894  
FXUS10 KWNH 171609  
PMDHMD  
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1108 AM 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...  
 
PLEASE REFER TO THE WPC QPFPFD PRECIPITATION FORECAST DISCUSSION  
FOR DETAILS ON MODEL PREFERENCES AND SYSTEM TIMING UNTIL THE NEXT  
PMDHMD IS ISSUED AT 1645 UTC.  
 
12Z MODEL EVALUATION INCLUDING PREFERENCES AND FORECAST CONFIDENCE  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
 
EXITING EASTERN US TROF AND COASTAL SURFACE LOWS  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 12Z NAM AND 00Z 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 SHOW A SLOWER WEAKENING PHASE WHILE  
TRANSLATING WELL OFF SHORE/NORTH OF BERMUDA. THE 00Z CMC ON THE  
OTHER HAND IS A BIT QUICKER TO BREAK DOWN THE WAVE AND ACCELERATES  
TO BE SLIGHTLY SLOWER THAN THE GFS. THE 12Z GFS CONTINUES 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 12Z NAM AND 00Z  
ECMWF/UKMET SHOULD PROVIDE SLIGHTLY ABOVE AVERAGE CONFIDENCE IN  
THE FORECAST.  
 
SHORTWAVE ROLLING THROUGH NORTHERN PLAINS TODAY WED INTO GREAT  
LAKES THURS AND 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: 12Z NAM/GFS WITH LOW WEIGHTED 00Z 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 00Z 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 12Z NAM/GFS  
AND 00Z UKMET ALSO GO THROUGH A STRETCHING AND EVENTUAL  
RECONSOLIDATION OVER THE SOUTHEAST MUCH FURTHER NORTH AND EAST.  
THE 00Z ENSEMBLE SPAGHETTI PLOT CONTINUES TO SHOW 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...WHILE THE UKMET IS FASTER AND FURTHEST EAST IN THE  
SPAGHETTI PLOT. THIS PROVIDES A LOW CONFIDENCE ENVIRONMENT THAT  
MAY START TO CONSOLIDATE WITH THE 12Z SOLUTIONS GIVEN THE WAVE IN  
EMERGING FROM TERRAIN. AS SUCH WILL FAVOR A 12Z GFS/NAM WITH SOME  
SLIGHT WEIGHT TO THE GEFS/00Z ECMWF SOLUTIONS.  
 
00Z ENSEMBLE SPAGHETTI ANALYSIS CONTINUE TO SHOW LARGE SPREAD IN  
THE SHAPE/TIMING OF THE TROF MOSTLY AFTER  
 
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 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 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 12Z NAM ALSO A BIT SLOWER TO WEAKEN SHIFTS  
SOUTHEASTWARD WHILE THE 00Z ECMWF/CMC SLOWLY SHIFT NORTHWARD UNDER  
STRONGER SWLY INFLUENCE FROM THE APPROACHING WAVE. THE 00Z 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 00Z 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 WITH THE LINGERING ARCTIC FRONT  
DRAPED BACK TO THE LEE OF THE CANADIAN ROCKIES WHERE 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 00Z UKMET APPEARS 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/00Z ECMWF AND 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 AND 00Z 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: 1/2 00Z ECMWF AND 1/2 12Z 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 00Z UKMET IS MUCH TOO  
QUICK SHOWING A GREATER BINARY INTERACTION WITH THE PRIOR FILLING  
WAVE ALONG THE COAST AND STICKS OUT WITHIN THE ENSEMBLE SUITE.  
THE 00Z CMC IS UNCHARACTERISTICALLY FURTHER EAST AS WELL BUT  
LIKELY DUE TO THE FACT IT IS MUCH WEAKER THAN THE SLOWER ECMWF OR  
12Z GFS/NAM. THE ECMWF IS A NICE MIDDLE GROUND BETWEEN THE FASTER  
AND WEAKER SOLUTIONS OF THE UKMET/CMC VS. THE DEEPER FASTER  
GFS/NAM BUT ALSO CLUSTERS WELL WITH THE SREF/ECENS AND WITH THE  
GEFS ALSO LEANING A BIT FASTER WILL SUPPORT A MUCH HIGHER  
WEIGHTING TO THE ECMWF IN A BLEND WITH THE 12Z NAM/GFS.  
CONFIDENCE IS AVERAGE IN THE BLEND GIVEN THE MODEST SPREAD.  
 
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PREVIOUS 500 MB FORECASTS AT  
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