EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 205 AM EDT WED JUL 19 2017 VALID 12Z SAT JUL 22 2017 - 12Z WED JUL 26 2017 ...PATTERN OVERVIEW AND WEATHER HIGHLIGHTS... WEAKENING UPPER HIGH OVER THE MID-MISSISSIPPI VALLEY ON SATURDAY WILL SLOWLY TRANSITION BACK TOWARD THE SOUTHWEST BY THE MIDDLE OF NEXT WEEK (ENDING UP NEAR THE NM/TX BORDER). THIS WILL FAVOR TROUGHING IN BOTH THE PAC NW AND THE NORTHEAST. A SURFACE FRONT WILL PUSH EASTWARD THROUGH NEW ENGLAND LATE SUNDAY INTO MONDAY WHILE ANOTHER SYSTEM ENTERS SW CANADA AND WASHINGTON STATE. LINGERING FRONTAL BOUNDARY WILL OSCILLATE THROUGH THE OHIO VALLEY ACTING AS A FOCUS FOR SHOWER AND STORMS. TEMPERATURES WILL BE VERY WARM/HOT AHEAD AND TO THE SOUTH OF THE FRONT (FROM THE PLAINS EASTWARD TO THE MID-ATLANTIC) UNTIL THE FRONTAL BOUNDARY DIPS THROUGH SOME AREAS. SCATTERED AFTERNOON SHOWERS/STORMS WILL BE A DAILY OCCURRENCE IN THE SOUTHWEST AND INTERMOUNTAIN WEST AROUND THE BACKSIDE OF THE UPPER HIGH. ...GUIDANCE EVALUATION/PREFERENCES... THE ENSEMBLES HAVE DRIFTED APART WITH RESPECT TO TIMING/EVOLUTION OF A FEW SYSTEMS ACROSS THE NORTHERN TIER (ESPECIALLY COMING OUT OF THE GULF OF ALASKA) THAT PREDOMINATELY MOVE THROUGH CANADA BUT HAVE AN AFFECT ON HOW MUCH COOLING TO BRING SOUTHWARD (AND WHEN) AND WHERE THE HEAVIEST RAIN MAY FOCUS. THE ECMWF AND ITS ENSEMBLES WERE NOTICEABLY QUICKER WITH A SHORTWAVE INTO THE GREAT LAKES LATE SATURDAY COMPARED TO THE GFS/UKMET WHILE THE PARALLEL GFS (SOON TO BE THE OPERATIONAL VERSION AS OF 12Z WED) WAS IN BETWEEN. TREND HAS SEEMINGLY BEEN QUICKER RATHER THAN SLOWER WITH SOME FEATURES OUT OF THE PACIFIC AND ACROSS SOUTHERN CANADA IN RECENT WEEKS SO OPTED TO RELY ON A BLEND OF THE QUICKER MODELS (GFSP AND ECMWF) WITH SOME WEIGHTING OF CONTINUITY AND THE ECMWF ENSEMBLE MEAN. THIS QUICKER PREFERENCE CARRIED DOWNSTREAM INTO THE NORTHEAST BY NEXT WEEK WHERE TROUGHING WILL AGAIN PUSH THROUGH, BRINGING IN SOME COOLER TEMPERATURES (BACK TO AVERAGE VALUES) NEXT TUE/WED. BACK UPSTREAM, SIMILAR DIFFERENCES EXISTED IN THE GULF OF ALASKA (ECMWF QUICKER) AS THE QUESTION WAS WHETHER TO BRING AN UPPER LOW EASTWARD INTO BRITISH COLUMBIA OR LEAVE IT TO LINGER IN THE GULF. AGAIN FAVORED SOMETHING MORE PROGRESSIVE GIVEN THE DOWNSTREAM PREFERENCE. THIS WILL CARRY A SURFACE FRONT EASTWARD THROUGH MONTANA ON TUESDAY BUT THE MAIN FORCING WILL STAY WELL NORTH OF THE US/CANADIAN BORDER SO THE FRONT MAY STALL SW-NE ALIGNED WITH THE UPPER FLOW NEXT WEEK. LINGERING BOUNDARY THROUGH THE OHIO VALLEY WILL LIFT AND THEN SINK AS A WARM FRONT OR COLD FRONT, RESPECTIVELY, AS THE SFC WAVE MOVES EASTWARD. TREND HERE HAS BEEN FARTHER NORTH WITH THE LOW -- UPPER RIDGE SEEMS TO HAVE WON AGAIN -- UNTIL NEXT WEEK WHEN THE RETROGRESSION OF THE UPPER HIGH BACK TOWARD NEW MEXICO SHOULD ALLOW THE FRONT TO SINK INTO THE CAROLINAS. FRACASSO