EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 132 AM EDT WED SEP 02 2015 VALID 12Z SAT SEP 05 2015 - 12Z WED SEP 09 2015 THERE WERE INDICATIONS AMONG THE RECENT GLOBAL ENSEMBLE MEANS THAT THE RELATIVE STABILITY--HENCE, PREDICTABILITY--OF THE FLOW PATTERN OVER CENTRAL NORTH AMERICA MAY BE DRAWING TO A CLOSE BY THE END OF THE MEDIUM RANGE. THERE WERE IMPORTANT SYNOPTIC DIFFERENCES BETWEEN THE 12Z/01 ECENS AND NAEFS MEANS ACROSS ALASKA AND CANADA THAT LIKELY PORTEND A SHUFFLING OF THE LONGWAVES BY THE MIDDLE OR END OF NEXT WEEK. MODEL DISPARITY AND VOLATILITY FREQUENTLY ATTEND SUCH PATTERN CHANGES, SO CONFIDENCE IN THIS FORECAST IS LOWER THAN OF LATE. USED A BLEND OF THE RECENT ENSEMBLE MEANS TO MITIGATE THIS UNCERTAINTY, AS WELL AS HONOR CONTINUITY. THE LARGE SWATH OF PRECIPITATION ASSOCIATED WITH THE TROUGH MOVING ACROSS THE NORTHERN PLAINS AND SOUTH-CENTRAL CANADA WILL SHRINK AND SINK SOUTHWARD INTO THE CENTRAL STATES ALONG THE ATTENDANT SURFACE POLAR FRONT. THIS FAIRLY RAPID TRANSITION WILL BE FACILITATED BY A KICKER SHORTWAVE COMING INTO THE PACIFIC NORTHWEST EARLY NEXT WEEK. THE GULF COAST AND SOUTHEAST SHOULD REMAIN SHOWERY, WITH STABLE HEIGHTS FOR NOW. THE SOUTHWEST MONSOON WILL GET SLOWLY SQUASHED BY THE STEADY POLAR FLOW ALONG THE WESTERN BORDER WITH CANADA. CISCO