EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 141 AM EDT WED AUG 24 2016 VALID 12Z SAT AUG 27 2016 - 12Z WED AUG 31 2016 WORKING FROM WEST TO EAST --- IN SHORT --- 45/35 OF THE 23/12Z ECENS/NAEFS MEANS RESPECTIVELY AND 20 PCT OF THE DETERMINISTIC ECMWF THROUGH THE END OF DAY 5 SHOULD WORK. FOR DAY 6-7 --- TRANSITION TO A 55/35 OF THE SAME MEANS AND 10 PCT OF THE ECMWF TO HOLD THE PATTERN DOWN. EVEN IF THERE REMAINS NO 'PERFECT PROG' SOLUTION AT DAY 7 ACROSS THE SOUTHEAST. WILL USE THIS FOR THE D4-5 AND D6-7 QPFS AND WITHIN REASON TAKE ANOTHER LOOK AT THE 24/00Z HI-RES 27KM ECMWF QPF TO MAKE SURE THINGS LOOK HALF WAY DECENT. THE FOLLOWING IS --- UNCERTAINTY ASSESSMENT AND ADDITIONAL INFORMATION OUTSIDE THE NATIONAL PACKAGE MODEL BLEND. THE 23/12Z CYCLE HAS SETTLED DOWN SOME ACROSS WEST CENTRAL CANADA THROUGH DAY 5 AND THOUGHT THE DETERMINISTIC ECMWF/UKMET HAVE THE RIGHT IDEA. IE...HOW TO EJECT THE ENERGY THROUGH THE GULF OF ALASKA AND DOWNSTREAM. AN ISSUE SEVERAL DAYS AGO --- SEEMS MUCH MORE AGREEABLE NOW AS THE DETERMINISTIC GUIDANCE (OUTSIDE OF THE 23/12Z CANADIAN) HAS SOME SHORTWAVE RIDGING CREATING SPACE BETWEEN SYSTEMS ACROSS THE CANADIAN ROCKIES AT DAY 5. THIS DOES ALLOW THE LEAD SYSTEM TO MIGRATE DOWNSTREAM AND SHEAR OUT ---OR AT THE VERY LEAST MIGRATE DOWNSTREAM AND EAST OF THE CANADIAN DIVIDE TO STOP THE SOLUTION ERRORS THAT WANTED TO CARVE COLD ADVECTION WEST OF THE DIVIDE. EVEN THE 23/12Z GFS LOOKED 'USE-ABLE' ALOFT FOR DAY 5 INTO DAY 6 ACROSS THE PACIFIC NORTHWEST AND NORTHERN DIVIDE. OVER THE NORTHERN PLAINS AND GREAT LAKES...THE DAY 3-4 WAVE EXITING NORTHERN COLORADO AND CENTRAL WYOMING LIFTS NORTHEASTWARD INTO THE UPPER GREAT LAKES AND WESTERN ONTARIO AND THE MID-LEVEL FRONT DOES DRAG ENOUGH OF A SURFACE BOUNDARY INTO THE CENTRAL PLAINS AND MIDWEST --- TO ACCEPT A GENERAL GFS/ECMWF COMPROMISE FOR DAY 5-6 FROM WESTERN KANSAS TO NEW ENGLAND. THEN THE QUESTION OVER THE SOUTHEAST ARISES --- AND WHICH CONCERNS ME. HOW DO YOU TRANSITION FROM SOMETHING THAT HAS SOME GFS BASIS ACROSS MID-LATITUDE NORTH AMERICA THROUGH DAY 5 --- TO A SOLUTION THAT SEEMS TO BE VERY 'NON-GFS BASED' WITH PROBABLY THE KEY SYNOPTIC-SCALE FEATURE IN THIS MEDIUM RANGE PERIOD (IN THE SOUTHEAST ATLANTIC BASIN). BASED ON RUN-TO-RUN CONTINUITY ALONE --- THE DIFFERENCES BETWEEN THE 23/12Z, 23/18Z AND 24/00Z GFS ARE SO VASTLY DIFFERENT --- WHY IS THIS EVEN A CONSIDERATION? BUT THAT'S ME. IN GENERAL ---THE 23/12Z GUIDANCE CONTINUES TO MOVE THE AREA OF DISTURBED WEATHER AT A 4-5 DEGREE-PER-DAY RATE IN A WEST-NORTHWESTWARD MANNER--- AND THIS WILL LIKELY BE THE CASE THROUGH THE EARLY PART OF THE UPCOMING MEDIUM RANGE --- BECAUSE THE 594DM RIDGE AXIS REMAINS OVER THE CAROLINAS AND SOUTHERN APPALACHIANS WITH A STATIONARY BOUNDARY DRAPE TO ITS IMMEDIATE SOUTH. THE RIDGE WILL BE VERY SLOW TO MOVE THROUGH DAY 5. THE MAIN THING THIS PACKAGE WANTS TO ACCOMPLISH --- IS GIVE THE LOCAL OFFICES SOME DECENT GRIDS TO WORK WITH. EVEN IF IT IS DAY 6 OR 7. I COULD EVEN CONSIDER WHAT THE 23/12Z CANADIAN AND THE 23/18Z HWRF (FOR INVEST 99L) OFFER SINCE BOTH DEVELOP ENERGY ALONG THE STATIONARY BOUNDARY --- BASICALLY SURFACE-BASED WAVES THAT ARE NOT PART OF THE 'DISTURBANCE'. THEY MAY ENHANCE RAINFALL ALONG THE STATIONARY BOUNDARY AND RE-SHAPE IT AHEAD OF WHATEVER FORMS BETWEEN SOUTH FLORIDA AND THE BAHAMAS. VOJTESAK