839  
FXUS02 KWBC 250645  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
144 AM EST SAT NOV 25 2017  
 
VALID 12Z TUE NOV 28 2017 - 12Z SAT DEC 02 2017  
   
..PATTERN ASSESSMENT AND MODEL CHOICES
 
 
INDIVIDUAL MODEL SOLUTIONS CONTINUE TO DIFFER BETWEEN EACH OTHER  
AND FROM RUN TO RUN FOR IMPORTANT DETAILS, BUT ULTIMATELY THE  
PATTERN EVOLUTION AND BEST APPROACH FOR REPRESENTING THE MOST  
LIKELY SCENARIO FOR EACH DAY DURING THE EXTENDED PERIOD HAVE  
CHANGED LITTLE OVER THE PAST 12-24 HOURS.  
 
GUIDANCE AGREEMENT MAINTAINS RELATIVELY HIGH CONFIDENCE FOR THE  
LARGEST SCALE ASPECTS OF THE FORECAST. EXPECT A PROGRESSIVE WAVE  
TRAIN TO BECOME ESTABLISHED ALL THE WAY FROM THE WESTERN PACIFIC  
ACROSS NORTH AMERICA, THEN SLOWING AND SPLITTING AROUND A  
PERSISTENT BLOCK OVER THE MIDDLE ATLANTIC OCEAN. TIME OF YEAR AND  
STRENGTH OF THE OVERALL POLE TO EQUATOR GRADIENTS OFFER POTENTIAL  
FOR EMBEDDED NOTEWORTHY OR EVEN STRONG LOWS TO SPIN UP WITHIN THE  
PROGRESSIVE FLOW, THOUGH FOR NOW THE DETERMINISTIC AND ENSEMBLE  
GUIDANCE SUGGESTS CYCLOGENESIS WILL FOR THE MOST PART REMAIN NORTH  
OF THE LOWER 48 STATES THROUGH NEXT WEEK. LATE IN THE PERIOD THE  
MAJORITY OF GUIDANCE STILL SHOWS AN AMPLIFYING UPPER TROUGH  
SETTLING NEAR THE WEST COAST, DOWNSTREAM FROM AN EAST-CENTRAL  
PACIFIC RIDGE. TELECONNECTIONS RELATIVE TO THE PACIFIC/WEST COAST  
PATTERN AS WELL AS ATLANTIC RIDGE SUGGEST THERE SHOULD BE A  
TENDENCY TOWARD ABOVE NORMAL HEIGHTS ALOFT OVER THE EASTERN U.S..  
 
EARLY IN THE PERIOD, GUIDANCE SHOWS SOMEWHAT IMPROVED CLUSTERING  
FOR NORTHERN STREAM FLOW ACROSS THE NORTHERN TIER U.S. AND  
SOUTHERN CANADA BUT MODEL/ENSEMBLE SPREAD IS STILL MUCH BROADER  
THAN DESIRED FOR THE SOUTHERN STREAM SHORTWAVE/POSSIBLE CLOSED LOW  
LIKELY TO BE NEAR THE FOUR CORNERS REGION AS OF EARLY TUE. THE  
18Z GFS/GEFS JUMPED WELL AHEAD OF THEIR CORRESPONDING 12Z RUNS. A  
STEADILY FASTER TREND IN THE ECMWF OVER THE 24 HOURS THROUGH ITS  
12Z/24 RUN LED TO SOME HESITATION ABOUT THE 12Z UKMET THAT BECAME  
THE SLOWEST OPERATIONAL SOLUTION BY WED. A COMPROMISE AMONG THE  
12Z GFS/ECMWF/CMC PROVIDED THE BEST INTERMEDIATE SOLUTION GIVEN  
ONGOING UNCERTAINTY, YIELDING AT MOST A MODEST NUDGE FROM  
CONTINUITY. NEW 00Z RUNS THUS FAR SEEM TO FAVOR STAYING IN THE  
MIDDLE OF THE SPREAD AS THE GFS/CMC HAVE SLOWED FROM THEIR 18Z/12Z  
RUNS RESPECTIVELY WHILE THE UKMET HAS ACCELERATED.  
 
THE UPSTREAM TROUGH APPROACHING THE WEST COAST ON TUE ALREADY HAS  
SOME DETAIL DIFFERENCES AT THAT TIME. THEN MODELS/ENSEMBLE  
MEMBERS CONTINUE TO VARY WIDELY REGARDING HOW MUCH ENERGY  
SEPARATES FROM THE SOUTHERN PART OF THE TROUGH OFF CALIFORNIA  
VERSUS HOW MUCH CONTINUES INLAND, AND THEN WHAT HAPPENS WITH THE  
OFFSHORE CALIFORNIA ENERGY AS UPSTREAM TROUGHING DEVELOPS AND  
SPECIFICS OF ENERGY THAT CONTINUES ACROSS THE LOWER 48. THUS FAR  
OPERATIONAL RUNS HAVE BEEN SUFFICIENTLY INCONSISTENT TO KEEP  
CONFIDENCE LOW IN ANY PARTICULAR SOLUTION. 00Z GFS TREND HAS  
ALIGNED IT WITH THE 12Z ECMWF OVER THE LOWER 48 BUT BOTH BECOME  
SOMEWHAT FASTER WITH THE TROUGH THAN THE MORE STABLE ENSEMBLE  
MEANS OVER THE EAST.  
 
IN TERMS OF BOTH DEPTH AND POSITION THE ECMWF MEAN RUNS CONTINUE  
TO PROVIDE THE MOST CONSISTENT EVOLUTION FOR THE UPPER TROUGH  
SETTLING NEAR THE WEST COAST BY DAYS 6-7 FRI-SAT. ECMWF/CMC RUNS  
HAVE GENERALLY SHARED SOME SIMILARITY WITH THE ECMWF MEAN IN  
PRINCIPLE. GFS RUNS HAVE TENDED TO BE SOMEWHAT SHARPER/SLOWER  
WHILE RECENT GEFS MEANS HAVE SEEMED TO WAFFLE BETWEEN ECMWF MEAN  
AND GFS IDEAS AND WITH HIGHER HEIGHTS THAN THE ECMWF MEAN. NOTE  
THAT THE 18Z/00Z GFS RUNS WEAKEN THE NORTHERN PART OF THE PACIFIC  
RIDGE MORE THAN OTHER GUIDANCE TOWARD DAY 7 SAT, LOWERING  
CONFIDENCE IN THE GFS FCST NEAR THE WEST COAST BY DAY 7 AND  
BEYOND. WITHIN THE 12Z/18Z COLLECTION OF MEANS THE CMC MEAN WAS A  
SLIGHTLY WEAKER VERSION OF THE ECMWF MEAN WHILE THE 12Z GEFS MEAN  
WAS CLOSER TO THE ECMWF MEAN THAN THE 18Z RUN. THIS FAVORS THE  
12Z RUN FOR THE GEFS COMPONENT OF THE FORECAST.  
 
BASED ON PREFERENCES FOR THE INITIAL FOUR CORNERS FEATURE THE  
UPDATED FORECAST STARTED WITH A BLEND OF 12Z GFS/ECMWF/CMC RUNS  
FOR DAYS 3-4 TUE-WED. THIS BLEND REPRESENTED THE MOST COMMON  
IDEAS OF GUIDANCE ELSEWHERE. CONTINUED DETAIL SPREAD LED TO A  
RAPID INCREASE OF ENSEMBLE WEIGHTING AROUND MID-PERIOD ON THE WAY  
TO AN EXCLUSIVELY ENSEMBLE MEAN FORECAST BY DAYS 6-7 FRI-SAT. FOR  
THIS LATTER TIME FRAME, PERSISTENCE OF THE ECMWF MEAN MERITED 2/3  
WEIGHTING RELATIVE TO THE 12Z GEFS MEAN.  
 
   
..WEATHER HIGHLIGHTS
 
 
EXPECT FAIRLY LOW COVERAGE OF SIGNIFICANT PRECIP ACROSS THE LOWER  
48 DURING THE PERIOD. THE FEATURE EMERGING FROM THE FOUR CORNERS  
REGION MAY PRODUCE A BRIEF PERIOD OF SNOW OVER THE ROCKIES  
INITIALLY, FOLLOWED BY RAINFALL ACROSS PARTS OF THE PLAINS/MS  
VALLEY. LACK OF AGREEMENT ON TIMING/TRACK OF THIS SYSTEM KEEPS  
CONFIDENCE LOW FOR INTENSITY AND PRECISE LOCATION OF THIS RAIN.  
LEADING FRONT CROSSING THE NORTHEAST AROUND MIDWEEK SHOULD BE  
FAIRLY DRY EXCEPT FOR EXTREME NORTHERN AREAS. NEXT FRONT ARRIVING  
INTO THE NORTHWEST ON TUE WILL BRING A PERIOD OF RAIN/SNOW WITH  
BEST FOCUS OVER THE FAVORED TERRAIN OF WESTERN WA/OR AND NORTHERN  
ROCKIES. PRECIP COVERAGE SHOULD EXPAND WITH THE FRONT AS IT  
CROSSES THE EAST LATER IN THE WEEK. ANY SNOW OVER THE EAST SHOULD  
BE CONFINED TO FAR NORTHERN AREAS/HIGH ELEVATIONS. DURING THE  
LATTER HALF OF THE PERIOD EXPECT THE UPPER TROUGH NEARING THE WEST  
COAST TO BRING AN INCREASE OF MOISTURE TO THE REGION, BUT WITH  
LOWER THAN DESIRED CONFIDENCE ON SPECIFICS DUE TO DETAIL  
UNCERTAINTIES WITH THE TROUGH ITSELF AS WELL AS  
STRENGTH/LOCATION/TIMING QUESTIONS WITH ENERGY THAT MAY BE  
LINGERING OFFSHORE CALIFORNIA AHEAD OF THE TROUGH.  
 
THERE WILL BE DECENT COVERAGE OF WELL ABOVE NORMAL TEMPS OVER THE  
PLAINS/EAST-CENTRAL U.S. ON TUE WITH HIGHEST ANOMALIES OF PLUS  
15-25F POSSIBLE FOR MINS THAT MORNING. PROGRESSION OF THE INITIAL  
GREAT LAKES-PLAINS FRONT WILL LOWER TEMPS TO MODESTLY ABOVE NORMAL  
LEVELS OVER MOST OF THE EAST AND THEN THE NEXT FRONT REACHING THE  
EAST BY THU-FRI SHOULD BRING READINGS DOWN TO A FEW DEGREES F  
EITHER SIDE OF NORMAL. HOWEVER SOME PLUS 10-15F ANOMALIES WILL  
LIKELY PERSIST OVER/NEAR THE NORTHERN PLAINS THROUGH THE PERIOD.  
NEAR TO SLIGHTLY ABOVE NORMAL TEMPS SHOULD TREND A LITTLE BELOW  
NORMAL OVER THE WEST COAST STATES BY NEXT SAT AS UPPER TROUGHING  
ARRIVES.  
 
RAUSCH/BURKE  
 

 
 
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