EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1203 PM EDT MON JUL 31 2017 VALID 12Z THU AUG 03 2017 - 12Z MON AUG 07 2017 ...RECORD-BREAKING HEAT APPEARS LIKELY FOR MUCH OF THE PACIFIC NORTHWEST ON THURSDAY... ...PATTERN OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES... THE LARGE-SCALE PATTERN ACROSS THE COUNTRY WILL GENERALLY FEATURE ABOVE NORMAL MID-LEVEL HEIGHTS ACROSS THE WEST COAST WHILE BROAD LONGWAVE TROUGHING EXTENDS ACROSS THE EASTERN HALF OF THE U.S. IN TIME...THE WESTERN RIDGE SHOULD FLATTEN/WEAKEN AS UPSTREAM TROUGHING HELPS LOWER HEIGHTS A BIT ACROSS THE WESTERN U.S. MEANWHILE...A SHORTWAVE MOVING THROUGH THE UPPER MIDWEST WILL HELP DRIVE AN AREA OF LOW PRESSURE THROUGH THE GREAT LAKES AND EVENTUALLY INTO SOUTHEASTERN CANADA BY FRIDAY/SATURDAY. THROUGH FRIDAY...THE MODELS HAVE A GOOD HANDLE ON THIS POSITION OF THIS SURFACE CYCLONE WITH SOME DEPTH DIFFERENCES AS THE 00Z ECMWF MAINTAINED A LOWER PRESSURE. EVENTUALLY THE 06Z GFS ENDS UP SOUTH OF THE CONSENSUS WHICH MAKES THE 00Z GFS MORE USEFUL AMONG THE PAIR OF RUNS. ENSEMBLE AGREEMENT IS QUITE REASONABLE DEEPER INTO THE FORECAST WITH THE 00Z GEFS/ECMWF ENSEMBLE MEANS IN LINE WITH ONE ANOTHER AS THE SYSTEM LIFTS FARTHER INTO QUEBEC. THE 00Z CMC/NAEFS ENSEMBLE MEANS WERE DISPLACED TO THE SOUTH FOR THE MOST PART. TOWARD THE DAY 6/7...AUGUST 6/7 TIME FRAME...LONGWAVE TROUGHING REMAINS FOCUSED ACROSS THE EASTERN HALF OF THE NATION WHILE GENERAL RIDGING CONTINUES TO BE A FIXTURE OVER THE WESTERN U.S. THERE IS A WEAK IMPULSE APPROACHING NORTHERN CA WHICH WOULD EFFECTIVELY DENT PART OF THIS MEAN RIDGE AXIS. GIVEN REASONABLE SYNOPTIC-SCALE AGREEMENT THROUGHOUT THE PERIOD...WAS ABLE TO AFFORD A DECENT AMOUNT OF THE 00Z GFS/ECMWF IN THE MODEL BLEND. WHILE THE 00Z CMC/UKMET HANDLED THE PROGRESSION OF THE GREAT LAKES CYCLONE REASONABLY WELL...THEY DIVERGED ON THE TRACK OF WHAT IS CURRENTLY TROPICAL STORM EMILY. GRADUALLY INCORPORATED MORE OF THE 00Z GEFS/ECMWF ENSEMBLE MEANS FARTHER INTO THE FORECAST PERIOD TO ACCOUNT FOR BUILDING UNCERTAINTIES. TOOK AN EQUAL SPLIT BETWEEN THE TWO CAMPS GIVEN BOTH FIT WELL WITHIN THE ENSEMBLE ENVELOPE THROUGH EARLY NEXT WEEK. ...WEATHER HIGHLIGHTS... GIVEN THE MID-LEVEL RIDGE FEATURING 500-MB HEIGHT ANOMALIES AROUND 2 STANDARD DEVIATIONS ABOVE AVERAGE CENTERED OVER THE WESTERN U.S...EXPECT MORE RECORD TEMPERATURES TO EXTEND INTO THE MEDIUM RANGE PERIOD. ON THURSDAY...THE CURRENT FORECAST HIGHS ARE IN THE TRIPLE DIGITS ACROSS WESTERN OR/WA. AT THIS POINT...THE RECORD HIGH ON THURSDAY IN SEATTLE WA IS ONLY 91 DEGREES WHICH SHOULD EASILY BE SHATTERED. THIS TREND IS NOTED TO POINTS SOUTHWARD WHICH INCLUDES PORTLAND AND MEDFORD OR. MANY OF THESE DAILY RECORDS COULD BE BROKEN BY OVER 5 DEGREES. IN TERMS OF DEPARTURES FROM NORMAL...SUCH TEMPERATURES ARE AROUND 20 TO 25 DEGREES ABOVE CLIMATOLOGY FOR EARLY AUGUST. AS THE UPPER RIDGE BEGINS TO WEAKEN...SUCH READINGS WILL COME DOWN BUT IT STILL REMAINS QUITE WARM THROUGHOUT THE PERIOD. FARTHER EAST...IT IS EXPECTED TO BE APPROXIMATELY 10 TO 15 DEGREES BELOW AVERAGE ACROSS THE CENTER OF THE U.S. THROUGH SATURDAY. THIS IS IN RESPONSE TO LOWER HEIGHTS GENERALLY REMAINING IN PLACE THROUGHOUT THE FORECAST. IN TERMS OF PRECIPITATION...THE BEST DAILY CHANCES FOR HEAVY RAINFALL WILL BE ALONG THE GULF COAST GIVEN A STALLED FRONTAL BOUNDARY LINGERING OVER THE REGION. THIS SHOULD WORK IN ACCORDANCE WITH ANY MESOSCALE SEA BREEZE CIRCULATIONS WHICH COULD AUGMENT LOCAL RAINFALL AMOUNTS. ELSEWHERE...ACTIVE MONSOONAL FLOW WILL BRING A THREAT FOR SEMI-ORGANIZED CONVECTION ACROSS THE DESERT SOUTHWEST/FOUR CORNERS REGION. SOME OF THESE THUNDERSTORMS COMPLEXES COULD TRANSLATE DOWNSTREAM INTO THE ADJACENT GREAT PLAINS AS A POTENTIAL MESOSCALE CONVECTIVE SYSTEM. RUBIN-OSTER