EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1159 AM EDT SAT OCT 07 2017 VALID 12Z TUE OCT 10 2017 - 12Z SAT OCT 14 2017 ...OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES... MODELS/ENSEMBLES TO SHOW BETTER THAN AVERAGE AGREEMENT/CONTINUITY REGARDING THE MEAN PATTERN, AS A STRONG RIDGE SETTLES OVER THE SOUTHEASTERN HALF OF THE LOWER 48 AND TROUGHING SETTLES OVER OR JUST INLAND FROM THE WEST COAST. THERE IS STILL SOME SPREAD AND OPPOSING TRENDS FOR A LEADING SHORTWAVE REACHING WESTERN NORTH AMERICA EARLY IN THE PERIOD AND EVENTUALLY EJECTING INTO CANADA. MEANWHILE GUIDANCE HAS CONVERGED SOMEWHAT REGARDING TRAILING ENERGY THAT RELOADS THE WESTERN TROUGH LATE IN THE WEEK. AHEAD OF THESE FEATURES, SOLUTIONS ARE MAKING GRADUAL PROGRESS IN RESOLVING THE SHORTWAVE TRACKING OUT OF THE CENTRAL PLAINS TUE ONWARD. OVER THE PAST COUPLE CYCLES MODELS HAVE BEEN SHOWING IMPROVED TIMING AGREEMENT AND CONSISTENCY FOR THE SHORTWAVE TRACKING OUT OF THE CENTRAL PLAINS AND ASSOCIATED MS VALLEY FRONTAL WAVE. HOWEVER BY DAY 4 WED THERE IS STILL MEANINGFUL SPREAD FOR THE STRENGTH/TRACK OF THE SURFACE WAVE RELATED TO SUBTLE STRENGTH/DETAIL DIFFERENCES ALOFT, WITH THE PAST COUPLE ECMWF/ECMWF MEAN RUNS ON THE NORTHERN SIDE OF THE SURFACE SPREAD DUE TO A STRONGER SHORTWAVE ALOFT. SOME 00Z GEFS/CMC ENSEMBLE MEMBERS SHOW POTENTIAL FOR A FARTHER NORTH SURFACE WAVE THAN THEIR OPERATIONAL RUNS/00Z UKMET AND SUCH SMALL SCALE DETAILS TEND TO HAVE LOW PREDICTABILITY THAT FAR OUT IN TIME SO AT LEAST AN INTERMEDIATE SOLUTION SEEMS REASONABLE. LEADING TROUGH ENERGY HEADING INTO THE WEST BY DAY 4 WED AND THEN EJECTING NORTHEASTWARD SHOWS A LOT OF TIMING SPREAD AMONG ENSEMBLE MEMBERS WITH THE RESULTING ENSEMBLE MEANS AND OPERATIONAL RUNS SHOWING SOME MIXED TRENDS. GFS RUNS HAD PREVIOUSLY BEEN EARLY ADVOCATES FOR FASTER EJECTION OF THE SHORTWAVE WITH OTHER GUIDANCE (ASIDE FROM THE CONSISTENTLY SLOW CMC) TENDING TO CATCH UP. HOWEVER IN THIS PAST CYCLE MOST MODEL RUNS HAVE ADJUSTED SLOWER WITH THE GFS RUNS EVEN SLOWER THAN THAN THE ECMWF. ON THE OTHER HAND ENSEMBLE MEANS HAVE TRENDED FASTER/BETTER DEFINED WITH THE SHORTWV, LEADING TO A MID-LATE PERIOD NORTHERN TIER SURFACE FRONT POSITION CLOSER TO WHAT MODELS WERE SHOWING YESTERDAY. GIVEN THE FAIRLY GOOD SIGNAL THAT UPSTREAM ENERGY WILL FEED INTO THE MEAN TROUGH, WOULD EXPECT THE LEADING SHORTWAVE TO PROGRESS AT A RATE THAT IS SOMEWHAT FASTER THAN THE SLOW END OF THE SPREAD. BEYOND THAT PREFER TO COMPROMISE BETWEEN LATEST GUIDANCE AND CONTINUITY TO ACCOUNT FOR POTENTIAL VARIABILITY IN FUTURE GUIDANCE RUNS. ADJUSTMENTS IN GUIDANCE OVER THE PAST DAY FOR THE WESTERN TROUGH LATE IN THE PERIOD HAVE YIELDED REMARKABLY GOOD AGREEMENT FOR THE TIME BEING. THE GFS HAS TONED DOWN ITS DEEP/CLOSED DEPICTION OF 24 HOURS AGO WHILE THE ECMWF/ECMWF MEAN HAVE TRENDED SOMEWHAT DEEPER/MORE AMPLIFIED. THE GEFS MEAN HAS BEEN FAIRLY STABLE WITH THE DEPTH OF THE TROUGH CORE BUT HAS NUDGED THE OVERALL TROUGH A LITTLE EASTWARD AND TRIMMED THE AMPLITUDE A BIT. THE COMBINATION OF GUIDANCE SPREAD FOR SOME FEATURES AND/OR DOUBTS OVER THE SUSTAINABILITY OF LATEST CYCLE TRENDS LEAD TO INCORPORATING 40 PCT WPC CONTINUITY FOR MOST OF THE PERIOD WITH THE REST OF THE INPUT COMING FROM VARIOUS IDEAS OF THE 00Z-06Z GFS/06Z GEFS MEAN AND 00Z ECMWF/ECMWF MEAN. THIS BLEND ENDED UP CLOSER TO THE ENSEMBLE MEANS AND 00Z ECMWF ACROSS THE NORTHERN TIER MID-LATE PERIOD AND PROVIDED A GOOD INTERMEDIATE SOLUTION FOR THE EAST-CENTRAL U.S. WAVE AROUND MIDWEEK. ...WEATHER HIGHLIGHTS/THREATS... FROM TUE ONWARD PARTS OF THE EASTERN HALF OF THE LOWER 48 WILL SEE AREAS OF LOCALLY ENHANCED PRECIP ALONG A WAVY FRONT PUSHING INTO THE SOUTHEAST BY THU. THE SHORTWAVE ALOFT TRACKING OUT OF THE CENTRAL PLAINS SHOULD PROMOTE AN AREA OF FOCUSED PRECIP OVER/NEAR THE MID MS VALLEY NEAR A MORE PRONOUNCED FRONTAL WAVE AROUND TUE BUT IT IS VERY UNCERTAIN HOW MUCH OF THIS MOISTURE REACHES THE GREAT LAKES/OH VALLEY BY TUE NIGHT-WED. EASTERLY FLOW OVER THE FLORIDA PENINSULA MAY PROMOTE PERIODS OF RAIN AND LATE IN THE PERIOD A WEAK FEATURE SURFACE/ALOFT MAY HELP TO ENHANCE ACTIVITY SOMEWHAT. EXPECT INCREASING COVERAGE OF RAIN AND HIGHER ELEVATION SNOW OVER THE NORTHWESTERN STATES AS MEAN TROUGHING ALOFT BECOMES ESTABLISHED. MOST ACTIVITY SHOULD BE LIGHT TO MODERATE GIVEN THE ABSENCE OF A LOWER LATITUDE MOISTURE CONNECTION, WITH BEST FOCUS OVER COASTAL LOCATIONS AND CASCADES/NORTHERN ROCKIES. THE FRONT CROSSING THE NORTHERN TIER WED ONWARD WILL LIKELY BE ACCOMPANIED BY FAIRLY LIGHT RAIN IF ANY FOR A WHILE, BUT IT MAY BECOME A FOCUS FOR HEAVIER RAINFALL BY FRI-SAT AS IT STALLS OVER THE NORTH-CENTRAL PLAINS/UPPER MIDWEST/GREAT LAKES. THE MEAN TROUGH ALOFT SETTLING OVER THE WEST WILL GRADUALLY EXPAND COVERAGE OF BELOW NORMAL TEMPS OVER THE WEST AND EXTENDING INTO THE NORTHERN PLAINS. SOME LOCATIONS SHOULD SEE MULTIPLE DAYS WITH HIGHS 10-20F BELOW NORMAL FROM WED ONWARD. CHILLY AIR OVER THE CENTRAL U.S. ON TUE WILL MODERATE CONSIDERABLY AS IT MOVES EASTWARD, SO THE WAVY FRONT CROSSING THE EAST WILL BRING A COOLING TREND AFTER A VERY WARM START TO THE PERIOD BUT ONLY TO THE EXTENT OF NEAR TO MODERATELY ABOVE NORMAL TEMPS. AS THE SOUTHEAST RIDGE/WESTERN TROUGH MEAN PATTERN BECOMES ESTABLISHED, EXPECT MUCH OF THE EASTERN U.S. AND SOUTHERN HALF OF THE PLAINS TO SEE ABOVE TO WELL ABOVE NORMAL TEMPS BY LATE WEEK/WEEKEND. RAUSCH