EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 245 AM EDT MON AUG 14 2017 VALID 12Z THU AUG 17 2017 - 12Z MON AUG 21 2017 ...PATTERN OVERVIEW/GUIDANCE EVALUATION/PREFERENCES... CONFIDENCE IN THE PARTICULARS OF THE FORECAST REMAINS WELL BELOW THAT FOR THE OVERALL MEAN PATTERN. THE MOST COMMON THEMES FOR THE LARGE SCALE EVOLUTION INVOLVE A MEAN TROUGH PROGRESSING FROM THE CENTRAL INTO EASTERN U.S. AND BROADLY CYCLONIC FLOW SPREADING FROM THE NORTHEASTERN PACIFIC INTO WESTERN NORTH AMERICA. GUIDANCE DIFFICULTIES IN RESOLVING HOW THE BLOCKY PATTERN OVER CANADA IN THE SHORT RANGE ULTIMATELY OPENS UP CONTINUES TO PLAGUE THE FORECAST FOR SURFACE EVOLUTION OVER THE NORTHEAST. AT THE SAME TIME THERE HAS BEEN A LOT OF SPREAD/RUN TO RUN VARIABILITY FOR FEATURES WITHIN THE CYCLONIC MEAN FLOW AFFECTING THE NORTHWEST. OVER THE NORTHEAST THE 12Z ECMWF CONTINUES THE MODEL'S RECENT TENDENCY TO BE ON THE FAST EDGE OF GUIDANCE FOR A WAVE TRACKING OUT OF THE UPPER MS VALLEY/GREAT LAKES. WHILE WITHIN THE FULL 12Z ENSEMBLE SPREAD THERE ARE NOT MANY INDIVIDUAL MEMBERS THAT ARE FASTER. ON THE OTHER HAND RECENT GFS RUNS HAVE REVERTED BACK TO A SLOWER/DEEPER SOLUTION FROM A COUPLE DAYS AGO AFTER THE DAYTIME SAT RUNS HAD BEEN MORE SIMILAR TO THE INTERMEDIATE ECMWF MEAN, WHICH AS A WHOLE HAS BEEN MORE STABLE THAN MOST OTHER GUIDANCE. AT LEAST FOR DAYS 3-4 THU-FRI THE 12Z CMC WAS THE CLOSEST MODEL IN PRINCIPLE TO THE 12Z ECMWF MEAN. ALONG WITH THE FAST TIMING OF THE ECMWF WAVE, THE MODEL IS ALSO QUESTIONABLE IN HOW IT AMPLIFIES EASTERN CANADA UPPER TROUGH ENERGY BEHIND IT FRI-SAT WHILE ENSEMBLE MEANS ADVERTISE RIDGING. EARLY STAGES OF THIS AMPLIFICATION MAY OPEN THE DOOR FOR QUICKER HEIGHT RISES BEHIND THE DEEP CANADIAN MARITIMES UPPER LOW, ALLOWING FOR THE FAST SURFACE WAVE. THE NEW 00Z UKMET IS ONE OTHER PIECE OF GUIDANCE THAT HAS REMOTE SIMILARITY TO THE 12Z ECMWF FOR THE EASTERN CANADA TROUGH LATE IN THE WEEK. BY THE LATTER HALF OF THE PERIOD THERE HAS RECENTLY BEEN A GENERAL TREND FOR THE OVERALL TROUGH ALOFT TO BECOME SOMEWHAT BROADER/MORE DIFFUSE LEADING TO GRADUALLY FASTER TRANSITION OF LOW PRESSURE EMPHASIS INTO THE ATLANTIC. CURRENT SPREAD AND CONTINUITY/TRENDS THROUGH THE PERIOD FAVOR PLACING HIGHEST WEIGHT ON THE 12Z ECMWF MEAN WHILE WAITING FOR BETTER GUIDANCE CLUSTERING. THE DISPARITY IN MODELS/ENSEMBLES FOR SHORTWAVE DETAILS FROM THE NORTHEAST PACIFIC INTO CANADA AND NORTHWEST U.S. CONTINUES TO FAVOR LEANING MORE TOWARD THE ENSEMBLE MEANS WHICH STILL HAVE SOME DETAIL DIFFERENCES BUT TO A LESS DRAMATIC DEGREE THAN OPERATIONAL MODEL RUNS. THUS FAR RECENT ECMWF RUNS HAVE BEEN MORE STABLE IN DEPICTING A CLOSED LOW REACHING JUST WEST OF HUDSON BAY BY DAY 7 MON WITH A TROUGH EXTENDING BACK INTO THE NORTHWEST U.S.. THE NEW 00Z GFS/UKMET/CMC ARE REMARKABLY SIMILAR FOR A STRONG FEATURE REACHING WESTERN CANADA BY EARLY SAT AND THOSE GFS/CMC RUNS END UP FAIRLY CLOSE TO THE 12Z ECMWF BY THE END OF THE PERIOD, SO PERHAPS SOME DETAILS MAY BE STARTING TO COME INTO BETTER FOCUS. COMBINED PREFERENCES LED TO GIVING HIGHEST WEIGHT TO THE 12Z ECMWF MEAN. THE 12Z CMC PROVIDED SOME INPUT OVER THE NORTHEAST DURING THE FIRST HALF OF THE PERIOD. SMALLER CONTRIBUTIONS BY THE 18Z GEFS MEAN AND 18Z GFS/12Z ECMWF ADDED VALUE TO WHERE CONSENSUS EXISTED WHILE NOT ADVERSELY AFFECTING THE FORECAST ELSEWHERE. ...WEATHER HIGHLIGHTS... SYSTEM TRACKING FROM THE GREAT LAKES INTO NORTHEAST WILL PROVIDE ONE FOCUS FOR MEANINGFUL RAINFALL BUT IMPORTANT DETAILS WILL TAKE FURTHER TIME TO RESOLVE. THE ASSOCIATED COLD FRONT TRAILING BACK INTO THE PLAINS SHOULD ALSO PRODUCE SOME CONVECTION. THE COMBINATION OF THIS FRONT STALLING OVER THE PLAINS AND PASSAGE OF ONE OR MORE IMPULSES ALOFT MAY LEAD TO SOME AREAS OF HEAVY RAINFALL OVER PARTS OF THE CENTRAL U.S.. MEANWHILE DIURNALLY FAVORED CONVECTION OVER THE SOUTHEAST MAY PRODUCE SOME POCKETS OF HEAVY RAIN. THERE IS SOME SIGNAL IN THE GUIDANCE FOR ONE OR MORE UPPER LEVEL FEATURES OVER THE AREA, WHICH COULD HELP TO ENHANCE ACTIVITY. EXPECT MONSOONAL CONVECTION OVER/NEAR THE FOUR CORNERS STATES TO BE LIGHTER/MORE SCATTERED THAN HAS RECENTLY BEEN THE CASE, WITH HIGHEST RAINFALL TOTALS CONFINED FARTHER SOUTH OVER MEXICO. FOR TEMPS THERE CONTINUES TO BE A RELATIVE LACK OF DOUBLE DIGIT ANOMALIES IN THE FORECAST, ESPECIALLY FOR HIGHS. AHEAD OF THE INITIAL GREAT LAKES SYSTEM THERE MAY BE SOME PLUS 10F OR GREATER ANOMALIES FOR LOW TEMPS FROM THE MIDWEST/GREAT LAKES INTO CENTRAL APPALACHIANS. LOWS SHOULD BE PERSISTENTLY A FEW DEGREES F ABOVE NORMAL ACROSS THE SOUTHERN TIER. RAUSCH