EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 250 AM EDT SAT AUG 12 2017 VALID 12Z TUE AUG 15 2017 - 12Z SAT AUG 19 2017 ...PATTERN OVERVIEW/GUIDANCE EVALUATION/PREFERENCES... EXPECT FLOW AT THE START OF THE PERIOD TUE TO FEATURE AN OMEGA BLOCK CONFIGURATION OVER CANADA WITH TROUGHING EXTENDING ACROSS THE WESTERN U.S. AND TO A LESSER DEGREE OVER THE EAST. AS GUIDANCE CLUSTERING HAS IMPROVED CONSIDERABLY FOR THE HANDLING OF A COMPACT UPPER LOW CROSSING EASTERN CANADA AND OVERALL TROUGH CROSSING THE NORTHEAST TUE-THU, THE DOMINANT FOCUS OF FORECAST UNCERTAINTY WILL BE ON THE WESTERN NORTH AMERICA TROUGH ENERGY AS IT HEADS INTO THE DOWNSTREAM RIDGE AND THEN LATER IN THE PERIOD DETAILS OF UPSTREAM FLOW ENTERING THE NORTHWEST U.S./WESTERN CANADA FROM THE NORTHEASTERN PACIFIC. FOR THE EVOLUTION OF WESTERN NORTH AMERICA TROUGH ENERGY AND ASSOCIATED SURFACE REFLECTION, OPERATIONAL MODELS AND ENSEMBLES HAVE BEEN DISPLAYING A WIDE VARIETY OF POSSIBILITIES. LACKING SUCH A SIGNAL IN PRIOR RUNS, THE 12Z ECMWF DEVELOPED A CLOSED LOW JUST NORTH OF THE NORTHERN PLAINS AND THEN TRACKED IT INTO THE GREAT LAKES. ITS MEAN WAS MORE SUBTLE BUT STILL HINTED AT A MORE SEPARATED DEPICTION OF ENERGY THAN MOST OTHER GUIDANCE. THE 12Z CMC WAS THE NEXT CLOSEST WITH A CLOSED LOW AT TIMES BUT FARTHER NORTHWARD. INTERESTINGLY THE NEW 00Z CMC HAS ADJUSTED VERY CLOSE TO THE 12Z ECMWF/ECMWF MEAN IN PRINCIPLE. WHILE THE GFS/GEFS MEAN AND CMC MEAN RUNS ARE MORE OPEN WITH THE TROUGH ENERGY, THEY SHARE A SIMILAR THEME IN BRINGING AN UPPER TROUGH INTO THE EAST-CENTRAL U.S. BY DAY 7 SAT WITH AN AREA OF LOW PRESSURE TRACKING INTO THE GREAT LAKES AND/OR SOUTHERN CANADA. TRENDS OVER THE PAST DAY HAVE BEEN TOWARD SOMEWHAT GREATER HEIGHT FALLS ALOFT CROSSING THE NORTHERN TIER, REFLECTED AT THE SURFACE AS BETTER DEFINED/FASTER LOW PRESSURE HEADING INTO THE GREAT LAKES AND A FARTHER SOUTHEAST PUSH OF THE TRAILING COLD FRONT. THE PAST FEW ECMWF MEAN RUNS HAVE BEEN MOST CONSISTENT IN DEPICTING THE EVOLUTION TOWARD WHICH OTHER SOLUTIONS ARE CURRENTLY TRENDING. WHILE NOT TO THE EXTENT OF THE 12Z ECMWF, LEANING SLIGHTLY TOWARD THE IDEA OF DEEPER UPPER MS VALLEY/GREAT LAKES TROUGHING THAN THE GFS/GEFS MEAN COULD BE REASONABLE GIVEN THAT THE STRENGTH OF THE HUDSON BAY RIDGE MAY BE SUFFICIENT TO HELP DIVERT ENERGY TOWARD THE NORTHERN TIER U.S.. WITHIN UPSTREAM PACIFIC FLOW THERE IS DECENT AGREEMENT ON A SHORTWAVE RIDGE REACHING THE WEST COAST AROUND THU FOLLOWED BY A TROUGH OF UNCERTAIN TIMING/AMPLITUDE. IT APPEARS TO BE MERE COINCIDENCE THAT THE 18Z GFS/12Z ECMWF WERE FAIRLY CLOSE WITH THE TROUGH GIVEN THE FULL ENSEMBLE SPREAD, BUT THEIR ENSEMBLE MEANS AT LEAST PROVIDE A HINT OF A TROUGH WITH SIMILAR TIMING INTO FRI. CONSULT THE NHC TROPICAL WEATHER OUTLOOK FOR LATEST INFO REGARDING AN AREA OF WEAK LOW PRESSURE CURRENTLY EAST OF THE SOUTHERN BAHAMAS. THIS FEATURE SHOULD RECURVE OFFSHORE AND BY EARLY IN THE EXTENDED TIME FRAME BECOME ABSORBED INTO THE WESTERN ATLANTIC FRONT THAT EXTENDS BACK INTO THE U.S.. VARYING WEIGHTS OF THE 18Z GFS/GEFS MEAN AND 12Z ECMWF/ECMWF MEAN PROVIDED A GOOD STARTING POINT TO REFLECT PREFERRED OR CONSENSUS IDEAS WHILE DOWNPLAYING LESS CONFIDENT SPECIFICS. EVEN OR GREATER OPERATIONAL WEIGHTING EARLY HELPED TO DEFINE THE UPPER LOW/TROUGH OVER EASTERN CANADA AND NORTHEAST U.S.. THEN ENSEMBLE MEAN WEIGHT INCREASED MID-LATE PERIOD, WITH MORE OF AN ECMWF MEAN TILT GIVEN ITS LONGER LEAD TIME FOR LOW PRESSURE EXPECTED TO TRACK INTO THE GREAT LAKES. THIS BLEND BALANCED DETAIL/UNCERTAINTY CONSIDERATIONS FOR UPSTREAM FLOW. ...WEATHER HIGHLIGHTS... COMBINATION OF A LEADING EASTERN U.S. TO PLAINS FRONT AND LOW PRESSURE/FRONTAL SYSTEM EMERGING ACROSS THE PLAINS AND TRACKING INTO THE GREAT LAKES WILL BRING THE POTENTIAL FOR SOME LOCALLY HEAVY CONVECTION ACROSS THE NORTHERN 2/3 OF THE PLAINS AND LOCATIONS TO THE EAST/NORTHEAST. CURRENTLY THE BEST PROBABILITY FOR HIGHEST 5-DAY TOTALS EXISTS OVER THE NORTHERN-CENTRAL PLAINS INTO MID-UPPER MS VALLEY. THE FRONT SETTLING OVER THE INTERIOR SOUTHEAST TUE-THU MAY SERVE AS A FOCUS PERIODS OF CONVECTION, SOME OF WHICH COULD BE LOCALLY HEAVY AS WELL. THE MOST NOTABLE ASPECT OF THE TEMPERATURE FORECAST IS THE RELATIVE LACK OF EXTREME READINGS, WITH AN OVERWHELMING MAJORITY OF MIN/MAX ANOMALIES REMAINING IN THE SINGLE DIGITS. LINGERING MONSOON CONVECTION EARLY IN THE PERIOD MAY LEAD TO SOME BELOW NORMAL HIGHS OVER/NEAR THE ROCKIES WHILE THE SHORTWAVE RIDGE ALOFT HEADING INTO THE WEST LATE WEEK MAY BRING MODERATELY ABOVE NORMAL TEMPS TO LOCATIONS INLAND FROM THE WEST COAST BUT WEST OF THE ROCKIES. MEANWHILE THE WARM SECTOR AHEAD OF THE FRONT PUSHING SOUTHEAST FROM THE NORTHERN PLAINS SHOULD CONTAIN ONE OR MORE DAYS OF ABOVE NORMAL TEMPS, PERHAPS A LITTLE MORE SO FOR MINS. RAUSCH