EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1149 AM EDT WED AUG 02 2017 VALID 12Z SAT AUG 05 2017 - 12Z WED AUG 09 2017 16Z UPDATE... NO MAJOR CHANGES WERE MADE TO THE WPC DAY 3-7 FORECAST. MODELS STAYED STABLE WITH THE OVERALL PERSISTENT PATTERN OF AN AMPLIFIED RIDGE ALONG THE WEST COAST AND BROAD TROUGHING ACROSS THE CENTRAL AND EASTERN U.S. THIS WEEKEND AND THROUGH MUCH OF NEXT WEEK. THERE CONTINUES TO BE UNCERTAINTY WITH THE DETAILS OF A SHORTWAVE AND SURFACE WAVE PROGRESSING FROM THE OHIO VALLEY TO THE NORTHEAST ON MONDAY...BUT WITH THE EXCEPTION OF THE CMC (WHICH APPEARED TO BE A STRONG/NORTHERN OUTLIER)...THE 00Z/06Z DETERMINISTIC GUIDANCE FIT WITHIN THE ENSEMBLE SPREAD WITH THIS FEATURE. THE UPDATED WPC FORECAST STARTED WITH A BLEND OF THE GFS/ECMWF/CMC/GEFS MEAN/ENSEMBLE MEAN/CONTINUITY. THE CMC WAS ELIMINATED FROM THE BLEND ONCE IT STRAYED FROM CONSENSUS DAY 5/MONDAY. ALSO...DETERMINISTIC GUIDANCE WAS WEIGHTED MORE HEAVILY THAN NORMAL DUE TO THE OVERALL ABOVE AVERAGE AGREEMENT WITH THE LONGWAVE PATTERN. GERHARDT ...PATTERN OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES... AN AMPLIFIED AND PERIODICALLY BLOCKING FLOW PATTERN FROM THE NORTH PACIFIC ACROSS NORTH AMERICA IS FORECAST BY ALL GUIDANCE TO PERSIST THROUGH THE MEDIUM RANGE. AS WAS THE CASE THIS TIME YESTERDAY, THE LARGE SCALE PATTERN ACROSS THE CONUS ON DAY 7 (WED) IS NOT SIGNIFICANTLY DIFFERENT THAN DAY 3 (SAT). AS LONG AS THIS PATTERN PERSISTS, RIDGING WILL REMAIN PREVALENT ALONG THE NORTH AMERICAN WEST COAST, WITH AN ANOMALOUS UPPER LOW ACROSS THE NORTH PAC/GULF OF AK, AND TROUGHING PREVALENT ACROSS THE CENTRAL AND EASTERN CONUS. GIVEN THE PERSISTENT PATTERN, CONFIDENCE IS FAIRLY HIGH WITH RESPECT TO THE LARGE-SCALE ASPECTS OF THE MEDIUM RANGE FORECAST, WITH CONFIDENCE DECREASING FOR SMALLER SCALE DETAILS, PARTICULARLY BEYOND DAY 4 (SUN). THIS IS PARTICULARLY TRUE FOR THE SHORTWAVE CROSSING THE MIDWEST/GREAT LAKES ON DAYS 5-7. THE ECMWF/CMC REMAIN ON THE FAST SIDE OF THE SPREAD WITH THIS WAVE (THE ECMWF HAS SLOWED SLIGHTLY FROM PREVIOUS RUNS), WHILE THE GFS REMAINS ON THE SLOW SIDE OF THE SPREAD (ALTHOUGH FASTER THAN PREVIOUS RUNS). THUS AT THIS TIME, THE PREFERENCE WAS SOMEWHERE IN BETWEEN THESE TWO CAMPS. ADDITIONALLY, SHORTWAVE ENERGY IS FORECAST BY ALL GUIDANCE TO UNDERCUT THE WESTERN RIDGE, MOVING INTO CALIFORNIA, AND PERHAPS THE GREAT BASIN BY LATE IN THE PERIOD. THIS WAVE WILL STRUGGLE TO MAINTAIN ITSELF AS IT MOVES INTO THE BROAD ANTICYCLONIC MEAN FLOW, AND WHILE MODELS ARE IN GENERAL AGREEMENT ON THE TIMING, THEY DIFFER WITH RESPECT TO HOW QUICKLY THIS WAVE WEAKENS AS IT MOVES INLAND. GIVEN THE DECENT LARGE SCALE AGREEMENT, THE FORECAST WAS INITIALLY BASED HEAVILY (70%) ON A BLEND OF DETERMINISTIC GUIDANCE, INCLUDING THE GFS/ECMWF/UKMET. FROM DAY 5 (MON) ONWARD, ENSEMBLE WEIGHTING WAS GRADUALLY BOOSTED IN THE FORECAST, TO 70% GEFS/ECENS BY DAY 7. ...WEATHER HIGHLIGHTS... THE PERSISTENT RIDGE WILL RESULT IN CONTINUED ABOVE AVERAGE TEMPERATURES, ESPECIALLY FOR THE NORTHWESTERN STATES, WHERE HIGH TEMPS ARE EXPECTED TO REMAIN 5 TO 15 DEG F ABOVE AVERAGE THROUGH THE MEDIUM RANGE. MONSOONAL MOISTURE WILL KEEP SCATTERED CONVECTION A POSSIBILITY ACROSS THE FOUR CORNERS REGION EAST INTO THE CENTRAL/SOUTHERN ROCKIES, SOME OF WHICH COULD BE LOCALLY HEAVY. UNDER THE CORE OF THE RIDGE, HOWEVER, DRY CONDITIONS WILL PREVAIL FROM THE PAC NW TO THE NORTHERN GREAT BASIN. FARTHER EAST, PERSISTENT TROUGHING AND INCURSIONS OF COOL CANADIAN AIR MASSES WILL KEEP TEMPERATURE BELOW AVERAGE FROM THE MIDWEST TO THE NORTHEAST. A FRONTAL SYSTEM WILL BRING SHOWERS AND STORMS TO THE GREAT LAKES AND NORTHEAST EARLY IN THE WEEKEND, WITH THE POTENTIAL FOR WIDESPREAD CONVECTION TO RETURN TO THE MID-ATLANTIC/NORTHEAST EARLY IN THE NEXT WORK WEEK AS A WAVY FRONTAL SYSTEM LINGERS OVERHEAD. FROM THE SOUTHERN/CENTRAL PLAINS TO THE SOUTHEAST, A LINGERING FRONTAL BOUNDARY THROUGH MUCH OF THE PERIOD WILL RESULT IN FREQUENT CONVECTION, SOME OF WHICH COULD BE HEAVY, AND NEAR OR BELOW AVERAGE TEMPS. RYAN