219  
FXUS02 KWBC 240643  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
142 AM EST FRI NOV 24 2017  
 
VALID 12Z MON NOV 27 2017 - 12Z FRI DEC 01 2017  
   
..PATTERN ASSESSMENT AND MODEL CHOICES
 
 
CONFIDENCE VARIES WIDELY DEPENDING ON THE SCALE OF INTEREST AND  
OTHER ASPECTS OF THE FORECAST. THERE IS GOOD AGREEMENT REGARDING  
THE IDEA OF PROGRESSIVE MEAN FLOW ACROSS THE LOWER 48 FOR MOST OF  
NEXT WEEK, TO THE SOUTH OF A CANADIAN UPPER LOW. THE MAJORITY OF  
GUIDANCE IS SIGNALING THAT A LARGER SCALE AND SLOWER MOVING MEAN  
TROUGH MAY BEGIN TO APPROACH THE WEST COAST BY DAY 7 FRI, AS  
SUPPORTED BY RIDGING THAT DEVELOPS OVER OR JUST EAST OF 160W. AS  
FOR THE LOWER CONFIDENCE DETAILS, MODELS AND INDIVIDUAL ENSEMBLE  
MEMBERS HAVE DIFFERED SIGNIFICANTLY BETWEEN EACH OTHER AND RUN TO  
RUN REGARDING SHORTWAVES EMBEDDED WITHIN THE PROGRESSIVE MEAN FLOW  
WITH SOME DIFFERENCES NOTED FOR THE EASTERN PACIFIC TROUGH NEAR  
THE END OF THE PERIOD AS WELL.  
 
REGARDING LEADING SHORTWAVE ENERGY REACHING WESTERN NORTH AMERICA  
BY EARLY MON, GFS RUNS CONTINUE TO OUTPACE OTHER SOLUTIONS  
INCLUDING THE GEFS MEAN ACROSS SOUTHERN CANADA. FROM THE START OF  
THE PERIOD THIS PRECLUDED USE OF THE OPERATIONAL GFS IN THE  
FORECAST BLEND. THE 00Z GFS HAS TRENDED FAVORABLY SLOWER BUT IS  
STILL ON THE LEADING SIDE OF THE ENVELOPE AS THE 00Z UKMET/CMC  
MAINTAIN CONTINUITY WITH THE PRIOR CONSENSUS. MEANWHILE THE  
12Z-18Z CYCLES OF OPERATIONAL MODEL RUNS HAVE TRENDED MORE  
STRONGLY TOWARD A SEPARATE SHORTWAVE/CLOSED LOW CROSSING THE  
SOUTHERN HALF OF THE WEST AND CONTINUING INTO THE PLAINS (12Z  
GFS/UKMET FASTEST AND 12Z ECMWF SLOWEST). NEW 00Z RUNS MAINTAIN A  
REASONABLE DEGREE OF CONTINUITY ON EXISTENCE OF THIS FEATURE BUT  
NOT SO MUCH FOR TIMING, WITH 12Z/23 ENSEMBLES A LOT MORE VARIED  
THAN THE CURRENT OPERATIONAL RUNS.  
 
THE NEXT SHORTWAVE IN THE SERIES SHOULD REACH THE WEST COAST  
AROUND LATE TUE-WED. ALREADY BY THAT TIME FRAME THE MODELS AND  
ENSEMBLES DEVELOP DISAGREEMENTS IN AMPLITUDE, SHARPNESS, AND  
WHERE/HOW MUCH STREAM SEPARATION COULD OCCUR, WITH DIFFERENCES  
INCREASING MORE GREATLY OVER THE REST OF THE FORECAST. THIS  
SPREAD AND THE DESIRE TO MAINTAIN REASONABLE CONTINUITY ARGUE FOR  
ADJUSTING THE FORECAST EMPHASIS LATER IN THE PERIOD TOWARD THE  
ENSEMBLE MEANS UNTIL BETTER CLUSTERING ARISES. FOR THE UPPER  
TROUGH NEARING THE WEST COAST LATE NEXT WEEK, ECMWF/ECMWF MEAN  
RUNS HAVE GENERALLY BEEN STEADIEST OVER THE PAST COUPLE DAYS.  
18Z/00Z GEFS MEANS HAVE MADE A NOTABLE TREND TOWARD THE ECMWF MEAN  
FOR THE DEPTH OF THE TROUGH, HELPING TO INCREASE CONFIDENCE  
SOMEWHAT. 12Z AND NEW 00Z CMC RUNS ARE SIMILAR IN PRINCIPLE AS  
WELL. RECENT GFS RUNS HAVE TENDED TO HAVE A SHARPER/SLOWER  
DEPICTION OF THE TROUGH.  
 
BASED ON THE ABOVE CONSIDERATIONS THE UPDATED FORECAST  
INCORPORATED IDEAS FROM THE 12Z ECMWF/UKMET/CMC AND 18Z GEFS MEAN  
FOR DAYS 3-4 MON-TUE. A MORE EVEN MODEL/ENSEMBLE MEAN WEIGHT  
(INTRODUCING THE 12Z ECMWF MEAN) SERVED AS THE DAY 5 WED STARTING  
BLEND ON THE WAY TO AN EXCLUSIVE ENSEMBLE MEAN SOLUTION BY DAYS  
6-7 THU-FRI.  
 
   
..WEATHER HIGHLIGHTS
 
 
ACROSS THE WEST, PROGRESSIVE EARLY-MID WEEK SYSTEMS MAY GENERATE A  
COUPLE PERIODS OF RAIN/HIGH ELEVATION SNOW. WITH UPPER LEVEL  
ENERGY POTENTIALLY CLOSING OFF A LOW THAT CROSSES THE FOUR CORNERS  
REGION, THE FIRST SYSTEM IS LIKELY TO BE THE MORE PRODUCTIVE ONE.  
LATE PERIOD EASTERN PACIFIC TROUGHING ALOFT MAY BRING ANOTHER AREA  
OF MOISTURE TO THE NORTHWEST TOWARD THE END OF THE WEEK. THE  
WASHINGTON OLYMPICS/CASCADES HAVE THE BEST PROBABILITY OF SEEING  
THE HIGHEST 5-DAY PRECIP TOTALS BUT UNCERTAINTY IN SHORTWAVE  
DETAILS ALOFT ALLOW POTENTIAL FOR OTHER AREAS TO SEE FOCUSED  
ACTIVITY AS WELL. LEADING SYSTEM EMERGING FROM THE WEST MAY  
GENERATE AN AXIS OF MEANINGFUL RAINFALL ACROSS THE CENTRAL PLAINS  
AROUND MIDWEEK. AMOUNTS WILL DEPEND ON STRENGTH/TIMING OF THE  
UPPER SYSTEM. THE NORTHEAST QUADRANT OF THE LOWER 48 SHOULD SEE  
LIGHT TO PERHAPS LOCALLY MODERATE PRECIP IN ASSOCIATION WITH  
FRONTAL PASSAGES. EXPECT SNOW CHANCES TO BE CONFINED TO NEAR THE  
CANADIAN BORDER.  
 
DOMINANCE OF PACIFIC FLOW ALOFT WILL PROMOTE NEAR TO ABOVE NORMAL  
TEMPS OVER MUCH OF THE LOWER 48 DURING THE PERIOD. THE MOST  
ANOMALOUS WARMTH WILL EXTEND FROM THE INTERIOR WEST THROUGH THE  
CENTRAL U.S. MON AND TO SOME EXTENT INTO TUE. MAX AND/OR MIN  
READINGS AS HIGH AS 20-30F ABOVE NORMAL SHOULD CHALLENGE DAILY  
RECORD VALUES AT SOME LOCATIONS. DOUBLE-DIGIT ANOMALIES WILL  
LIKELY TAPER OFF IN COVERAGE FOR THE LATTER HALF OF THE WEEK.  
 
RAUSCH  
 

 
 
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