198  
FXUS02 KWBC 280645  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
144 AM EST TUE NOV 28 2017  
 
VALID 12Z FRI DEC 01 2017 - 12Z TUE DEC 05 2017  
   
..OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES
 
 
NEARLY ZONAL MEAN FLOW ACROSS THE LOWER 48 AT THE START OF THE  
PERIOD WILL BECOME MORE AMPLIFIED AS A TROUGH MOVES INTO THE  
WESTERN STATES, MOST LIKELY BECOMING ORIENTED FROM THE NORTHERN  
PLAINS SOUTHWESTWARD BY DAY 7 NEXT TUE. A DOWNSTREAM RIDGE WILL  
PROGRESS FROM THE ROCKIES/PLAINS INTO THE EAST WHILE A MEAN TROUGH  
BECOMES ESTABLISHED OVER THE WESTERN ATLANTIC.  
 
WHILE ALL GUIDANCE HAS BEEN SIGNALING THE EXISTENCE OF THE WESTERN  
TROUGH OVER THE PAST FEW DAYS, MODELS/ENSEMBLE MEMBERS HAVE BEEN  
DIFFERING ON VARIOUS ASPECTS. MULTIPLE GFS RUNS THROUGH THE 18Z  
CYCLE HAD TENDED TO PULL OFF SOME ENERGY FROM THE SOUTHWEST PART  
OF THE TROUGH WHILE STILL OVER THE PACIFIC, LEADING TO A FLATTER  
TROUGH THAN THE GUIDANCE MAJORITY (INCLUDING GEFS MEANS) UPON  
ARRIVAL INTO THE WEST. THE NEW 00Z GFS HAS ADJUSTED AWAY FROM  
THAT QUESTIONABLE EVOLUTION BUT NOW HOLDS THE TROUGH BACK ON THE  
SLOWER SIDE OF THE SPREAD AND FORMS AN EMBEDDED CLOSED LOW EARLIER  
THAN CONSENSUS. BY THE LATTER HALF OF THE PERIOD SOLUTIONS  
DISPLAY INCREASING DIVERGENCE REGARDING EVOLUTION OF EMBEDDED  
TROUGH ENERGY AND RESULTING TIMING. ON ONE EXTREME THE 12Z/18Z  
GFS RUNS STRAYED FASTER THAN THE GEFS/ECMWF/CMC MEANS AND OTHER  
SOLUTIONS WITH A LARGE PART OF THE TROUGH, WHILE THE 12Z ECMWF  
HELD BACK A DEEP CLOSED LOW OVER THE EXTREME SOUTHWEST. BOTH ARE  
WITHIN THE FULL ENSEMBLE ENVELOPE THOUGH IN THE MINORITY DEPENDING  
ON HEIGHT VALUE COMPARED. INTERESTINGLY THE NEW 00Z CMC HAS  
ADJUSTED TOWARD A SLOWER CLOSED LOW VERSUS ITS PRIOR RUN THOUGH  
NORTH OF THE 12Z ECMWF. TELECONNECTIONS RELATIVE TO THE UPSTREAM  
RIDGE (POSITIVE HEIGHT ANOMALY CENTER JUST OFF VANCOUVER ISLAND)  
OFFER SUPPORT FOR A LINGERING WEAKNESS OVER THE EXTREME SOUTHWEST  
AS PER THE ENSEMBLE MEANS BUT ARE NOT CONVINCING FOR AN UPPER LOW  
WITH THE COMBINED DEPTH/PERSISTENCE OF THE 12Z ECMWF OR 00Z CMC.  
MEANWHILE IN SPITE OF QUESTION MARKS IN THE 00Z GFS AS THE TROUGH  
REACHES THE WEST COAST, IT COMPARES MUCH MORE FAVORABLY TO THE  
MEANS FARTHER INLAND VERSUS THE 12Z/18Z RUNS. GOOD AGREEMENT FOR  
THE TROUGH IN PRINCIPLE BUT CONTINUING RUN TO RUN CHANGES IN THE  
MODEL DETAILS FAVOR A CONSENSUS/INTERMEDIATE APPROACH THROUGH THE  
PERIOD, REPRESENTED BY AN OPERATIONAL MODEL BLEND EARLY AND THEN  
TRANSITIONING TOWARD THE ENSEMBLE MEANS LATE.  
 
SOUTHEAST OF THE MAIN TROUGH THE GUIDANCE IS STILL QUITE VARIED  
WITH UPPER LOW ENERGY THAT MAY COLLECT OFF BAJA CALIF BY THE START  
OF THE PERIOD AND EVENTUALLY EJECT NORTHEASTWARD. AT THIS TIME  
THE MOST COMMON THEME IS THAT THIS ENERGY SHOULD BE BETTER DEFINED  
AND SLOWER THAN GFS RUNS.  
 
FARTHER EAST, CONSENSUS IS FURTHERING THE RECENT FASTER TREND WITH  
THE SHORTWAVE INITIALLY NEAR THE EAST COAST AND ASSOCIATED SURFACE  
DEVELOPMENT WHICH NOW LOOKS TO HAVE MINIMAL IMPACT ON THE  
NORTHEAST. ON THE OTHER HAND GUIDANCE HAS SHOWN A STRONGER TREND  
WITH UPSTREAM ENERGY THAT AMPLIFIES AS IT EMERGES FROM THE PLAINS.  
THROUGH THE 12Z/18Z CYCLES MOST SOLUTIONS WEAKENED ITS SURFACE  
REFLECTION BEYOND THE UPPER GREAT LAKES BUT NOW THE 00Z GFS/UKMET  
DEVELOP LOW PRESSURE OFF THE EAST COAST SUN IN CONTRAST TO HAVING  
THE ENERGY SIMPLY REINFORCE THE INITIAL SYSTEM FARTHER OFFSHORE.  
THE REFERENCED BLEND FOR THE WESTERN TROUGH REPRESENTED THE MOST  
COMMON IDEAS OF GUIDANCE OVER THE EAST AS OF THE 12Z/18Z CYCLES.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
AREAS OF RAIN AND HIGHER ELEVATION SNOW INITIALLY OVER THE  
NORTHWEST WILL SPREAD SOUTHEASTWARD WITH THE APPROACH/ARRIVAL OF  
THE UPPER TROUGH. MANY AREAS OF FAVORED TERRAIN OVER THE WEST  
SHOULD SEE A PERIOD OF ENHANCED PRECIP BUT SIGNIFICANT DETAIL  
UNCERTAINTIES FOR FLOW ALOFT KEEP CONFIDENCE LOWER THAN DESIRED  
FOR PINPOINTING WHERE THE HIGHEST TOTALS FOR THE FULL EVENT WILL  
BE. AS THE UPPER TROUGH MOVES INLAND EXPECT ABOVE NORMAL TEMPS  
EARLY IN THE PERIOD TO FALL TO NEAR/BELOW NORMAL VALUES BY LATE  
WEEKEND/EARLY NEXT WEEK, WITH SNOW LEVELS LIKEWISE DECREASING.  
THE LEADING FRONT HEADING INTO THE PLAINS WILL PROMOTE AN  
EXPANDING AREA OF PRECIP OVER PARTS OF THE PLAINS/EAST-CENTRAL  
U.S. EARLY NEXT WEEK. MOST SHOULD BE IN THE FORM OF RAIN EXCEPT  
PERHAPS OVER THE NORTHERN PLAINS AFTER FRONTAL PASSAGE. AHEAD OF  
THE FRONT THERE WILL BE A FAIRLY BROAD AREA OF TEMPS 10-20F ABOVE  
NORMAL. TOWARD THE EAST COAST, CYCLONIC MEAN FLOW ALOFT WILL  
LIKELY KEEP TEMPS WITHIN A FEW DEGREES F ON EITHER SIDE OF NORMAL  
THROUGH THE WEEKEND. SEPARATE FRI AND WEEKEND SYSTEMS MAY BRING  
SOME RAIN AND POSSIBLY HIGHER ELEVATION SNOW TO THE NORTHEAST.  
TEMPS OVER THE EAST SHOULD TREND A LITTLE WARMER BY TUE AS THE  
UPPER RIDGE REACHES THE AREA.  
 
RAUSCH  
 

 
 
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