EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1200 PM EDT SAT AUG 12 2017 VALID 12Z TUE AUG 15 2017 - 12Z SAT AUG 19 2017 ...PATTERN OVERVIEW/GUIDANCE EVALUATION/PREFERENCES... TO COMMENCE THE PERIOD ON DAY 3/TUESDAY...AMPLIFIED FLOW ACROSS THE COUNTRY WILL BE IN PLACE WITH A PAIR OF UPPER TROUGHS ANCHORING NEW ENGLAND AND THE WESTERN U.S...RESPECTIVELY. WITHIN THE FORMER REGION...A COMPACT UPPER LOW IS FORECAST TO DESCEND FROM CENTRAL QUEBEC WITH HEIGHT FALLS POSSIBLY SKIRTING NORTHERN ME ON WEDNESDAY. THE 00Z GFS FORECAST 500-MB HEIGHT ANOMALIES PLACE THIS CLOSED LOW ANYWHERE FROM 2 TO 3 STANDARD DEVIATIONS BELOW AVERAGE. WHILE THE CORE OF THE SYSTEM SHOULD STAY NORTH OF THE U.S./CANADIAN BORDER...THE ATTENDANT SURFACE BOUNDARY WILL SWEEP THROUGH NEW ENGLAND...THUS REINFORCING THE TREND OF COOLER TEMPERATURES. LOOKING TO THE SOUTH...A WEAK SHORTWAVE IS FORECAST TO PUSH THROUGH THE MID-ATLANTIC ON TUESDAY WITH A SUBTLE WAVE OF LOW PRESSURE EXITING THE DELMARVA PENINSULA THAT MORNING. MODELS TEND TO KEEP THIS SYSTEM ON THE WEAKER SIDE WHICH IS IN CONTRAST TO SOLUTIONS A COUPLE OF DAYS AGO. LURKING FURTHER OFF THE COAST IS A POTENTIAL TROPICAL SYSTEM WHICH THE NATIONAL HURRICANE CENTER CURRENTLY HAS A 70 PERCENT CHANCE FOR DEVELOPMENT DURING THE NEXT 5 DAYS. THE 00Z CMC REMAINS THE MOST AGGRESSIVE AS IT HAS BEEN FOR DAYS WHILE THE 00Z UKMET DEPICTS ITS EXISTENCE...BUT IN A MORE DIMINISHED FASHION. ACROSS THE WESTERN U.S...AN ELONGATED LONGWAVE TROUGH WILL EXTEND OVER THE REGION WITH ENSEMBLE SPAGHETTI PLOTS NOTING SOME MODEL SPREAD EARLY ON. MOST NOTABLY...THE 00Z GEFS MEMBERS ARE MORE AMPLIFIED AND SLOWER TO EJECT THE TROUGH EASTWARD TOWARD THE GREAT PLAINS. THIS MAINLY IMPACTS THE SOUTHWESTERN FLANK OF THE LONGWAVE PATTERN AS MANY SOLUTIONS SHOW THE TROUGH BEING SLOWER TO PUSH DOWNSTREAM. THE EXCEPTION HAS BEEN THE PAST COUPLE OF RUNS OF THE ECMWF WHICH DEPICT A MORE SUBTLE FEATURE WHICH MOVES AHEAD OF THE PACK. ULTIMATELY THIS FLATTER SOLUTION LEADS TO WEAKER HEIGHT RISES ACROSS THE MS RIVER VALLEY WHILE THE 06Z/00Z GFS FAVORS MORE RIDGING DOWNSTREAM. THE AXIS OF LOWER HEIGHTS SHOULD EVENTUALLY REACH THE EASTERN THIRD OF THE U.S. BY THE END OF THE PERIOD. THERE HAS BEEN A NOTABLE TREND IN THE ENSEMBLE SPAGHETTI PLOTS TO BE MORE AMPLIFIED THAN PREVIOUSLY ADVERTISED. UNTIL THE 00Z CYCLE...PREVIOUS ENSEMBLE MEMBERS WERE ALL OVER THE PLACE WITH BEING ZONAL IN NATURE VS. MORE AMPLIFICATION. DIFFICULT TO SAY IF THIS TREND WILL HOLD UP DURING THE NEXT MODEL CYCLE. WHILE LOWER HEIGHTS ENTRENCH THE NORTHEASTERN U.S...THERE IS FAIRLY GOOD AGREEMENT ON A PERSISTENT LONGWAVE TROUGH SETTING UP ACROSS MUCH OF WESTERN NORTH AMERICA WITH HEIGHT FALLS EXTENDING INTO THE PACIFIC NORTHWEST. ENSEMBLE MEANS ARE MUCH FLATTER IN NATURE WITH THE LONGWAVE PATTERN BUT THERE ARE MANY OPERATIONAL SOLUTIONS FAVORING SOME FAIRLY IMPRESSIVE EMBEDDED SHORTWAVES PUSHING INTO WESTERN CANADA. GIVEN THE 00Z MODEL CYCLE MADE SOME MARKED CHANGES RELATIVE TO THE PREVIOUS RUNS...DID NOT WANT TO PUT COMPLETE STOCK IN ITS SOLUTION IN SPITE OF DECENT CLUSTERING OF THE ENSEMBLE MEMBERS. FAVORED KEEPING 20 PERCENT OF THE OVERNIGHT CONTINUITY WHILE GENERALLY SPLITTING THE DIFFERENCE BETWEEN THE 00Z GFS/GEFS MEAN AND 00Z ECMWF/ECMWF ENSEMBLE MEAN THROUGHOUT. DID INCORPORATE A TOKEN 10 PERCENT OF THE 00Z UKMET TO ACCOUNT FOR THE POTENTIAL TROPICAL CYCLONE OVER THE WESTERN ATLANTIC. REMOVED MUCH OF THE OPERATIONAL MODEL INFLUENCE BEYOND DAY 5/THURSDAY GIVEN GROWING UNCERTAINTIES IN THE SYNOPTIC-SCALE PATTERN. ...WEATHER HIGHLIGHTS... OVERALL...TEMPERATURES ACROSS THE NATION SHOULD RESIDE SOMEWHERE NEAR CLIMATOLOGY TO BELOW AVERAGE GIVEN THE LACK OF ANY STAGNANT/PERSISTENT UPPER RIDGE. FOR THE MOST PART...HIGHS IN THE 70S AND 80S SHOULD BE COMMONPLACE EAST OF THE ROCKIES. THE EXCEPTION BEING FROM THE SOUTHERN PLAINS EASTWARD ACROSS THE SOUTHERN U.S. AND INTO FL. HIGH TEMPERATURES IN THE LOW 90S ARE LIKELY EACH DAY WITH READINGS CLOSER TO THE 100 DEGREE MARK EXPECTED ACROSS SOUTH TX. FARTHER WEST...THE USUAL HOT SPOT WILL BE THE DESERT SOUTHWEST ALTHOUGH FORECAST TEMPERATURES SHOULD SIT VERY CLOSE TO CLIMATOLOGY...105 TO 110 DEGREES. THE MOST ACTIVE AREA FOR HEAVY RAINFALL SHOULD BE ACROSS THE NORTHERN PLAINS/UPPER MIDWEST...GENERALLY IN ADVANCE OF THE SLOW-MOVING UPPER TROUGH. THIS ACTIVITY WILL SPREAD EASTWARD INTO THE GREAT LAKES AND EVENTUALLY INTO NEW ENGLAND LATER IN THE FORECAST. MODELS SEEM TO MAINTAIN THE HIGHEST FORECAST AMOUNTS ACROSS THE NORTH-CENTRAL U.S. WITH POCKETS OF 24-HOUR TOTALS IN THE 2 TO 3 INCH RANGE. ELSEWHERE...FRONTAL CONVECTION IS LIKELY FROM THE MID-SOUTH TO THE SOUTHEASTERN U.S. WHICH WILL BE ENHANCED BY THE DIURNAL HEATING CYCLE. ACROSS THE FOUR CORNERS REGION...CONDITIONS SHOULD BE MUCH DRIER THAN THEY HAVE BEEN WITH PERHAPS SOME CONVECTION ACROSS THE SOUTHERN ROCKIES. RUBIN-OSTER