259  
FXUS10 KWNH 171846  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
146 PM EST WED JAN 17 2018  
 
VALID JAN 17/1200 UTC THRU JAN 21/0000 UTC  
 
...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR  
INGEST...  
 
FINAL 12Z MODEL EVALUATION INCLUDING PREFERENCES AND FORECAST  
CONFIDENCE  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
~~~~~  
 
EXITING EASTERN US TROF AND COASTAL SURFACE LOWS  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 17/12Z NAM AND 17/12Z ECMWF/UKMET BLEND  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
GUIDANCE TREND CONTINUES TO SLOW THE BASE OF THE TROF EXITING THE  
CAROLINAS LEADING TO A MORE WOUND UP SOLUTION OFF SHORE...THE 12Z  
NAM CONTINUED ITS TREND OF DEEPER WOUND UP SOLUTION BUT IS A SHADE  
FURTHER NORTH WITH THE SURFACE WAVE EMERGING OFF THE CAROLINA  
COAST. THIS PAIRS NICELY WITH THE DEEPER/SLOWER SOLUTION OF THE  
ECMWF AND ECENS MEAN...WHICH CONTINUED TO SHOW A SLOWER WEAKENING  
PHASE WHILE TRANSLATING WELL OFF SHORE/NORTH OF BERMUDA IN THE  
17/12Z MODEL CYCLE.  
 
THE 17/12Z CMC ON THE OTHER HAND WAS A BIT QUICKER TO BREAK DOWN  
THE WAVE AND ACCELERATES TO BE SLIGHTLY SLOWER THAN THE GFS.  
WHILE THE 17/12Z CMC WAS A BIT SLOWER TO BREAK DOWN THE WAVE  
COMPARED WITH ITS PREVIOUS RUN...IT REMAINED ON THE FASTER SIDE OF  
ENVELOPE OF SOLUTIONS. THE 17/12Z GFS CONTINUED TO BE FASTEST AND  
LEAST WOUND UP (THOUGH CONTINUED TO TREND TOWARD THE DEEPER/SLOWER  
SOLUTIONS...PROVIDING MORE CONFIDENCE IN SELECTING THIS SLOWER  
SOLUTION).  
 
OVERALL A BLEND OF THE 17/12Z NAM AND 17/12Z ECMWF/UKMET SHOULD  
PROVIDE SLIGHTLY ABOVE AVERAGE CONFIDENCE IN THE FORECAST.  
 
SHORTWAVE ROLLING THROUGH NORTHERN PLAINS THIS AFTERNOON INTO  
GREAT LAKES/NEW ENGLAND BY FRI  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
GOES-WV LOOP DEPICTS THE SOUTHEAST PORTION OF THE STRETCHED  
NEGATIVE TILT TROF STARTING TO WORK THROUGH THE APEX OF THE LARGER  
SCALE MEAN RIDGE OVER THE SOUTHERN CANADIAN ROCKIES. NOW THAT THE  
WAVE HAS REACHED THIS APEX THERE IS A MUCH BETTER AGREEMENT IN  
TIMING AND LATITUDE AT WHICH THE WAVE EMERGES INTO THE  
WEST-NORTHWESTERLY FLOW. STILL SOME SMALL TIMING DIFFERENCES  
REMAIN THOUGH MORE IN-LINE WITH TRADITIONAL TIMING DIFFERENCES:  
CMC/ECMWF SLOWER THAN THE NAM/GFS AND THE UKMET LEADING SLIGHTLY.  
OVERALL THIS SPREAD IS SMALL ENOUGH TO SUPPORT A GENERAL MODEL  
BLEND AT SLIGHTLY ABOVE AVERAGE CONFIDENCE.  
 
SHORTWAVE DEVELOPING ACROSS TX BY EARLY THURS...SLIDING EAST ALONG  
GULF COAST FRI/SAT  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 17/12Z NAM/GFS WITH LOW WEIGHT GIVEN TO THE 17/12Z  
ECMWF  
CONFIDENCE: SLIGHTLY BELOW AVERAGE  
 
MAIN SHORTWAVE OF EVENTUAL BROAD UPPER TROF THAT WILL EVOLVE OVER  
CENTRAL TX/NE MEXICO IN THE NEXT DAY OR SO REMAIN OVER THE HIGHER  
PLATEAUS OF THE FOUR CORNERS REGION AND CHIHUAHUA. THE  
WAY/DIRECTION THE TWO MAIN PIECES OF VORT ENERGY EMERGES FROM THE  
TERRAIN AND THE STRENGTH OF THE SWLY FLOW FROM THE EASTERN PACIFIC  
LEAD TO MODEST SPREAD IN HOW THE WAVE STRETCHES INTO A POSITIVE  
TILT TROF WHILE CROSSING THE GULF COAST INTO FRI.  
 
THE SLOWER/FURTHER SOUTH GUIDANCE IS PRESENTED BY THE 17/12Z ECMWF  
AND CMC. HOWEVER...THE CMC IS A BIT MORE ELONGATED WITH GREATER  
POSITIVE TILTING ALLOWING FOR A MORE CONSOLIDATED INTERNAL WAVE  
THAN THE EC BY LATE SAT. THE FASTER SOLUTIONS OF THE 17/12Z  
NAM/GFS AND 17/12Z UKMET ALSO GO THROUGH A STRETCHING AND EVENTUAL  
RECONSOLIDATION OVER THE SOUTHEAST MUCH FURTHER NORTH AND EAST.  
THE 17/12Z ENSEMBLE SPAGHETTI PLOT SHOWED THE LARGE SPREAD IN THIS  
EVOLUTION OF STRETCHING AND RECONSOLIDATION WITH A BULK OF ECENS  
MEMBERS CENTERING FASTER AND EAST THAN THE OPERATIONAL  
RUN...THOUGH THE OPERATIONAL DOES REPRESENT THE EC MEAN WELL. THE  
GEFS ARE CENTERED MUCH CLOSER TO THE GFS/NAM SOLUTION AND A SMALL  
CLUSTERING OF ECENS MEMBERS BRINGING SOME CONFIDENCE TO THAT  
SOLUTION. THE UKMET REMAINED FASTER AND STAYED FURTHEST EAST IN  
THE SPAGHETTI PLOT.  
 
AS SUCH WILL FAVOR A 17/12Z GFS/NAM WITH SOME SLIGHT WEIGHT TO THE  
GEFS AND 17/12Z ECMWF SOLUTIONS.  
 
DEEP LAYERED TROF APPROACHING VANCOUVER ISLAND BY LATE WED/EARLY  
THURS WITH COLD FRONT SINKING SOUTH THROUGH FRI  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: NON-UKMET BLEND  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
GUIDANCE INCLUDING THE 17/12Z NAM/GFS CONTINUE TO CONSOLIDATE ON  
STRENGTH/SHAPE AND TIMING OF THE DEEP LAYERED MEAN CYCLONE  
EMERGING FROM THE CENTRAL PACIFIC TODAY...AND SETTLING BETWEEN  
VANCOUVER ISLAND AND THE QUEEN CHARLOTTE ISLANDS...WHERE IT SLOWLY  
FILLS/DECAYS. THE AGREEMENT IS VERY STRONG THROUGH ABOUT 00Z  
FRIDAY WHEN INFLUENCES FROM A UPSTREAM DEEPENING CYCLONE EITHER  
ACCELERATE THE WEAKENING OR SHIFT THE LOW NORTH OR SOUTH. THE  
17/12Z GFS REMAINS MOST STUBBORN IN WEAKENING THE LOW SUPPORTING  
THE SFC LOW TO SLOWLY DRIFT E STAYING IN THE CHANNEL BETWEEN THE  
ISLANDS...THE 17/12Z NAM ALSO A BIT SLOWER TO WEAKEN SHIFTS  
SOUTHEASTWARD WHILE THE 17/12Z ECMWF/CMC SLOWLY SHIFT NORTHWARD  
UNDER STRONGER SWLY INFLUENCE FROM THE APPROACHING WAVE. NOTE  
THAT THE 17/12Z ECMWF MAINTAINED GOOD CONTINUITY WITH ITS PREVIOUS  
RUNS WITH THIS FEATURE. THE LATEST UKMET IS MOST DRAMATIC IN A  
SHIFT NORTHWARD AS THE UPSTREAM WAVE IS MUCH FASTER THAN ANY OTHER  
GUIDANCE LEADING TO GREATER BINARY INTERACTION. THIS SEEMS LESS  
LIKELY GIVEN THE ENSEMBLE SUITE...AND THE DIFFERENCE BETWEEN THE  
TIMING OF WEAKENING APPEARS TO SIT UPON A RAZOR'S EDGE ON WHICH  
WAY IT WILL FALL OUT. A BLEND OF THOSE REMAINING SOLUTIONS SEEMS  
A GOOD COMPROMISE. FURTHER SOUTH THE COLD FRONT REMAINS WELL  
TIMED WITH SIMILAR STRENGTH/MOISTURE WITHIN THE ATMOSPHERIC RIVER  
CONVEYOR THURS INTO FRI...PLEASE SEE SECTIONS BELOW FOR MORE  
DETAILS OF THE PERIPHERAL SHORTWAVES ENTERING THE US.  
 
LEAD SHORTWAVE ENERGY FROM DEEP LAYERED TROF INTERACTS WITH ARCTIC  
STREAM TO PRODUCE BROAD TROF SPURRING SURFACE CYCLONE ACROSS  
CENTRAL CANADA CLIPPING THE NORTHERN TIER LATE THURS INTO FRI AND  
NORTHERN NEW ENGLAND LATE SAT  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 12Z GFS AND 12Z ECMWF/CMC BLEND  
CONFIDENCE: AVERAGE  
 
AS THE DEEP LAYERED CYCLONE BREAKS DOWN LATE THURS INTO  
FRIDAY...THE LEADING SHORTWAVE ENERGY NEAR THE TRIPLE POINT  
(INTERSECTION OF SSWLY FLOW AND TROWAL BACK TO THE NW) PRESSES  
NORTHEAST INTO THE SOUTHERN CANADIAN ROCKIES. COMBINATION FROM  
DOWNSTREAM LINGERING ENERGY FROM THE PREVIOUS TROF AS WELL AS  
DESCENDING SHORTWAVE OUT OF THE ARCTIC FLOW WILL PRODUCE A BROADLY  
CYCLONIC SHORTWAVE INTERACTION ACROSS CENTRAL CANADA INTO FRIDAY.  
 
GUIDANCE HAS BEEN CONSOLIDATING ON A COMMON TIMING OF THE  
INTERACTION OF THE PACIFIC ENERGY AND THE ARCTIC SHORTWAVE...WITH  
A FASTER PACIFIC FLOW SUPPORTING A BROAD SURFACE CYCLONE THAT  
CROSSES CENTRAL CANADA QUICKLY AND AN ARCTIC FRONT DRAPED BACK TO  
THE LEE OF THE CANADIAN ROCKIES. FROM THAT POINT...A SECOND  
WEAKER WAVE WILL SLIDE SOUTHEAST BRING THE COLD FRONT ACROSS THE  
GREAT LAKES MIDDAY TO LATE SAT WHILE CLIPPING NORTHERN NEW  
ENGLAND. THE 12Z UKMET WAS A BIT FASTER EMERGING THROUGH THE  
RIDGE IN THE ROCKIES...AND IS FURTHER SOUTH WITH THE LEADING  
SURFACE WAVE... THE 12Z NAM DEPICTS A STRONGER SHORTWAVE THAT IS  
SLOW THROUGH THE RIDGE AND UNLIKE THE 12Z GFS/ECMWF/CMC IS FURTHER  
SOUTH AS WELL. WHILE THERE REMAINS MODEST SPREAD IN THE  
TIMING/MAGNITUDE OF THE INTERACTION BETWEEN THE ARCTIC AND PACIFIC  
STREAMS...A BLEND OF THE 12Z GFS/ECMWF/CMC SEEMS TO REPRESENT THE  
MEAN ENSEMBLE SOLUTION WHILE ALSO HANDLING THE TREND TOWARD A  
FASTER PACIFIC FLOW REGIME.  
 
CONFIDENCE IS AVERAGE GIVEN THE CONTINGENCY ON TIMING/STRENGTH OF  
BOTH STREAMS.  
 
BASE OF DEEP LAYERED CYCLONE CROSSING INTERMOUNTAIN WEST LEADING  
TO LEE CYCLONE BY LATE FRI IN SE COLORADO  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
AS THE DEEP LAYERED CYCLONE BREAKS DOWN LATE THURS INTO  
FRIDAY...THE BASE OF THE TROF BEGINS TO FOLD SLIGHTLY SHARPENING  
THE TROF ENTERING CENTRAL CA AND SLIDING SOUTH. THIS DIRECTS THE  
MOISTURE STREAM INTO THE HIGHER TERRAIN WITH VERY GOOD MODEL  
AGREEMENT IN ORIENTATION AND TIMING. MODEL AGREEMENT REMAINS  
STRONG THROUGH THE REMAINING SHORT-TERM FORECAST EVEN AS THE TROF  
BECOMES MORE NEUTRALLY TILTED AND BEGINS TO WIND UP MORE  
CYCLONICALLY ACROSS SE NV/SW UT AND NW AZ AT 00Z SUN. THE  
AGREEMENT IS STRONG ENOUGH TO SUPPORT A GENERAL MODEL BLEND AT  
SLIGHTLY ABOVE AVERAGE CONFIDENCE.  
 
NEXT CLOSED LOW APPROACHING VANCOUVER ISLAND/NW WA BY LATE SAT  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 50 PCT ECMWF AND 50 PCT NAM/GFS BLEND  
CONFIDENCE: AVERAGE  
 
THE NEXT UPSTREAM CLOSED LOW WILL APPROACH VANCOUVER ISLAND TOWARD  
LATE SAT INTO SUNDAY AND WILL QUICKLY BREAK DOWN THE PRECEDING  
WAVE...HOWEVER IT WILL CONTINUE TO DIRECT THE SOUTHWESTERLY  
MOISTURE STREAM ASHORE THOUGH A LITTLE LESS FOCUSED THAT THE PRIOR  
WAVE INITIALLY GIVEN THE PROXIMITY TO THE LOW/STRENGTH OF FLOW  
WILL BE LIMITED AT LEAST THROUGH DAY 3. THE 12Z CMC WAS STILL  
FURTHER EAST AS WELL BUT LIKELY DUE TO THE FACT IT IS MUCH WEAKER  
THAN THE SLOWER ECMWF OR 12Z GFS/NAM. GIVEN GOOD RUN TO RUN  
CONTINUITY...THE 17/12Z ECMWF WAS A NICE MIDDLE GROUND BETWEEN THE  
FASTER AND WEAKER SOLUTIONS OF THE UKMET/CMC VS. THE DEEPER FASTER  
GFS/NAM. IT ALSO CLUSTERED WELL WITH THE SREF/ECENS AND WITH THE  
GEFS WHICH WERE ALSO LEANING A BIT FASTER.  
 
MODEL TRENDS AT WWW.WPC.NCEP.NOAA.GOV/HTML/MODEL2.SHTML  
500 MB FORECASTS AT WWW.WPC.NCEP.NOAA.GOV/H5PREF/H5PREF.SHTML  
 
GALLINA/BANN  
 

 
 
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