959  
FXUS02 KWBC 270650  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
149 AM EST MON NOV 27 2017  
 
VALID 12Z THU NOV 30 2017 - 12Z MON DEC 04 2017  
   
..OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES
 
 
THE EXTENDED FORECAST CONTINUES TO FOCUS PRIMARILY ON TWO  
FEATURES, A SHORTWAVE TROUGH PROGRESSING THROUGH THE EAST FROM  
NEAR THE MS VALLEY AND AN AMPLIFYING LARGE SCALE TROUGH MOVING  
INTO THE WEST FROM THE EASTERN PACIFIC. AFTER DAYTIME SAT RUNS  
HAD DEVELOPED TREMENDOUS DIFFERENCES ON IMPORTANT DETAILS OF THE  
SYSTEM AFFECTING THE EAST, GUIDANCE HAS ADJUSTED TOWARD SOMEWHAT  
MORE SIMILAR IDEAS IN THE PAST COUPLE CYCLES. ON THE OTHER HAND  
FORECASTS OF THE WESTERN TROUGH HAVE BEEN SOMEWHAT MORE STABLE IN  
GENERAL BUT WITH SOME LOCALLY IMPORTANT DETAIL DIFFERENCES THAT  
PERSIST.  
 
IT APPEARS THAT THE GFS/ECMWF HAVE ABANDONED THEIR BRIEF  
SUGGESTION FROM SAT RUNS THAT THE SYSTEM CROSSING THE EAST COULD  
FORM A SLOWER CLOSED LOW ALOFT, THOUGH SOME GEFS MEMBERS ARE STILL  
HANGING ONTO THE IDEA OF A SLOWER TROUGH THAN LATEST CONSENSUS.  
MEANWHILE THE 12Z ECMWF/ECMWF MEAN ARE A BIT ON THE PROGRESSIVE  
SIDE AS THE TROUGH REACHES THE EAST COAST. GIVEN THE INCONSISTENT  
HISTORY OF GUIDANCE FOR THIS SYSTEM, PREFERENCES LEAN TOWARD  
INTERMEDIATE TIMING BETWEEN THE ECMWF AND SLOWER GFS (ALBEIT WITH  
THE 18Z RUN TRENDING CLOSER TO OTHER GUIDANCE THAN THE 12Z RUN).  
THE COMBINATION OF MODEL/ENSEMBLE GUIDANCE NOW SIGNALS THE BEST  
OPPORTUNITY FOR SOME FLOW SEPARATION ALOFT TO BE FROM NEW ENGLAND  
INTO THE CANADIAN MARITIMES LATE FRI ONWARD. INDIVIDUAL MODEL  
RUNS SHOW POTENTIAL THAT UPSTREAM SHORTWAVE ENERGY COULD FEED INTO  
THE OVERALL MEAN TROUGH AND MAINTAIN A CLOSED LOW NEAR THE  
CANADIAN MARITIMES INTO THE START OF NEXT WEEK. ENSEMBLE SURFACE  
LOW CENTERS ARE STILL QUITE SCATTERED BUT CONFINED TO LOCATIONS  
OVER OR OFFSHORE NEW ENGLAND AND CANADIAN MARITIMES.  
 
THE AMPLIFYING TROUGH MOVING INTO THE WEST STILL SHOWS SOME  
AMPLITUDE/TIMING DIFFERENCES. AS THE TROUGH ARRIVES, GFS RUNS  
HAVE TENDED TO BE ON THE SLOWER/MORE SHALLOW SIDE OF THE SPREAD  
DUE TO MORE ENERGY BEING PULLED OFF FROM THE SOUTHWEST PART OF THE  
TROUGH WHILE THE FEATURE IS OVER THE PACIFIC. GEFS MEANS HAVE  
BEEN CLOSER TO THE MAJORITY CLUSTER REPRESENTED BY THE PAST COUPLE  
ECMWF RUNS AND MULTIPLE DAYS OF ECMWF MEAN/CMC/CMC MEAN RUNS.  
AMONG RECENT GFS RUNS THE 12Z VERSION WAS A LITTLE CLOSER TO  
CONSENSUS THAN THE 18Z RUN, BRINGING IN THE TROUGH A LITTLE FASTER  
TOWARD OTHER SOLUTIONS NEXT WEEKEND AND THEN HOLDING BACK ON  
LEADING HEIGHT FALLS REACHING THE PLAINS BY DAY 7 MON. THE NEW  
00Z GFS HAS GENERALLY IMPROVED ON THE 18Z RUN BUT MAY STILL BE A  
BIT FAST LATE IN THE PERIOD. BASED ON RECENT OPERATIONAL MODEL  
RUNS AND HINTS FROM THE ENSEMBLE MEANS, THERE APPEARS TO BE A  
BETTER THAN EVEN CHANCE THAT THE TROUGH WILL CONTAIN AN EMBEDDED  
CLOSED LOW FOR A PORTION OF THE PERIOD WITH A TRACK OVER OR  
SOUTH/SOUTHWEST OF THE GREAT BASIN. A MULTI-MODEL/ENSEMBLE MEAN  
CONSENSUS PROVIDES REASONABLE CONTINUITY FOR THIS SYSTEM'S  
EVOLUTION ALOFT AND ASSOCIATED SURFACE REFLECTION, AND DOWNPLAYS  
LESS CONFIDENT DETAILS LATER IN THE PERIOD-- SUCH AS THE FARTHER  
SOUTHWEST CLOSED LOW IN THE 12Z ECMWF NEXT MON.  
 
AHEAD OF THIS TROUGH, GUIDANCE CONTINUES TO MAKE SOME ADJUSTMENTS  
FOR AN UPPER LOW EXPECTED TO BE OFFSHORE BAJA CALIF AS THE START  
OF THE PERIOD THU, WITH A NOTABLE SHIFT TO THE SOUTH COMPARED TO  
24 HOURS AGO. AS A RESULT THE EVENTUAL NORTHEASTWARD EJECTION AND  
ASSOCIATED PRECIP OVER THE WEST ALSO TAKE A MORE SOUTHERN ROUTE.  
MEANINGFUL TIMING DIFFERENCES FOR THIS EJECTION STILL EXIST AND  
WILL LIKELY TAKE A WHILE TO RESOLVE.  
 
GIVEN PREFERENCES TOWARD AN INTERMEDIATE/CONSENSUS APPROACH FOR  
SIGNIFICANT FEATURES, THE UPDATED FORECAST INCORPORATED IDEAS FROM  
THE 18Z GFS AND 12Z ECMWF/UKMET/CMC DAYS 3-4 THU-FRI. THEN AS  
DETAIL UNCERTAINTIES INCREASED AND THE 12Z GFS COMPARED BETTER TO  
CONSENSUS THAN THE 18Z RUN, THE BLEND TRANSITIONED TO THE 12Z  
GFS/ECMWF/CMC AND 18Z GEFS/12Z ECMWF MEAN COMPONENTS.  
 
   
..WEATHER HIGHLIGHTS
 
 
EXPECT THE UPPER TROUGH ARRIVING FROM THE EASTERN PACIFIC TO  
INCREASE PRECIP COVERAGE ACROSS THE WEST WITH TIME, WITH HIGHEST  
TOTALS OVER FAVORED WINDWARD TERRAIN. UNRESOLVED DETAILS FOR FLOW  
ALOFT WITHIN THE TROUGH AND LEADING UPPER LOW WEST OF BAJA CALIF  
MAINTAIN UNCERTAINTY FOR EXACTLY WHICH LOCATIONS WILL SEE THE BEST  
FOCUS FOR ACTIVITY. INITIALLY NEAR TO ABOVE NORMAL TEMPS WILL  
DECREASE TO MODERATELY BELOW NORMAL VALUES FROM SAT THROUGH MON  
AND SNOW LEVELS WILL TREND LOWER. IN THE WARM SECTOR AHEAD OF THE  
LEADING COLD FRONT THERE WILL BE AN EXPANDING AREA OF ABOVE NORMAL  
TEMPS (ANOMALIES REACHING PLUS 10-20F) FROM THE FOUR CORNERS  
STATES INTO MUCH OF THE PLAINS AND MS VALLEY. LATE IN THE PERIOD  
A NORTHERN TIER WAVE/FRONT MAY HELP TO FOCUS PRECIP IN THAT  
REGION. THE FRONTAL SYSTEM CROSSING THE EAST LATE IN THE WEEK IS  
LIKELY TO BE FAIRLY PROGRESSIVE WITH LIGHT-MODERATE PRECIP (MOSTLY  
RAIN) OVER MOST AREAS. CURRENTLY THE ONE REGION OF UNCERTAINTY IS  
NEW ENGLAND WHERE PROXIMITY OF LOW PRESSURE MAY SUPPORT HEAVIER OR  
LONGER DURATION ACTIVITY. SNOW IS POSSIBLE OVER HIGHER ELEVATIONS  
DEPENDING ON DEPTH OF THE SYSTEM. THERE MAY BE A PERIOD OF BREEZY  
CONDITIONS OVER THE NORTHEAST AS WELL. FRONTAL PASSAGE WILL BRING  
SLIGHTLY BELOW NORMAL TEMPS INTO THE EAST, PERSISTING INTO THE  
WEEKEND.  
 
RAUSCH  
 

 
 
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