EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1157 AM EDT THU SEP 17 2015 VALID 12Z SUN SEP 20 2015 - 12Z THU SEP 24 2015 ...ANOTHER ANOMALOUS SURGE OF TROPICAL MOISTURE INTO THE SOUTHWEST... THE GLOBAL NUMERICAL MODELS INDICATE A RELOADING OF TROPICAL MOISTURE ALONG THE PACIFIC COAST OF MEXICO THIS WEEKEND, WITH A SURGE INTO THE SOUTHWESTERN UNITED STATES NEXT WEEK AS THE MEAN-LAYER FLOW GOES SOUTHERLY IN RESPONSE TO STRONG HEIGHT FALLS ALONG THE CANADIAN BORDER. AS WITH THE RECENT HEAVY RAINFALL EPISODE IN THE SOUTHWEST, THE RAINFALL COVERAGE AND SYNOPTIC-SCALE QUANTITIES IN THE ENSEMBLE MEANS ARE IMPRESSIVE FROM SOUTHERN CALIFORNIA TO THE GREAT BASIN AND FOUR CORNERS REGION NEXT WEEK, WITH GEFS PRECIPITABLE WATER ANOMALIES PEAKING BETWEEN +2 AND +4. THERE ARE MANY IMPORTANT DETAILS THAT ARE STILL UNCERTAIN, THE SCALE OF WHICH MAY ADD OR SUBTRACT WFO AREAS TO THE GREATEST THREAT. ANOTHER POSITIVE-TILT TROUGH IS EXPECTED TO ANCHOR DOWN THE APPALACHIAN CHAIN AT THE MEDIUM RANGE, WITH A GENERALLY COOL, SHOWERY PERIOD ON TAP FROM THE MOUNTAIN CRESTS TO THE ATLANTIC COAST. RELIANCE ON THE ENSEMBLE MEANS SHOULD MITIGATE THE "NOISE" IN ANY GIVEN OPERATIONAL MODEL RUN--E.G., THE CANADIAN TROPICAL STORM HEADING TOWARD THE GULF OF MAINE. CISCO