EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1202 PM EDT THU AUG 18 2016 VALID 12Z SUN AUG 21 2016 - 12Z THU AUG 25 2016 ...OVERVIEW/GUIDANCE EVALUATION/MODEL PREFERENCES... A SERIES OF TROUGHS WILL PASS THROUGH THE SOUTHERN PROVINCES OF CANADA AND THE NORTHERN TIER OF U.S. THE FIRST TROUGH WILL PASS OVER THE EASTERN THIRD OF THE U.S. AND DRIVE A COLD FRONT THROUGH THE GREAT LAKES AND OHIO/TENNESSEE VALLEYS THIS SUN 21 AUG. A FEW MODELS INDICATE THE POSSIBILITY OF A WAVE OF LOW PRESSURE FORMING ALONG THE FRONT AS IT CROSSES NEW ENGLAND MON 22 AUG. OVERALL, THERE IS GOOD CLUSTERING OF THE GFS/ECMWF AND THEIR ENSEMBLE MEANS WITH THE UPPER TROUGH AND SFC FRONT. A SECOND TROUGH AND SURFACE FRONT WILL TRACK FROM THE GULF OF ALASKA AND ACROSS THE PACIFIC NORTHWEST, THE NORTHERN ROCKIES/PLAINS AND THE CANADIAN PRAIRIE; HOWEVER, THE UPPER LOW/TROUGH DOWNSTREAM NEAR THE HUDSON BAY BEGINS TO DIVERGE. THE 00Z 18 UKMET AND 00Z 18 ECMWF BOTH DEPICT A DEEPER SOLUTION THAN THE REST OF THE GUIDANCE. LOW CONFIDENCE PERSISTS WITH BOTH ENSEMBLE AND DETERMINISTIC SOLUTIONS THROUGH DAY 7 ON WHETHER THE EASTERN PACIFIC MID-UPPER LEVEL HIGH RIDGE REMAIN AMPLIFIED OR IF SHORTWAVES RIDING OVER THE TOP OF THE RIDGE LOWER HEIGHTS AND REINFORCE THE LOWER HEIGHTS OVER SOUTHWEST CANADA AND THE NORTHERN PLAINS. THEREFORE, DAYS 3 AND 4 FORECAST WERE CONSTRUCTED OF EQUAL PARTS OF THE 00Z 18 ECMWF AND THE ITS ENSEMBLE MEAN, THE 06Z 18 GFS AND ITS ENSEMBLE MEAN AND THE 00Z 18 UKMET. DAY 5 HAD SLIGHTLY LESS WEIGHTING OF THE EC AND GFS; WHEREAS, DAYS 6 AND 7 WERE HEAVILY WEIGHTED WITH THE 00Z 18 ECMWF AND THE 06Z 18 GFS ENSEMBLE MEAN. A SLOW-MOVING MID-LEVEL WEAKNESS TRACKS SLOWLY FROM THE DESERT SOUTHWEST TOWARD THE SOUTHERN PLAINS DURING THE ENTIRE MEDIUM RANGE PERIOD. A RATHER ELONGATED TROUGH APPEARS IN MANY OF THE DETERMINISTIC SOLUTIONS WHICH REALLY TEND TO DAMPEN THIS SIGNAL. ...SENSIBLE WEATHER HIGHLIGHTS... A MAJOR COOL DOWN IS FORECAST ON DAY 3/SUNDAY 21 AUGUST. MAXIMUM TEMPERATURES WILL BE SEVERAL DEGREES COOLER THAN AVERAGE FOR THE NORTHERN HIGH PLAINS, UPPER MISSISSIPPI VALLEY AND THE UPPER GREAT LAKES REGION. THE COOLER AIR WILL EXPAND FURTHER SOUTH IN THE MISSISSIPPI VALLEY AND THE SURROUNDING PLAINS ON MONDAY AS THE COLD FRONT PUSHES THROUGH. TEMPERATURES WILL REBOUND BRIEFLY TO NEAR OR ABOVE NORMAL ACROSS PORTIONS OF INTERMOUNTAIN WEST AND THE NORTHERN PLAINS BEFORE THE NEXT SYSTEM APPROACHES. ON DAY 5, TUESDAY 23 AUGUST A WARM FRONT MOVING THROUGH THE PLAINS WILL SPREAD HIGHER TEMPERATURES AS FAR AS THE UPPER MISSISSIPPI VALLEY - COOLER TEMPERATURES WILL CONTINUE OVER THE MID TO LOWER MISSISSIPPI VALLEY AND CENTRAL APPALACHIANS. ON DAY 6, WEDNESDAY 24 AUGUST...THE NEXT COLD FRONT USHERS IN COOLER AIR INTO THE NORTHERN ROCKIES AND HIGH PLAINS. THE THREAT OF SHOWERS/STORMS ACROSS THE CENTRAL TO SOUTHERN PLAINS LEADS TO A GOOD CHANCE OF TEMPERATURES REMAINING BELOW NORMAL IN MUCH OF THE SOUTHERN TO CENTRAL PLAINS. FINALLY ON DAY 7, THURSDAY 25 AUGUST...THE COOLER THAN NORMAL TEMPERATURES WILL SPREAD SOUTH AND EAST OF THE NORTHERN HIGH PLAINS ACROSS THE REST OF THE NORTHERN TO CENTRAL PLAINS AND CENTRAL ROCKIES. THE HIGH PRESSURE WILL RETREAT TOWARD THE EAST COAST, ALLOWING TEMPERATURES IN LOCATIONS EAST OF THE ADVANCING FRONT TO REBOUND. THERE WILL BE MODERATE TO LOCALLY HEAVY RAINFALL ALONG THE ADVANCING COLD FRONT ACROSS NEW YORK AND NEW ENGLAND ON DAY 3 BEFORE THE FRONT MOVES OFF THE COAST. THE FRONT MAY HANG UP AND LEAD TO INCREASED CHANCES FOR HEAVY RAINFALL OVER TEXAS ON SUNDAY AND THE THREAT MAY CONTINUE INTO MONDAY. SHOWERS AND THUNDERSTORMS WILL LIKELY INCREASE IN ADVANCE OF THE COLD FRONT IN THE NORTHERN TO CENTRAL PLAINS ON DAY 6 AND CONTINUE UP INTO THE UPPER AND MID MISSISSIPPI VALLEY ON DAY 7/THURSDAY 25 AUGUST. CAMPBELL