EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 127 AM EDT MON JUL 17 2017 VALID 12Z THU JUL 20 2017 - 12Z MON JUL 24 2017 ...PATTERN OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES... AN UPPER HIGH IS POISED TO SPREAD EASTWARD OUT OF THE MIDDLE MISSISSIPPI VALLEY AND INTO THE SOUTHEAST THU-SAT BEFORE BUILDING OFFSHORE. TROUGHING WILL BE CONFINED TO THE PACIFIC NORTHWEST AND ALONG THE US/CANADIAN BORDER BEFORE SETTLING INTO THE NORTHEAST BY THE END OF THE PERIOD. THE MODELS/ENSEMBLES REMAIN IN GOOD AGREEMENT AND A CONSENSUS BLEND OFFERED A REASONABLE STARTING POINT. 12Z GFS LIED ON THE EDGES (OR PAST) OF THE ENSEMBLE SPREAD BY THE WEEKEND INTO NEXT WEEK SO ITS WEIGHTING WAS ZEROED OUT BY THEN. LINGERING QUESTION IN THE FORECAST IS HOW TO EVOLVE THE SHORTWAVE OR WEAK UPPER LOW OFF VANCOUVER ISLAND AT THE START OF THE FORECAST (THURSDAY) AS IT EVENTUALLY SHEARS INTO THE BASE OF THE TROUGH IN EASTERN CANADA. THIS WILL TAKE A COLD FRONT EASTWARD LATE WEEK INTO THE WEEKEND WHILE A STATIONARY BOUNDARY ATOP THE UPPER HIGH LINGERS UNTIL LIT GETS COAXED ALONG VIA THE SYSTEM TO ITS NORTH. TREND HAS BEEN TOWARD STRONGER UPPER RIDGING IN THE SOUTHEAST WHICH WOULD ACT TO STOP ANY SOUTHWARD MOVING FRONTS. ...WEATHER HIGHLIGHTS... HOT TEMPERATURES FOR MOST OF THE COUNTRY THIS PERIOD BUT PERHAPS JUST SHORT OF DAILY RECORDS (WHICH ARE MOSTLY AT THEIR PEAK FOR THE YEAR). BEST CHANCE OF RECORD MAXES LIE ACROSS THE SOUTHERN PLAINS EASTWARD AND SOUTHWARD TO THE INLAND CAROLINAS. PERSISTENCE OF THE WELL ABOVE AVERAGE TEMPERATURES MAY BE THE STORY BY NEXT WEEKEND (ESPECIALLY AROUND MISSOURI). LOCALLY HEAVY RAINFALL WILL STILL BE POSSIBLE ACROSS THE FOUR CORNERS REGION IN ASSOCIATION WITH MONSOONAL CONVECTION. PRECIPITABLE WATER VALUES ARE FORECAST TO BE AS HIGH AS +2 TO 2.5 STANDARD DEVIATIONS. THE FRONTAL BOUNDARY ACROSS THE NORTHERN TIER WILL SERVE AS A FOCUS FOR MULTIPLE ROUNDS OF POTENTIAL CONVECTION FROM THE NORTHERN PLAINS TO THE GREAT LAKES AND OHIO VALLEY, BUT CONFIDENCE IS LOW WITH RESPECT TO TIMING OR LOCATION. THE POTENTIAL FOR HEAVY RAINFALL WILL EXIST, BUT WILL DEPEND LARGELY ON MESO/CONVECTIVE SCALE PROCESSES WITH LIMITED PREDICTABILITY IN THE MEDIUM RANGE. JUST THE OPPOSITE WILL BE TRUE FOR AREAS UNDER THE UPPER HIGH WHERE NARY A DROP OF RAIN CAN BE BOUGHT. FRACASSO