EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1215 PM EDT MON AUG 22 2016 VALID 12Z THU AUG 25 2016 - 12Z MON AUG 29 2016 ...PATTERN OVERVIEW... SUBTROPICAL RIDGING WILL HOLD STRONG OVER THE SOUTHEASTERN U.S. LATE THIS WEEK AND THROUGH THE WEEKEND...WHILE GENERAL TROUGHING SHOULD PERSIST OVER THE WESTERN U.S.. AT THE SURFACE...A COLD FRONT WILL PRESS THROUGH THE GREAT LAKES AND NORTHEAST THURSDAY-FRIDAY BEFORE THE TAIL END OF THE BOUNDARY WEAKENS AND STALLS ACROSS THE SOUTHEAST AND TENNESSEE VALLEY THIS WEEKEND. OTHER SURFACE BOUNDARIES COULD POTENTIALLY CROSS THE NORTH CENTRAL U.S. THIS WEEKEND AND INTO EARLY NEXT WEEK...BUT THE DETAILS OF THESE FEATURES AND THE SHORTWAVE ENERGY MOVING THROUGH ALOFT REMAIN VERY UNCERTAIN. ...UNCERTAINTY ASSESSMENT/MODEL PREFERENCES... MODEL GUIDANCE GUIDANCE IS REASONABLE WELL CLUSTERED WITH THE GENERAL PATTERN OVER THE LOWER 48 THURSDAY AND FRIDAY...BUT THEN CONFIDENCE IN THE FORECAST QUICKLY DECREASES BY THE WEEKEND AS MODEL SPREAD BECOMES UNUSUALLY LARGE WITH THE EVOLUTION OF FLOW FROM ALASKA TO THE NORTHWESTERN U.S.. THE LATEST WPC MEDIUM RANGE FORECAST RELIED HEAVILY ON CONTINUITY AND ENSEMBLE MEAN GUIDANCE TO ACCOUNT FOR THE UNCERTAINTY LATER IN THE FORECAST PERIOD...WITH LESS WEIGHT GIVEN TO THE 00Z ECMWF ENSEMBLE MEAN SINCE IT APPEARED TO AN OUTLYING SOLUTION WITH BUILDING A STRONG UPPER RIDGE INTO THE ALASKA EARLY NEXT WEEK. PLEASE REFER TO WPC'S ALASKA DISCUSSION (PMDAK) FOR MORE DETAILS ON MODEL UNCERTAINTY IN THIS REGION. ...WEATHER HIGHLIGHTS... RAIN AND THUNDERSTORMS WILL BE POSSIBLE WITH THE COLD FRONT PRESSING FROM THE GREAT LAKES TO THE NORTHEAST THURSDAY AND FRIDAY...BUT THEN PRECIPITATION CHANCES SHOULD DECREASE AS THE BOUNDARY WEAKENS OVER THE SOUTHEAST AND TENNESSEE VALLEY THIS WEEKEND. PERIODS OF HEAVY RAIN AND THUNDERSTORMS WILL ALSO BE POSSIBLE ALONG THE GULF COAST AROUND AN UPPER/SFC HIGH AS WELL AS OVER THE UPPER MISSISSIPPI VALLEY WHERE MOISTURE AND INSTABILITY SHOULD INCREASE AHEAD OF A POSSIBLE SHORTWAVE LIFTING THROUGH THE NORTH CENTRAL U.S. THIS WEEKEND. GERHARDT