EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1129 AM EDT FRI OCT 13 2017 VALID 12Z MON OCT 16 2017 - 12Z FRI OCT 20 2017 ...OVERALL FLOW PATTERN/GUIDANCE EVALUATION/PREFERENCES... MEDIUM RANGE GUIDANCE CONTINUES TO SHOW A PROGRESSIVE AND WAVY FLOW PATTERN IN THE BEGINNING OF NEXT WEEK TRANSITIONING TO ONE OF INCREASED WESTERN U.S. TROUGHING AND BUILDING HEIGHTS OVER THE EASTERN STATES. THERE IS PRETTY GOOD AGREEMENT ON THIS OVERALL FLOW PATTERN BETWEEN THE DETERMINISTIC GFS/ECMWF AND THEIR RESPECTIVE ENSEMBLE MEANS...WITH MODEL DIFFERENCES RELATED TO THE LESS PREDICTABLE TIMING AND STRENGTH OF EMBEDDED SHORTWAVES. THE 06Z DETERMINISTIC GFS APPEARS TO BE AN OUTLIER WITH ITS DEPICTION OF A NEGATIVELY TILTED SHORTWAVE ACROSS CENTRAL CANADA MID-WEEK...AND CONSEQUENTLY A FASTER PROGRESSION OF TROUGHING BEHIND IT MOVING INTO THE WESTERN U.S.. THE PREVIOUS RUN (00Z) OF THE GFS BETTER SUPPORTS THE REST OF THE GUIDANCE SHOWING A MORE PROGRESSIVE/SLIGHTLY AMPLIFIED CANADIAN SHORTWAVE AND SLOWER/LATER ARRIVAL OF THE WESTERN U.S. TROUGH. THE 00Z ECMWF/CMC ARE QUITE A BIT MORE AMPLIFIED WITH THE CANADIAN SHORTWAVE AS WELL AS IT MOVES INTO EASTERN CANADA BY THE END OF THE WEEK. WHILE THE 00Z EC ENSEMBLE MEAN ALSO HINTS AT A SLIGHTLY MORE AMPLIFIED SHORTWAVE... IT IS STILL IN LINE WITH THE REST OF THE GUIDANCE. ELIMINATING THE CMC AND ITS ENSEMBLE...THIS CYCLE OF THE WPC MEDIUM RANGE PROGS WAS BASED ON A BLEND OF THE 00Z GFS/ECMWF WITH THE 06Z GEFS MEAN AND 00Z EC MEAN. THE BLEND FEATURES A MAJORITY DETERMINISTIC MODEL WEIGHT IN THE BEGINNING OF THE PERIOD TRANSITIONING TO A NEARLY EQUAL BLEND OF THE ENSEMBLES BY THE END OF THE PERIOD. ...SENSIBLE WEATHER HIGHLIGHTS/THREATS... A COLD FRONT MOVING OFF THE EAST COAST ON MONDAY WILL PUSH RAINFALL OFF THE COAST AS WELL AS USHER IN A BRIEF RETURN TO COOLER MORE FALL-LIKE TEMPERATURES. THE SOUTHERN PORTION OF THIS FRONT MAY STALL WHICH SHOULD KEEP RAIN CHANCES IN THE FORECAST THROUGH THE WEEK ACROSS FLORIDA AND THE GULF COAST. AN UPPER LOW SETTLING INTO ALASKA WILL PUSH THE STORM TRACK SOUTHWARD INTO BRITISH COLUMBIA EARLY NEXT WEEK...THEN EVENTUALLY INTO THE PACIFIC NORTHWEST AND INTERIOR WEST BY THURSDAY/FRIDAY. THIS WOULD START A RATHER STORMY PERIOD FOR THE NORTHWESTERN U.S. BY LATER IN THE WEEK AS A SERIES OF SHORTWAVES TAPS INTO AVAILABLE PACIFIC MOISTURE TO PRODUCE WIDESPREAD PRECIPITATION WITH SOME MODERATE TO HEAVY TERRAIN ENHANCED PRECIPITATION EXPECTED. THIS WILL INCLUDE A THREAT FOR HEAVY MOUNTAIN SNOWS...PLEASE SEE THE WPC WINTER WEATHER OUTLOOK PROBABILITIES FOR ADDITIONAL DETAILS. OTHERWISE...THE REST OF THE COUNTRY NEXT WEEK WILL SEE MAINLY DRY CONDITIONS AND NEAR TO ABOVE AVERAGE TEMPERATURES. SANTORELLI