EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1158 AM EDT FRI SEP 01 2017 VALID 12Z MON SEP 04 2017 - 12Z FRI SEP 08 2017 ...OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES... MODELS/ENSEMBLES CONTINUE TO AGREE UPON SIGNIFICANT LONG WAVE FLOW PATTERN AMPLIFICATION ACROSS NORTH AMERICA DURING THE MEDIUM RANGE. EXPECT RIDGING TO EXTEND NORTH FROM THE WESTERN U.S. INTO ARCTIC WESTERN CANADA EARLY IN THE PERIOD WHILE AN UPPER TROUGH AMPLIFIES ACROSS THE EASTERN U.S.. THE PATTERN SHOULD ACHIEVE ITS GREATEST AMPLITUDE AROUND MIDWEEK. LATEST MODELS AND ENSEMBLE MEMBERS SHOW DECENT CLUSTERING INTO ABOUT DAY 5 WED BUT THEN DIVERGE REGARDING THE TROUGH ENERGY NEARING THE WEST COAST, STRENGTH OF THE WESTERN RIDGE, AND CHARACTER OF THE EASTERN U.S. MEAN TROUGH. THE LATEST FORECAST UPDATE INCORPORATED VARIOUS WEIGHTS OF THE 06Z GFS/GEFS MEAN AND 00Z ECMWF/ECMWF MEAN WITH MORE OPERATIONAL INPUT EARLY AND THEN MORE ENSEMBLE INPUT LATE AS DETAILS BECOME MORE UNCERTAIN. CONSIDERATIONS NEAR THE WEST COAST YIELDED SOMEWHAT MORE OF A TILT TOWARD THE ECMWF/ECMWF MEAN. OFF THE WEST COAST GUIDANCE SHOWS EVENTUAL EJECTION OF AN OFFSHORE UPPER LOW, CONSISTING OF T.S. LIDIA (WEAKENING TO POST-TROPICAL STATUS IN A COUPLE DAYS) AND SEPARATE ENERGY TO THE WEST. GFS/UKMET RUNS ARE FASTER WITH THE NORTHWARD PROGRESSION OF THIS UPPER LOW THAN THE ECMWF/CMC TO VARYING DEGREES WITH A COMPROMISE OFFERING A GOOD STARTING POINT. LATER IN THE PERIOD GUIDANCE SPREAD INCREASES WITH GFS/GEFS MEMBERS TENDING TO LOWER HEIGHTS OVER THE NORTHWEST TO A GREATER EXTENT THAN OTHER GUIDANCE. THE 06Z GFS HAS TRENDED SLOWER THAN THE 00Z RUN WITH THE TROUGH AXIS NEAR THE WEST COAST THOUGH. STRENGTH OF THE WESTERN RIDGE AND TELECONNECTIONS RELATIVE TO THE ASSOCIATED POSITIVE HEIGHT ANOMALY CENTER IN D+8 MULTI-DAY MEANS SEEM TO FAVOR LEANING SOMEWHAT MORE TOWARD THE SLOWER EROSION OF THE WESTERN RIDGE AS PER THE ECMWF-BASED SOLUTIONS. THE MOST SIGNIFICANT DIFFERENCE WITHIN THE EASTERN TROUGH ARISES BY DAYS 6-7 THU-FRI WITH INDIVIDUAL MODELS/ENSEMBLE MEMBERS EITHER MAINTAINING AN OPEN TROUGH OR FORMING A CLOSED LOW. OVER THE PAST COUPLE DAYS BOTH GFS AND ECMWF RUNS HAVE DEPICTED EITHER OPTION. THE 00Z ECMWF REVERTED BACK TO THE OPEN TROUGH SCENARIO AFTER THE PREVIOUS TWO RUNS WERE AMONG THE MOST AGGRESSIVE SOLUTIONS IN TERMS OF DIGGING THE TROUGH AND CUTTING OFF AN UPPER LOW OVER THE LOWER MS VALLEY. THE 00Z GFS HAD CLOSED OFF A LOW A LITTLE FARTHER EAST BUT THE 06Z RUN RETURNED TO AN OPEN TROUGH. WHILE THERE IS ROOM IN THE MEAN PATTERN FOR AN UPPER LOW, EXPECTED PERSISTENCE OF THE WESTERN RIDGE AND TELECONNECTIONS RELATIVE TO THE CORE OF NORTHERN ATLANTIC POSITIVE HEIGHT ANOMALIES SEEM TO PROVIDE MORE SUPPORT FOR AN OPEN TROUGH. THE PREFERRED FORECAST BLEND INCORPORATING THE 06Z GFS/GEFS MEAN AND 00Z ECMWF/ECMWF MEAN MAINTAIN THE OPEN TROUGH SCENARIO. THE MAIN ADJUSTMENT FROM CONTINUITY IS A MODESTLY SLOWER TREND FOR THE UPPER TROUGH AND LEADING SURFACE FRONT WHICH MAY HAVE SOME EMBEDDED WAVES. NOTE THAT CONFIDENCE IS STILL NOT PARTICULARLY HIGH GIVEN THE INCONSISTENT GUIDANCE SIGNALS AND SENSITIVITY TO UPSTREAM DETAILS. ...WEATHER HIGHLIGHTS... THE RIDGE ACROSS WESTERN NORTH AMERICA, LIKELY STRONGEST DURING DAYS 3-5 MON-WED, WILL SUPPORT WELL ABOVE-AVERAGE TEMPERATURES ACROSS THE WESTERN U.S. WITH HIGH TEMPERATURES 10 TO 20 DEGREES ABOVE AVERAGE. ISOLATED HIGHER ANOMALIES MAY BE POSSIBLE OVER PARTS OF THE NORTHWEST WHICH IS THE REGION MOST LIKELY TO SEE READINGS APPROACH OR BREAK DAILY MAX/WARM MIN RECORDS ON MULTIPLE DAYS. MEANWHILE THE AMPLIFYING EASTERN U.S. TROUGH ALOFT WILL MOVE A SURFACE FRONTAL SYSTEM FROM THE MIDWEST TO THE EAST COAST. THE FRONT SHOULD BRING A PERIOD OF PRECIPITATION ALONG/AHEAD OF IT DAYS 3-6 MON-THU. COVERAGE/INTENSITY/DURATION OF ASSOCIATED RAINFALL WILL BE SENSITIVE TO UNCERTAIN DETAILS ALOFT. THE MOST LIKELY SCENARIO OF AN OPEN TROUGH ALOFT BUT WITH SOME WAVINESS ALONG THE SURFACE FRONT MAY SUPPORT A PERIOD OF ENHANCED RAINFALL FROM THE EASTERN OH/TN VALLEYS INTO THE MID ATLANTIC/NORTHEAST AS WELL AS POSSIBLY THE EASTERN HALF OF THE GULF COAST/FLORIDA. ANY CLOSING OFF OF FLOW ALOFT AS INCONSISTENTLY SIGNALED BY SOME GUIDANCE WOULD ALLOW FOR MORE PROLONGED INFLOW OF GULF/ATLANTIC MOISTURE AND MUCH HIGHER RAINFALL TOTALS OVER PARTS OF THE EAST. EITHER WAY A MUCH COOLER AIR MASS WILL PROGRESS INTO THE EASTERN U.S. IN THE WAKE OF THE COLD FRONT, BRINGING HIGH TEMPERATURES FOR MANY AREAS 5 TO 15 DEGREES BELOW AVERAGE BY DAYS 5-7 WED-FRI. RAUSCH/RYAN