EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1158 AM EDT SUN OCT 08 2017 VALID 12Z WED OCT 11 2017 - 12Z SUN OCT 15 2017 ...OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES... MULTI-DAY MEANS ARE FAIRLY SIMILAR AND STABLE IN DEPICTING AN OPEN WESTERN U.S. TROUGH BETWEEN RIDGES OVER THE SOUTHEAST U.S./SOUTHERN PLAINS AND CENTRAL PACIFIC. TELECONNECTIONS RELATIVE TO THE POSITIVE HEIGHT ANOMALY CENTERS ASSOCIATED WITH EACH RIDGE YIELD FAIRLY CONSISTENT RESULTS IN TERMS OF AREAS EXPECTED TO HAVE ABOVE/BELOW NORMAL HEIGHTS SO CONFIDENCE IN THE LARGE SCALE FORECAST REMAINS ABOVE AVERAGE. AS FOR EMBEDDED DETAILS, GUIDANCE CLUSTERING HAS IMPROVED INTO DAY 6 SAT BUT THEN INDIVIDUAL MODELS/ENSEMBLES RAPIDLY DIVERGE THEREAFTER. OVER THE PAST DAY MOST GUIDANCE HAS GRAVITATED TO ECMWF/ECMWF MEAN RUNS THAT HAD DEPICTED A STRONGER MID LEVEL FEATURE HEADING INTO THE GREAT LAKES AS OF DAY 3 WED, CORRESPONDING TO A FARTHER NORTH SURFACE WAVE. THIS FEATURE SHOULD STEADILY WEAKEN AFTER WED AS THE SHORTWAVE SHEARS OUT AND THE SURFACE WAVE APPROACHES STRONG NORTHEASTERN HIGH PRESSURE. BEHIND THIS SYSTEM THERE IS ALSO IMPROVED CLUSTERING FOR LEADING WESTERN TROUGH ENERGY THAT SHOULD EJECT INTO CANADA AND SUPPORT STRENGTHENING CANADIAN LOW PRESSURE THAT WILL BRING A COLD FRONT ACROSS THE PLAINS AND THEN LIKELY INTO THE GREAT LAKES/NEW ENGLAND. THE LATEST MODEL/ENSEMBLE MEAN CONSENSUS IS SIMILAR TO OR JUST A FRACTION SLOWER THAN PREVIOUS FORECAST. GUIDANCE AGREES THAT THIS FRONT WILL STALL ACROSS THE PLAINS-GREAT LAKES REGION AS FLOW ALOFT BECOMES PARALLEL TO THE FRONT AND TROUGHING RELOADS OVER THE WEST. THERE ARE STILL SOME DETAIL DIFFERENCES REGARDING THE WESTERN TROUGH IN THE FRI-SAT TIME FRAME WITH 00Z/06Z GFS RUNS SHOWING GREATER SOUTHWARD ELONGATION OF ENERGY VERSUS THE PAST COUPLE ECMWF RUNS. ENSEMBLE SPREAD IS GREATEST OVER THE WEST COAST STATES/GREAT BASIN DURING THIS PERIOD SO MAINTAINING AN INTERMEDIATE SOLUTION AMONG THE GFS/ECMWF AND THEIR MEANS APPEARS MOST REASONABLE AT THIS TIME. AFTER DAY 6 SAT GUIDANCE BECOMES INCREASINGLY DIVERGENT WITH RESPECT TO EJECTION OF THIS ENERGY, DUE TO A COMBINATION OF THE FRI-SAT DETAIL DIFFERENCES AND SPREAD FOR DETAILS OF UPSTREAM PACIFIC FLOW THAT BECOMES INCREASINGLY STRONG WITH TIME. FOR DAY 7 SUN THIS LEADS TO A BROAD ENVELOPE FOR A POTENTIAL SURFACE SYSTEM BETWEEN THE CENTRAL PLAINS AND SOUTHERN ONTARIO/QUEBEC. GFS RUNS ARE ON THE WESTERN SIDE OF THIS SPREAD AND 00Z ECMWF/ECMWF MEAN ARE MORE TOWARD THE EASTERN SIDE. MULTI-RUN AVERAGES OF THE MODELS/MEANS WOULD SUGGEST A MOST LIKELY SURFACE LOW POSITION APPROACHING THE WESTERN GREAT LAKES BY EARLY SUN. A BLEND OF THE FASTER 00Z ECMWF/ECMWF MEAN AND SLOWER 00Z GEFS MEAN/OLD 12Z ECMWF PROVIDES THE BEST TEMPLATE FOR THIS IDEA. A BLEND OF THE 00Z-06Z GFS/00Z ECMWF AND 00Z GEFS/ECMWF MEANS REPRESENTED A CONSENSUS/INTERMEDIATE SOLUTION FOR DAYS 3-6 WED-SAT. THIS BLEND EMPHASIZED THE OPERATIONAL RUNS BY 80 PCT WED-THU AND 60 PCT FRI-SAT. THE FORECAST TRANSITIONED TO THE AFOREMENTIONED BLEND OF THE PAST TWO ECMWF RUNS AND 00Z GEFS/ECMWF MEANS FOR DAY 7 SUN TO INCORPORATE PREFERENCES FOR THE POTENTIAL PLAINS/GREAT LAKES SYSTEM. ...WEATHER HIGHLIGHTS/THREATS... THE FEATURE OVER THE OH VALLEY/GREAT LAKES AND FRONT PUSHING INTO THE SOUTHEAST WILL SUPPORT RAINFALL/CONVECTION EARLY IN THE PERIOD. SOME POCKETS OF LOCALLY HEAVY RAINFALL ARE POSSIBLE BUT DO NOT ANTICIPATE WIDESPREAD HEAVY ACTIVITY. THE FLORIDA PENINSULA SHOULD SEE PERIODS OF RAIN IN EASTERLY FLOW. A SURFACE/UPPER LEVEL WAVE APPROACHING FROM THE BAHAMAS LATE WEEK/WEEKEND MAY ENHANCE RAINFALL OVER/NEAR FLORIDA. THROUGH FRI THE COLD FRONT CROSSING THE NORTHERN U.S. SHOULD CARRY LIGHT IF ANY PRECIP. HOWEVER STRENGTHENING MOISTURE RETURN AS THE FRONT STALLS, FOLLOWED BY POSSIBLE SURFACE DEVELOPMENT LATE IN THE PERIOD, SUGGEST INCREASING POTENTIAL FOR SOME LOCALLY HEAVY RAINFALL WITHIN AN AREA FROM THE NORTH-CENTRAL PLAINS THROUGH THE GREAT LAKES BY NEXT WEEKEND. NORTHWESTERN STATES WILL SEE PERIODS OF MOSTLY LIGHT-MODERATE RAIN AND HIGH ELEVATION SNOW DURING THE PERIOD UNDER MEAN TROUGHING ALOFT THAT WILL CONTAIN MULTIPLE SHORTWAVES. EXPECT HIGHEST PRECIP TOTALS ALONG FAVORED TERRAIN. THE PERSISTENT LARGE SCALE PATTERN WILL SUPPORT VERY COOL TEMPS OVER THE WEST INTO NORTHERN PLAINS WITH MINUS 10-20F ANOMALIES FOR HIGHS MOST COMMON FROM THE GREAT BASIN/INTERIOR NORTHWEST INTO NORTHERN HIGH PLAINS. ON THE OTHER HAND THE SOUTH AND EAST WILL BE WELL ABOVE AVERAGE FOR TEMPS, ESPECIALLY DAYS 5-7 FRI-SUN. EXPECT BROAD COVERAGE OF MAX TEMPS UP TO 10-20F ABOVE NORMAL WITH PERHAPS EVEN GREATER EXTENT OF SUCH ANOMALIES FOR MINS. RAUSCH