EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 240 AM EDT SUN AUG 13 2017 VALID 12Z WED AUG 16 2017 - 12Z SUN AUG 20 2017 ...PATTERN OVERVIEW/GUIDANCE EVALUATION/PREFERENCES... FROM THE TELECONNECTION PERSPECTIVE, MEAN RIDGING OVER CENTRAL-NORTHERN CANADA/HUDSON BAY CONTINUES TO BE THE PRIMARY FEATURE THAT CAN BE KEYED UPON FOR NORTH AMERICAN FLOW. TELECONS RELATIVE TO THE CORRESPONDING POSITIVE HEIGHT ANOMALY CENTER TO THE NORTH/NORTHEAST OF HUDSON BAY IN RECENT D+8 MULTI-DAY MEANS OFFER SUPPORT FOR THE CONSENSUS MEAN TROUGH FORECAST TO REACH THE NORTHEASTERN U.S. LATE IN THE PERIOD AFTER TRACKING EASTWARD FROM THE PLAINS. MEANWHILE THE FAVORED FLOW REGIME IS ALSO CONSISTENT WITH THE MODEST MEAN TROUGH WHICH ENSEMBLE MEANS AND SOME OPERATIONAL MODELS HAVE SETTLING OVER THE WEST COAST NEXT WEEKEND BEHIND A PROGRESSIVE SHORTWAVE RIDGE. SOME IMPORTANT LARGER SCALE ASPECTS OF THE FORECAST OVER THE EASTERN 2/3 OF NORTH AMERICA SEEM TO BE GAINING SOME CLARITY IN SPITE OF CONTINUED DISCREPANCIES AND TRENDING FOR SOME OF THE EMBEDDED DETAILS. GFS/GEFS MEAN TRENDS OVER THE PAST DAY HAVE BROUGHT THOSE SOLUTIONS MUCH MORE IN LINE WITH MOST PRIOR ECMWF/ECMWF MEAN RUNS THAT WERE DEVELOPING A SPLIT FLOW PATTERN ACROSS SOUTHERN CANADA WITH A PERSISTENT RIDGE OVER HUDSON BAY AND TROUGHING HEADING INTO THE GREAT LAKES AND VICINITY. INTERESTINGLY BY THE LATTER HALF OF THE PERIOD THE ECMWF MEAN HAS ACTUALLY WEAKENED THE CANADA RIDGE A BIT SO THAT THE GEFS MEAN IS NOW A LITTLE STRONGER. BETTER CLUSTERING FOR THIS ASPECT OF THE FORECAST IMPROVES CONFIDENCE IN THE LARGE SCALE BUT THERE IS STILL CONSIDERABLE UNCERTAINTY FOR THE TYPICALLY MORE CHALLENGING SURFACE FORECAST. THIS IS ESPECIALLY TRUE CONSIDERING THE POTENTIAL CONVECTIVE CONTRIBUTION OF ONE OR MORE PIECES OF ENERGY ALOFT COMPRISING THE OVERALL TROUGH ALOFT. MULTI-DAY TRENDS HAVE BEEN SHOWING FASTER PROGRESSION/EASTWARD ELONGATION OF LOW PRESSURE ACROSS THE NORTHEASTERN QUADRANT OF THE LOWER 48. THE 12Z ECMWF/CMC WERE THE FAST EXTREMES WITH RESPECT TO A LEADING WAVE EMERGING INTO THE ATLANTIC. CONTINUED TRENDING COULD ULTIMATELY LEAD TO SUCH AN OUTCOME BUT THE 12Z/18Z GUIDANCE CONSENSUS (WITH THE 18Z GFS NOT FAR FROM THE 12Z ECMWF MEAN IN PRINCIPLE FOR SOME FORECAST HOURS) ALREADY PROVIDED A DECENT NUDGE FASTER VERSUS CONTINUITY SO PREFERRED TO PLACE LESS WEIGHT ON THE OPERATIONAL 12Z ECMWF SPECIFICS. THE NEW 00Z GFS/UKMET/CMC SEEM TO VALIDATE THAT APPROACH FOR THE TIME BEING. AHEAD OF THIS EVOLUTION THERE CONTINUES TO BE REASONABLE AGREEMENT FOR THE FAIRLY DEEP CLOSED LOW TRACKING JUST NORTH OF NEW ENGLAND EARLY IN THE PERIOD, WITH ASSOCIATED COLD FRONT CROSSING NEW ENGLAND AROUND WED. FARTHER WEST THE GUIDANCE AGREES FAIRLY WELL UP TO THE POINT WHEN THE PROGRESSIVE RIDGE BEGINS MOVING INTO THE WEST THU-FRI. THEN MODELS/ENSEMBLES DIVERGE ON UPSTREAM SHORTWAVE DETAILS. OF PARTICULAR NOTE, THE RIDGE WHICH THE 12Z GFS BROUGHT TO THE NORTHWEST U.S. AND WESTERN CANADA BY SUN WAS FAIRLY EXTREME COMPARED TO THE FULL ENSEMBLE SPREAD. THE 18Z GFS COMPARED SOMEWHAT BETTER BUT COULD STILL BE SOMEWHAT HIGH WITH ITS HEIGHTS ALOFT ALONG THE WEST COAST BY THE END OF THE PERIOD. AS NOTED ABOVE TELECONNECTIONS PROVIDE SUPPORT FOR THE MODEST WEST COAST TROUGH WHICH THE ENSEMBLE MEANS DEPICT BY NEXT WEEKEND. BASED ON THE ABOVE CONSIDERATIONS THE UPDATED FORECAST PLACED MORE EMPHASIS ON THE 12Z ECMWF MEAN/18Z GFS/18Z GEFS MEAN THAN 12Z ECMWF. MODEL/MEAN WEIGHT WAS CLOSE TO EVEN EARLY-MID PERIOD AND THEN LEANED MORE TO THE MEANS BY DAYS 6-7 SAT-SUN. ...WEATHER HIGHLIGHTS... THE UPPER TROUGH AND ASSOCIATED SURFACE SYSTEM MIGRATING EASTWARD FROM THE PLAINS WILL BE A PROMINENT FOCUS FOR EPISODES OF CONVECTION WITH LOCALLY HEAVY RAINFALL. ENHANCED AMOUNTS WILL BE POSSIBLE FROM THE UPPER MS VALLEY INTO THE GREAT LAKES/NORTHEAST IN ASSOCIATION WITH ONE OR MORE SURFACE WAVES. IN ADDITION THE TRAILING COLD FRONT WILL LIKELY STALL OVER THE CENTRAL PLAINS AND DECELERATE AS IT HEADS INTO THE MID MS/OH VALLEY REGIONS, POTENTIALLY SUPPORTING SIGNIFICANT RAINFALL OVER THESE AREAS AS WELL. THE SOUTHEAST WILL SEE PERIODS OF DIURNALLY FAVORED CONVECTION THAT MAY BE LOCALLY HEAVY THOUGH SOMEWHAT LESS ORGANIZED THAN THE ACTIVITY TO THE NORTH/NORTHWEST. SOME MONSOONAL CONVECTION MAY BE POSSIBLE OVER THE SOUTHERN HALF OF THE ROCKIES BUT EXPECT IT TO BE LIGHTER/MORE SCATTERED THAN IN RECENT EPISODES. FOR THE MOST PART TEMPS ACROSS MUCH OF THE LOWER 48 SHOULD REMAIN WITHIN 10F OF NORMAL DURING THE PERIOD. THE BROADEST AREA OF ANOMALOUS READINGS (PLUS 5-10F OR SO VERSUS NORMAL) WILL LIKELY BE FOR MIN TEMPS AHEAD OF THE SYSTEM TRACKING OUT OF THE PLAINS. AROUND LATE WEEK THE INTERIOR WEST SHOULD SEE A COUPLE DAYS OF MODERATELY ABOVE NORMAL TEMPS WITH A PROGRESSIVE RIDGE ALOFT. RAUSCH