EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 201 AM EDT TUE JUL 18 2017 VALID 12Z FRI JUL 21 2017 - 12Z TUE JUL 25 2017 ...PATTERN OVERVIEW... AN UPPER HIGH INITIALLY CENTERED OVER THE TENNESSEE VALLEY ON FRIDAY WILL WEAKEN AND SPLIT AS TROUGHING DEEPENS INTO THE PACIFIC NORTHWEST AND THE NORTHEAST CONUS EARLY NEXT WEEK. THIS WILL CARRY A SURFACE LOW EASTWARD ACROSS THE NORTHERN TIER ALONG A WAVY STATIONARY BOUNDARY STRETCHED FROM THE CENTRAL PLAINS TO NEW ENGLAND AND THE MID-ATLANTIC. ...GUIDANCE EVALUATION/PREFERENCES... THE MODELS AND ENSEMBLES HAVE REMAINED IN GOOD SYNOPTIC AGREEMENT FOR SEVERAL DAYS RUNNING WITH THE OVERALL PATTERN. THROUGH ABOUT SUNDAY, A MODEL/ENSEMBLE CONSENSUS BLEND WOULD SUFFICE. A LINGERING QUESTION IN THE FORECAST LIES OVER SW CANADA WITH AN INCOMING AND WEAKENING UPPER LOW. MODELS HAVE BEEN WAVERING ON SPEED/STRENGTH FOR SEVERAL DAYS BUT A BLEND OF THE VARIOUS SOLUTIONS SEEMS TO KEEP RUN-TO-RUN CHANGES TO A MINIMUM. 12Z ECMWF WAS ON THE QUICKEST SIDE OF THE SPREAD (ECMWF ENSEMBLES IN GENERAL WERE QUICKER WITH THIS FEATURE AND ALL THE WAY UPSTREAM TO THE GULF OF ALASKA, TOO) SO OPTED TO INCLUDE THE SLOWER 00Z/17 ECMWF ALONG WITH CONTINUITY BUT DECREASED THE DETERMINISTIC WEIGHTING FOR NEXT MON/TUE AS DIFFERENCES AMPLIFY INTO THE NORTHEAST (STRONGER TROUGH PER THE 12Z ECMWF OR FLATTER FLOW PER THE GFS). TO THE NORTHWEST, ECMWF AND ITS ENSEMBLES WERE QUICKER TO LOWER HEIGHTS INTO WA/OR THAN THE GEFS MEMBERS AND THE 12Z/18Z GFS RUNS. WITH THE UPPER RIDGE FORECAST TO REASSERT ITSELF BACK OVER THE CENTRAL ROCKIES, FELT THAT THE ECMWF-LED GUIDANCE MAY BE TOO AMPLIFIED BUT NOT UNREASONABLE. ENSEMBLE TREND THE PAST TWO DAYS MAY BE TOWARD LOWER HEIGHTS INTO THE PAC NW BUT RECENT VERIFICATION HAS SHOWN THE UPPER RIDGING TO HOLD STRONG. FOR NOW, A 50/50 GFS/GEFS TO ECMWF/ECENS MEAN BLEND WAS REASONABLE. ...WEATHER HIGHLIGHTS... WELL ABOVE AVERAGE TEMPERATURES OVER THE PLAINS EASTWARD TO THE MID-ATLANTIC (SOME AREAS NEAR 100F) LATER THIS WEEK WILL EASE WITH TIME AS THE UPPER HIGH WEAKENS AND THE SURFACE BOUNDARY FINALLY SINKS A BIT SOUTHWARD. INLAND PACIFIC NORTHWEST WILL HEAT UP AGAIN THIS WEEKEND INTO NEXT WEEK BEFORE THE APPROACHING TROUGHING. OTHERWISE, NEAR TO JUST BELOW AVERAGE TEMPERATURES ARE EXPECTED ACROSS THE FOUR CORNERS REGION (WHERE MONSOONAL-DRIVEN CONVECTION WILL PERSIST) AND THROUGH THE UPPER GREAT LAKES INTO NEW ENGLAND TO THE NORTH OF THE FRONT. PRECIPITATION WILL LARGELY FOLLOW ALONG THE FRONT AND TO ITS NORTH AS THE SFC LOW PUSHES EASTWARD. THIS WILL BE MODULATED BY AFTERNOON AND OVERNIGHT CONVECTION WHICH MEANS LOCALLY HEAVY RAIN FOR SOME BUT NOT MUCH FOR OTHERS. FRACASSO