGER WITH THIS RUN...MAKING IT LESS FAVORABLE. THE  
UKMET CONTINUES TO BE MORE PHASED OVERALL AND TOO FAST. GIVEN THE  
POSITIVE TREND IN THE ECMWF AND COMPROMISE BETWEEN THE 12Z  
ECMWF/GFS WILL REACH A MORE PREFERABLE SOLUTION IN TIMING AS WELL  
AS EVOLUTION COMPARED TO THE INITIAL PREFERENCE. CONFIDENCE IS  
INCREASED BUT GIVEN THE LOW PREDICTABILITY OF THE PATTERN AND THE  
FEATURES STILL WELL OUT OF THE MORE DENSE DATA NETWORK...WILL KEEP  
A SLIGHTLY BELOW AVERAGE CONFIDENCE IN THE GFS/ECMWF BLEND.  
 
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