EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1101 AM EDT TUE MAY 09 2017 VALID 12Z FRI MAY 12 2017 - 12Z TUE MAY 16 2017 ...OVERVIEW AND GUIDANCE EVALUATION... UPPER PATTERN WILL EVOLVE INTO A BROAD AND RELOADING OMEGA BLOCK ACROSS THE NORTHERN TIER OF THE LOWER 48, ANCHORED WITH CLOSED LOWS ALOFT IN THE NW AND NE WITH RIDGING THROUGH THE HIGH PLAINS. THE MODELS/ENSEMBLES REMAIN IN DECENT AGREEMENT ON THE LARGE SCALE PATTERN BUT DETAIL DIFFERENCES PERSIST FROM MODEL TO MODEL AND BETWEEN MODELS AND THEIR ENSEMBLE MEANS ACROSS THE GFS/GEFS/ECMWF/ECMWF ENSEMBLES AND CANADIAN GLOBAL/CANADIAN GLOBAL ENSEMBLES. A BLEND AMONG THE 00Z GFS/06Z GEFS AND 00Z ECMWF /ECMWF ENSEMBLE MEAN WAS USED FOR THE FORECAST ACROSS THE COUNTRY...WITH MORE WEIGHTING ON THE ENSEMBLE MEANS DAYS 6-7(MAY 15-16). THE INITIAL CLOSED LOW ACROSS THE LAKES IS FORECAST TO MERGE WITH A SOUTHERN STREAM WAVE WITH THE MODELS FORECASTING LOW PRESSURE IN THE OH/TN VALLEYS 12Z FRI TO MOVE EAST WITH COASTAL LOW DEVELOPMENT OVER EASTERN NC/VA BY 12Z SAT AND THE LOW MOVING OFF THE COAST THEREAFTER. DIFFERENCES GROW OVER THE WEEKEND AS THE 00-06Z GFS INDICATE A DEEPER CYCLONE AND THE 00Z ECMWF SHOWS A DEEPENING OCCLUSION THAT LINGERS OVER EASTERN NEW ENGLAND INTO EARLY NEXT WEEK. THE 00Z ECMWF LOOKS LIKE A LOW PROBABILITY SOLUTION AND GIVEN ITS RADICAL RUN TO RUN CHANGES FROM A FASTER/PROGRESSIVE TROUGH TO THE SLOWEST...MUCH MORE WEIGHTING WAS GIVEN TO THE 00Z ECMWF ENSEMBLE MEAN/06Z GEFS MEAN...WHICH CLUSTER WELL. IN THE WEST, THE LEAD CLOSED LOW ALOFT SYSTEM WILL DRIFT EAST AND ONSHORE INTO WA SAT 13 MAY BEFORE WEAKENING SUNDAY 13 MAY. A CIRCULATION INITIALLY NEAR KODIAK ISLAND DIVES SOUTHEAST OFF THE WEST COAST OF CANADA AND VANCOUVER ISLAND SUNDAY AND POSSIBLY INTO WA/OR LATER MON INTO TUE 15 MAY. THE ECMWF AND GFS HAVE ALTERED HOW FAR SOUTH EACH RUN DEVELOPS THIS LOW...BUT ALL RUNS HAVE THE LOW NOW. LIKEWISE...THE OPERATIONAL RUNS OF THE ECMWF/GFS DIG THE LOW FURTHER SOUTH THAN THEIR RESPECTIVE ENSEMBLE MEANS...WITH A CONSENSUS BASED APPROACH TAKEN TO MITIGATE DIFFERENCES. THE HIGH AMPLITUDE UPSTREAM RIDGE IN THE EASTERN PACIFIC WITH POSITIVE ANOMALIES CENTERED ON 150W FAVORS THE PERSISTENCE OF THE WESTERN US TROUGH INTO THE MIDDLE OF NEXT WEEK. ...WEATHER HIGHLIGHTS... PRECIPITATION WILL FOCUS ALONG THE WAVY FRONT AND DEVELOPING SFC LOW FROM THE SOUTHEAST/CAROLINAS AND THEN NORTHEASTWARD ACROSS THE MID ATLANTIC INTO SOUTHERN NEW ENGLAND LATER THIS WEEK INTO THE WEEKEND. PRECIPITATION ALSO FOCUSES ALONG THE FRONT IN THE PACIFIC NORTHWEST (COASTAL RANGE AND CASCADES) INTO MONTANA/IDAHO OVER THE WEEKEND AND THEN AGAIN NEXT WEEK AS THE REINFORCING UPPER TROUGH DIGS IN. SNOW IS EXPECTED IN THE HIGHER PEAKS OF THE RANGES OF THE PACIFIC NORTHWEST AND NORTHERN ROCKIES. ABOVE AVERAGE TEMPERATURES WILL PRECEDE THE FRONT IN MT/WY/CO WITH COOLER THAN AVERAGE TEMPERATURES BEHIND THE FRONT FURTHER WEST ACROSS THE NORTHERN ROCKIES AND PACIFIC NORTHWEST/CALIFORNIA AND THE CENTRAL TO NORTHERN GREAT BASIN. GENERALLY COOLER THAN AVERAGE TEMPERATURES WILL PERSIST IN THE NORTHEAST BUT WITH A SLOW CLIMB BACK TOWARDS TYPICAL LEVELS THE MIDDLE OF NEXT WEEK. PETERSEN