EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1207 PM EDT MON AUG 14 2017 VALID 12Z THU AUG 17 2017 - 12Z MON AUG 21 2017 ...PATTERN OVERVIEW/GUIDANCE EVALUATION/PREFERENCES... CONFIDENCE IN THE DETAILS OF THE FORECAST REMAINS BELOW THAT FOR THE OVERALL MEAN PATTERN. THE DAY 3 TO 7 (THURS TO MON) OVERALL PATTERN FEATURES A MEAN TROUGH PROGRESSING FROM THE CENTRAL INTO THE EASTERN U.S. AND BROADLY CYCLONIC FLOW SPREADING FROM THE NORTHEASTERN PACIFIC INTO WESTERN CANADA AND THE U.S.. DESPITE THE AGREEMENT REGARDING THE OVERALL FLOW PATTERN...THERE CONTINUES TO BE RUN TO RUN CONTINUITY AND SPREAD REGARDING SPECIFIC FEATURES/DETAILS OF BOTH SYSTEMS. STARTING IN THE EAST...THE 00Z ECMWF CONTINUES TO BE ON THE FASTER SIDE FOR A WAVE TRACKING OUT OF THE UPPER MISSISSIPPI/GREAT LAKES AND INTO THE NORTHEAST. THE LATEST RUN OF THE GFS SHOWS A SECONDARY SHORTWAVE DIGGING INTO THE NORTHERN PLAINS ON DAY 4...WHICH A FEATURE NOT NEARLY AS DEFINED IN THE OTHER OPERATIONAL MODELS. BY DAY 7...THIS SHORTWAVE SEEMS TO DISAPPEAR/BECOME ABSORBED INTO THE BROADER TROUGH OVER THE NORTHEAST WITH THE GFS DAY 7 SOLUTION FAIRLY CLOSE TO THAT OF THE 00Z ECMWF AT THIS POINT. THE 00Z CANADIAN IS MORE CLOSELY ALIGNED WITH THE ECMWF AND THUS WAS USABLE FOR AT LEAST THE FIRST HALF OF THE PERIOD BUT BY DAYS 6 AND 7 IS FASTER THAN EITHER THE ECMWF/GFS AND THUS WAS NOT USED AFTER DAY 5 IN THE OVERALL MODEL BLEND. THE ENSEMBLE MEANS WOULD SUPPORT A SOLUTION CLOSER TO THAT OF THE ECMWF/CMC AND DO SHOW OVERALL GOOD AGREEMENT AND MINIMAL SPREAD THROUGHOUT THE ENTIRE PERIOD. OUT WEST...THE OPERATIONAL MODELS SEEM TO BE COMING INTO A LITTLE BETTER AGREEMENT WITH THE EVOLUTION OF A SHORTWAVE DIGGING INTO WESTERN CANADA THIS WEEKEND...CLOSING OFF INTO AN UPPER LOW AND REACHING WEST OF THE HUDSON BAY BY 12Z MONDAY. THERE REMAINS SOME DIFFERENCES IN DETAIL BUT THE ECWMF CONTINUES TO SHOW THE BEST RUN TO RUN CONTINUITY WITH THIS FEATURE. DESPITE SOME SPREAD AMONGST INDIVIDUAL ENSEMBLE MEMBERS...THE ENSEMBLE MEANS SHOW GOOD AGREEMENT WITH EACH OTHER AND THUS WOULD CONTINUE TO FAVOR A MOSTLY ENSEMBLE MEAN BLEND WITH THIS SYSTEM TO LET DETAILS COME INTO BETTER FOCUS WITH SUBSEQUENT FORECASTS. COMBINED PREFERENCES FROM BOTH SYSTEMS DISCUSSED LED TO THE WPC FORECAST FAVORING A MAJORITY EC/GEFS ENSEMBLE MEAN BLEND THROUGHOUT THE PERIOD. THE 00Z CMC DID PROVIDE SOME HELP PARTICULARLY OVER THE NORTHEAST DURING THE BEGINNING OF THE PERIOD. ADDED SMALLER CONTRIBUTIONS TO THE 06 GFS/00Z ECMWF THROUGHOUT THE PERIOD TO TRY TO PICK UP SOME DETAILS WITHOUT DEGRADING THE OVERALL FORECAST. ...WEATHER HIGHLIGHTS... THE SYSTEM TRACKING FROM THE GREAT LAKES INTO THE 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 WITH 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...EXPECT ABOVE NORMAL TEMPS AHEAD OF BOTH THE EASTERN AND WESTERN U.S. SYSTEMS BUT NOTHING TOO EXTREME AS THERE CONTINUES TO BE A RELATIVE LACK OF DOUBLE DIGIT ANOMALIES IN THE FORECAST. SANTORELLI/RAUSCH