EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1121 AM EDT WED AUG 16 2017 VALID 12Z SAT AUG 19 2017 - 12Z WED AUG 23 2017 ...PATTERN OVERVIEW/GUIDANCE EVALUATION/PREFERENCES... GUIDANCE SHOWS A MODERATELY PROGRESSIVE PATTERN ACROSS NORTHERN U.S. AND SOUTHERN CANADA. A TROUGH CROSSING THE EAST WILL LIFT AWAY AFTER THE WEEKEND WHILE AN UPSTREAM TROUGH WITH EMBEDDED CLOSED LOW WILL CONTINUE STEADILY EASTWARD FROM WESTERN CANADA. THIS TROUGH SHOULD AMPLIFY SOME AS IT REACHES EASTERN NORTH AMERICA, LIKELY COVERING THE NORTHEASTERN QUADRANT OF THE LOWER 48 BY DAY 7 WED. BEYOND THE RIDGE BUILDING INTO WESTERN-CENTRAL CANADA AND NORTHERN ROCKIES/HIGH PLAINS, ANOTHER TROUGH WILL APPROACH THE PACIFIC NORTHWEST AND WESTERN CANADA DURING THE LATTER HALF OF THE PERIOD. OVER MORE SOUTHERN LATITUDES ENERGY ROUNDING THE EASTERN PACIFIC RIDGE WILL SETTLE INTO A TROUGH/UPPER LOW NEAR THE CALIFORNIA COAST AND A RETROGRADING ATLANTIC RIDGE WILL RAISE HEIGHTS FROM THE SOUTHEAST INTO SOUTHERN PLAINS. MODEL SPREAD IS INITIALLY RELATIVELY LOW DURING DAYS 3-4 (SAT-SUN), AND A MULTI-MODEL DETERMINISTIC BLEND INCLUDING THE 06Z GFS AND THE 00Z ECMWF/UKMET/CMC SERVED AS A GOOD STARTING POINT FOR THE FORECAST. SPREAD INCREASES SIGNIFICANTLY BY DAYS 6-7 (TUES-WED), PRIMARILY WITH RESPECT TO THE NORTHERN STREAM FLOW EVOLUTION. THE GREATEST UNCERTAINTY EXISTS WITH RESPECT TO TROUGH/UPPER LOW REACHING THE PAC NW/WESTERN CANADA BY WED. THE EVENTUAL EVOLUTION OF THIS SYSTEM DEPENDS ON THE INTERACTION BETWEEN ENERGY CURRENTLY ALONG THE ARCTIC COAST OF NW ALASKA TODAY AND THE REMNANTS OF TYPHOON BANYAN. DUE TO THE COMPLEX NATURE OF THIS INTERACTION, MODEL/ENSEMBLE SPREAD IS HIGH AND DETERMINISTIC SOLUTIONS SHOW MODERATE TO HIGH RUN-TO-RUN VARIABILITY. ADDITIONALLY, SPREAD INCREASES BY DAYS 6-7 WITH RESPECT TO THE TROUGH/UPPER LOW AMPLIFYING ACROSS EASTERN CANADA AND THE GREAT LAKES, WHICH IMPACTS THE LOCATION/TIMING OF A SURFACE FRONTAL BOUNDARY CROSSING THE MS VALLEY/MIDWEST/GREAT LAKES. THUS, THE WPC FORECAST TRENDED TOWARD SUBSTANTIALLY HEAVIER ENSEMBLE WEIGHTING (06Z GEFS/00Z ECENS/00Z NAEFS) BY DAYS 6-7. ...WEATHER HIGHLIGHTS... THE FRONT NEAR THE EAST COAST AS OF EARLY SAT WILL SETTLE OVER THE SOUTHEAST AND POSSIBLY HELP TO ENHANCE DIURNALLY FAVORED CONVECTION OVER THE REGION. THE GULF COAST AND FLORIDA MAY ALSO SEE SOME EPISODES OF LOCALLY HEAVY CONVECTION. THE COLD FRONT PROGRESSING SOUTHEAST FROM THE NORTHERN PLAINS FROM THE WEEKEND ONWARD WILL LIKELY GENERATE SHOWERS/STORMS WITH BEST HEAVY RAIN POTENTIAL EXPECTED OVER PARTS OF THE MIDWEST, WHICH MAY BE ADDITIONALLY INFLUENCED BY THE TRAILING PART OF THE SOUTHEASTERN FRONT THAT SHOULD EXTEND BACK INTO THE CENTRAL PLAINS FOR A PERIOD OF TIME. MONSOONAL CONVECTION OF VARYING INTENSITY SHOULD BE MOST COMMON OVER THE SOUTHERN HALF OF THE ROCKIES. LIGHTER AND MORE SCATTERED ACTIVITY MAY OCCUR OVER THE REST OF THE FOUR CORNERS REGION AND PARTS OF THE INTERIOR WEST AND NORTHERN ROCKIES. ADDITIONALLY, CONVECTION COULD INCREASE IN COVERAGE ACROSS SOUTH FLORIDA BY TUES-WED NEXT WEEK IN ASSOCIATION WITH A TROPICAL WAVE MOVING FROM THE BAHAMAS INTO THE FLORIDA STRAITS. THE MOST PRONOUNCED AREA OF TEMPERATURE ANOMALIES SHOULD BE AHEAD OF THE NORTHERN TIER FRONT WITH MAX/MIN READINGS TENDING TO BE 5-10F ABOVE NORMAL AND PERHAPS LOCALLY A LITTLE WARMER. EXPECT SOUTHERN STATES TO SEE MIN TEMPS PERSISTENTLY ABOVE NORMAL BY A FEW DEGREES F. RYAN/RAUSCH