EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1258 AM EDT SAT AUG 19 2017 VALID 12Z TUE AUG 22 2017 - 12Z SAT AUG 26 2017 ...OVERVIEW... NORTHERN HEMISPHERE WILL DO A LITTLE ATMOSPHERIC SHUFFLING NEXT WEEK WHICH WILL BE FORCED BY STRONG BLOCKING IN THE NORTH ATLANTIC AND INCREASED TROUGHING INTO THE NORTHEAST. RIDGING IN THE PAC NW WILL ONLY TEMPORARILY YIELD AS TROUGHING FROM THE GULF OF ALASKA TRIES TO DIP INTO SOUTHERN BRITISH COLUMBIA. THIS WILL BRING A COLD FRONT EASTWARD AND SOUTHWARD, USHERING IN COOLER AIR (NEAR TO A FEW DEGREES BELOW AVERAGE) BY NEXT WEEKEND IN THE GREAT LAKES AND NORTHEAST. ...GUIDANCE EVALUATION/PREFERENCES... MODELS/ENSEMBLES SHOW BETTER AGREEMENT WITH THE SFC LOW EXITING OUT OF THE GREAT LAKES ON TUESDAY THROUGH ONTARIO AND QUEBEC THOUGH THE 12Z GFS WAS SLOWER/DEEPER THAN THE OTHER 12Z MODELS AND ENSEMBLE MEANS. A BLEND OF THE 18Z GFS/GEFS MEAN AND THE 12Z ECMWF/ECENS MEAN OFFERED A GOOD STARTING POINT FOR NEXT WED-FRI. QUESTION REMAINS HOW TO HANDLE THE UPPER LOW EXITING THE GULF OF ALASKA -- STRONGER/DEEPER AND FARTHER SOUTH INTO SOUTHERN BRITISH COLUMBIA (GFS AND PERHAPS THE UKMET IF EXTRAPOLATING, AS WELL AS PREVIOUS ECMWF RUNS) OR FARTHER NORTH (ECMWF/CANADIAN AND PREVIOUS GFS RUNS). GIVEN THE FLIP FLOPPING, OPTED TO SPLIT THE DIFFERENCE BETWEEN THE TWO CAMPS WHICH ONLY MODIFIED THE FRONTAL TIMING THROUGH THE PAC NW INTO IDAHO/MONTANA A BIT. FARTHER EAST, STILL EXPECT THE COLD FRONT TO DROP ALL THE WAY TO THE GULF COAST NEXT FRI/SAT WITH AT LEAST SOME MODEST COOLING/DRYING INTO THE DEEP SOUTH. ...WEATHER HIGHLIGHTS... TUE-THU - FRONT WILL PUSH EASTWARD AND DROP SOUTHWARD AS THE SFC LOW LIFTS INTO SOUTHERN CANADA. TEMPERATURES WILL WARM AHEAD OF THE BOUNDARY WELL INTO THE 80S OR NEAR 90 INTO NEW ENGLAND (WITHIN A FEW DEGREES OF RECORD HIGHS) AND STAYING IN THE 90S AND LOW 100S OVER TEXAS. PRECIPITATION COULD BE HEAVIEST THROUGH THE LOWER/EASTERN LAKES AS THE SFC LOW LIFTS NORTHEASTWARD. FLORIDA SHOULD ALSO SEE AN INCREASE IN RAINFALL AS A TROPICAL WAVE MOVES THROUGH THE STATE WED/THU. THU-SAT - PAC NW WILL SEE AN INCREASE IN SHOWERS AS THE COLD FRONT PUSHES INLAND BUT AMOUNTS SHOULD BE GENERALLY LIGHT. COOLER TEMPERATURES ARE EXPECTED WITH THE INCREASE IN CLOUD COVER BUT ONLY BRIEFLY. FARTHER SOUTH, COMBINATION OF THE WEAKENING UPPER LOW/TROUGH PUSHING INTO SOUTHERN CALIFORNIA AND THE UPPER RIDGE FOCUSING OVER TEXAS SHOULD SUPPORT INCREASED SHOWERS/CONVECTION OVER THE FOUR CORNERS REGION, MOSTLY OVER NEW MEXICO AND SOUTHERN COLORADO PER THE ENSEMBLES. FRACASSO