EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1159 AM EDT TUE JUL 25 2017 VALID 12Z FRI JUL 28 2017 - 12Z TUE AUG 01 2017 ...PATTERN OVERVIEW/GUIDANCE EVALUATION/PREFERENCES... THE LATEST CYCLE OF GUIDANCE MAINTAINS GOOD CONTINUITY FOR THE OVERALL MEAN PATTERN BUT WITH TYPICAL EMBEDDED DETAIL UNCERTAINTIES. WHILE A CLOSED LOW/TROUGH PERSISTS OVER THE NERN PACIFIC, WRN U.S. RIDGING WILL STRENGTHEN DURING THE FIRST HALF OF THE PERIOD AND LIKELY PERSIST THROUGH THE FIRST HALF OF NEXT WEEK. DOWNSTREAM EXPECT MEAN TROUGHING OVER THE EAST, WITH A LEADING FEATURE/COMBINATION OF FEATURES LIFTING AWAY FROM THE EAST EARLY NEXT WEEK AS CANADIAN FLOW AMPLIFIES INTO THE GREAT LAKES AND VICINITY TOWARD MON-TUE. MORE DIFFUSE ENERGY AMPLIFYING OVER THE SOUTHEAST EARLY IN THE FCST SHOULD REMAIN OVER THE REGION THEREAFTER, AND POTENTIALLY BEGIN TO RETROGRADE BY MON-TUE IN RESPONSE TO A RIDGE BUILDING WWD OVER THE ATLANTIC. THE MOST SIGNIFICANT UNCERTAINTIES IN TERMS OF GUIDANCE SPREAD/CONTINUITY AND SENSIBLE WEATHER LIE WITHIN THE ERN U.S. MEAN TROUGH. THUS FAR GUIDANCE HAS BEEN HAVING DIFFICULTY IN RESOLVING THE EVOLUTION OF A FRI-SAT GRTLKS/MID ATLC SHRTWV (WHOSE SMALL SCALE LENDS ITSELF TO LOW PREDICTABILITY) AND ERN CANADA CLOSED LOW ALONG WITH POTENTIAL FOR THEIR INTERACTION, ALONG WITH ASSOC SFC REFLECTION. ENSEMBLE SPAGHETTI PLOTS STILL SHOW A FAIR AMOUNT OF SPREAD FOR BEING SO EARLY IN THE MEDIUM RANGE TIME FRAME. THE 00Z ECMWF TRENDED SOMEWHAT AWAY FROM ITS PRIOR TWO RUNS THAT SPUN UP A RATHER CONCENTRATED SFC SYSTEM IN RESPONSE TO PHASING OF THE TWO FEATURES BUT THE 00Z UKMET STILL HAS A STRONG SYSTEM SFC/ALOFT. IN CONTRAST TO THE ECMWF, LATEST GFS RUNS HAVE TRENDED STRONGER. AT THIS TIME PREFER TO MAINTAIN A CONSERVATIVE BLENDED MODEL/ENSEMBLE MEAN/CONTINUITY APPROACH, GIVEN BELOW AVG PREDICTABILITY AND OCCASIONAL TENDENCY FOR OPERATIONAL MODELS TO OVER-DEVELOP SFC SYSTEMS IN THE WARM SEASON DUE TO FEEDBACK AND/OR OTHER PROCESSES. HOWEVER THIS IS WITH THE UNDERSTANDING THAT THERE IS STILL POTENTIAL FOR DYNAMICS ALOFT TO EVOLVE IN SUCH A WAY AS TO SUPPORT STRONGER DEVELOPMENT. LATER IN THE PERIOD ENSEMBLES/INDIVIDUAL MODELS SHOW FURTHER INCREASE IN SPREAD AS UPSTREAM ENERGY BEINGS TO AMPLIFY INTO THE MEAN TROUGH POSN. BY THIS POINT IN TIME PREFER A GREATER EMPHASIS ON THE ENSEMBLE MEANS TO YIELD BETTER CONTINUITY. ACROSS THE PAC NW THE PRIMARY FCST ISSUE WILL BE THE SEWD EXTENT OF ENERGY ON THE LEADING SIDE OF THE NERN PAC TROUGH. DURING THE WEEKEND SOME ECMWF ENSEMBLE MEMBERS AND 00Z UKMET RESIDE IN THE SERN SIDE OF THE SPREAD FOR PARTICULAR HGT VALUES. TENDENCIES FOR ONE MODEL/ENSEMBLE SYSTEM BECOME LESS NOTICEABLE BY LATE IN THE PERIOD. MODELS/MEANS HAVE OSCILLATED OVER RECENT RUNS WITH NO WELL DEFINED TREND TOWARD HIGHER OR LOWER HGTS OVER THE NORTHWEST, FAVORING AN INTERMEDIATE SOLN. THE PREFERRED BLEND APPROACH BASED ON ABOVE CONSIDERATIONS INCORPORATED IDEAS FROM THE 00Z-06Z GFS/06Z GEFS MEAN AND 00Z ECMWF/ECMWF MEAN AND CONTINUITY. OPERATIONAL SOLNS WERE GIVEN SOMEWHAT MORE WEIGHT FOR DAYS 3-5 FRI-SUN BEFORE EMPHASIS TRANSITIONED MORE TO THE MEANS. ...WEATHER HIGHLIGHTS... PERSISTENT MONSOONAL MOISTURE SHOULD SUPPORT PERIODS OF LOCALLY HEAVY SHWRS/TSTMS OVER THE FOUR CORNERS STATES THROUGH THE PERIOD. POTENTIAL FOR SOME OF THIS ACTIVITY TO EXTEND INTO THE SRN HIGHS PLAINS MAY INCREASE BY THE FIRST HALF OF NEXT WEEK. ANY POSSIBLE CONNECTIONS WITH MOISTURE FROM ERN PACIFIC TROPICAL SYSTEMS WILL HAVE TO BE MONITORED. THE OTHER AREA WITH GREATEST HEAVY PRECIP POTENTIAL WILL EXTEND FROM THE EAST COAST TO THE GULF COAST WITH WHAT COULD BE A FAIRLY VIGOROUS NRN MID ATLC/NERN U.S. WAVE LATE THIS WEEK AND TRAILING FRONT SETTLING OVER THE SOUTHEAST/GULF COASTS. WITHIN THIS BROAD AREA, CURRENTLY EXPECT HIGHEST 5-DAY RAINFALL TOTALS TO BE NEAR THE SOUTHEAST COAST WHERE THE AFOREMENTIONED FRONT MAY SERVE AS A CONVECTIVE FOCUS FOR MULTIPLE DAYS AND PRECIPITABLE WATER VALUES SHOULD REMAIN MOST CONSISTENTLY HIGH. A FRONT DROPPING INTO THE NRN TIER BY MID-PERIOD MAY SUPPORT SOME SCATTERED CONVECTION AS IT CONTINUES SWD/SEWD. AWAY FROM MONSOONAL CONVECTION OVER THE FOUR CORNERS STATES, RIDGING ALOFT OVER THE WEST WILL SUPPORT ABOVE NORMAL TEMPS THROUGH THE PERIOD ACROSS THE INTERIOR WEST/NRN ROCKIES WITH SOME POCKETS OF PLUS 10-15F ANOMALIES FOR HIGHS OVER THE NRN ROCKIES/INLAND PAC NW AND AT TIMES OVER NRN CA. ON THE OTHER HAND EXPECT NEAR TO BELOW NORMAL TEMPS TO PREVAIL FROM THE PLAINS THROUGH THE EAST. MOST ANOMALIES SHOULD BE IN THE SINGLE DIGITS BUT SOME LOCALLY COOLER HIGH TEMPS ARE POSSIBLE WHERE CLOUDS/RAIN HAPPEN TO PERSIST FOR MOST OF THE DAYTIME HOURS ON ANY PARTICULAR DAY. RAUSCH