EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1159 AM EDT SAT SEP 17 2016 VALID 12Z TUE SEP 20 2016 - 12Z SAT SEP 24 2016 ...PATTERN OVERVIEW/UNCERTAINTY ASSESSMENT... GLOBAL MODELS WERE IN DECENT AGREEMENT WITH THE LARGE SCALE FLOW PATTERN OVER THE LOWER 48 NEXT WEEK WHICH WILL FEATURE AN UPPER TROUGH AMPLIFYING OVER THE WESTERN U.S. AND A DOWNSTREAM RIDGE EXPANDING OVER THE MIDWEST...OHIO VALLEY...AND MID-ATLANTIC STATES. HOWEVER...THERE REMAINS UNCERTAINTY WITH THE EXACT EVOLUTION OF THE UPPER TROUGH CARVING OUT OVER THE WEST AND A POSSIBLE ASSOCIATED DEEP SURFACE CYCLONE SPINNING UP OVER THE CENTRAL/NORTHERN PLAINS LATE NEXT WEEK. THE GFS/GEFS MEAN SOLUTIONS CONTINUE TO BE LEADERS IN THE FAIRLY DRASTIC TREND TOWARDS A SLOWER/MORE AMPLIFIED UPPER TROUGH IN THE WEST...SO THE WPC DAY 3-7 FORECAST WAS HEAVILY WEIGHTED TOWARDS THE 06Z GFS...WHICH APPEARED TO BEST FIT WITHIN THE MIDDLE OF THE SLOWER SIDE OF MODEL SPREAD. THIS RESULTED IN A SLOWER/MORE AMPLIFIED SHIFT FROM THE PREVIOUS WPC FORECAST. ...WEATHER HIGHLIGHTS/THREATS... A NOTICEABLE COOLING TREND IS EXPECTED BENEATH THE UPPER TROUGH CARVING OUT OVER THE WESTERN HALF OF THE NATION NEXT WEEK...WITH THE POTENTIAL FOR A SIGNIFICANT HEAVY RAIN/MOUNTAIN SNOW EVENT ACROSS THE NORTHERN PLAINS/NORTHERN ROCKIES AS HEIGHT FALLS PROGRESS EASTWARD LATE NEXT WEEK. PERIODS OF HEAVY RAIN WILL BE POSSIBLE OVER THE SOUTHWESTERN U.S. EARLY TO MID NEXT WEEK AS AN WEAKENING UPPER LOW OFF THE SOUTHERN CALIFORNIA COAST LIFTS NORTHEASTWARD AND INTERACTS WITH ANOMALOUS MOISTURE ACROSS THE REGION. THE WEAKENING UPPER LOW COULD ALSO HELP TRIGGER HEAVY CONVECTIVE RAIN WHILE IT CROSSES THE MIDDLE/UPPER MISSISSIPPI VALLEY NEXT WEDNESDAY/THURSDAY. GERHARDT