EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1209 PM EDT TUE AUG 23 2016 VALID 12Z FRI AUG 26 2016 - 12Z TUE AUG 30 2016 ...PATTERN OVERVIEW... SUB-TROPICAL RIDGING WILL HOLD STRONG OVER THE SOUTHERN MID-ATLANTIC STATES LATE THIS WEEK AND THROUGH THE WEEKEND...WHILE GENERAL TROUGHING WILL PERSIST IN THE WESTERN U.S.. WITHIN THE MEAN FLOW...SHORTWAVE ENERGY LIFTING NORTH OF THE GREAT LAKES ON FRIDAY WILL HELP PUSH A COLD FRONT THROUGH THE NORTHEAST...BEFORE THE TAIL END OF THE BOUNDARY STALLS AND WEAKENS WHILE TRYING TO PRESS INTO THE STRONG RIDGING OVER THE SOUTHERN MID-ATLANTIC STATES THIS WEEKEND. UPSTREAM SHORTWAVES COULD ALSO TRACK THROUGH THE LOWER 48 THIS WEEKEND...ONE CROSSING THE NORTH CENTRAL U.S. AND SECOND DROPPING TROUGH BRITISH COLUMBIA...BUT THE DETAILS OF THESE FEATURES REMAIN UNCERTAIN. ALSO OF NOTE...THE NATIONAL HURRICANE CENTER IS CURRENTLY MONITORING AN AREA OF SHOWERS AND THUNDERSTORMS EAST OF THE LEEWARD ISLANDS WHICH COULD POTENTIALLY IMPACT THE SOUTHEASTERN U.S. EARLY NEXT WEEK. ...UNCERTAINTY ASSESSMENT/MODEL PREFERENCES... THE GREATEST AREA OF UNCERTAINTY CONTINUES TO BE WITH THE EVOLUTION OF FLOW FROM ALASKA TO THE NORTHWESTERN U.S. THIS WEEKEND. ENSEMBLE MEANS HAVE COME INTO BETTER AGREEMENT ACROSS THE REGION...BUT THERE REMAINS AN UNUSUALLY LARGE AMOUNT OF SPREAD BETWEEN DETERMINISTIC RUNS AND INDIVIDUAL ENSEMBLE MEMBERS TO SUGGEST THIS IS STILL A LOW CONFIDENCE FORECAST. A SECOND AREA OF FORECAST UNCERTAINTY IS WITH THE POSSIBLE TROPICAL SYSTEM APPROACHING THE SOUTHEASTERN U.S. THIS WEEKEND/EARLY NEXT WEEK...ESPECIALLY SINCE THE SLIGHTEST DIFFERENCE IN TRACK AND STRENGTH WILL HAVE A LARGE IMPACT ON THE SENSIBLE WEATHER. THE WPC FORECAST STARTED WITH A GENERAL CONSENSUS BLEND OF THE GFS/ECMWF...BUT THEN TRANSITIONED TO ENSEMBLE MEAN GUIDANCE LATER IN THE FORECAST PERIOD TO ACCOUNT FOR THE UNCERTAINTY OVER THE GULF OF ALASKA/WESTERN CANADA/NORTHWESTERN U.S. AND SOUTHEASTERN U.S.. ...WEATHER HIGHLIGHTS... SHOWERS AND THUNDERSTORMS WILL BE POSSIBLE WITH THE COLD FRONT PRESSING THROUGH THE NORTHEAST ON FRIDAY...BUT THEN THE CHANCE FOR PRECIPITATION DIMINISHES AS THE BOUNDARY WEAKENS OVER THE MID-ATLANTIC STATES THIS WEEKEND. SHOWERS AND THUNDERSTORMS WILL ALSO BE POSSIBLE ACROSS THE MIDDLE TO UPPER MISSISSIPPI VALLEY THIS WEEKEND...AHEAD OF THE SHORTWAVE CROSSING THE NORTH CENTRAL U.S.. FINALLY...HEAVY RAINS COULD THREATEN PORTIONS OF THE SOUTHEASTERN U.S. BY EARLY NEXT WEEK...DEPENDING ON THE DETAILS OF THE POTENTIAL TROPICAL SYSTEM. PLEASE REFER TO THE NATIONAL HURRICANE CENTER FOR THE LATEST INFORMATION ON THIS FEATURE (WWW.NHC.NOAA.GOV). GERHARDT