EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1222 PM EDT WED AUG 09 2017 VALID 12Z SAT AUG 12 2017 - 12Z WED AUG 16 2017 16Z UPDATE... NO MAJOR CHANGES WERE MADE TO THE UPDATED WPC DAY 3-7 FORECAST. MODEL GUIDANCE CONTINUES TO SHOW A WIDE RANGE OF SOLUTIONS WITH THE DETAILS OF A PATTERN CHANGE EXPECTED OVER THE NORTHWESTERN U.S. EARLY NEXT WEEK, WHERE A ONCE PERSISTENT UPPER RIDGE OVER THE REGION WILL SHIFT EASTWARD AND GET REPLACED BY ANOMALOUS TROUGHING. THE ECMWF, WHICH HAD BEEN CONSISTENT WITH ADVERTISING A SLOWER/MORE AMPLIFIED UPPER TROUGH OVER THE NORTHWEST, SHIFTED TOWARDS THE MORE PROGRESSIVE GFS SOLUTION IN ITS 00Z RUN. HOWEVER, THERE CONTINUED TO BE QUITE A FEW ECWMF ENSEMBLE MEMBERS THAT SUPPORTED THE POTENTIAL FOR A DEEP LOW TO CLOSE OFF OVER THE NORTHWEST NEXT MONDAY INTO TUESDAY IN THE 00Z SUITE OF ENSEMBLE DATA. UNTIL DETERMINISTIC GUIDANCE COMES INTO BETTER AGREEMENT WITH THIS FEATURE, THE WPC FORECAST STAYED CLOSE TO THE AGREEABLE GEFS/ECWMF ENSEMBLE MEANS, WHICH MATCHED WELL WITH CONTINUITY. IN REGARDS TO THE AREA OF DISTURBED WEATHER EAST OF THE LEEWARD ISLANDS, THERE IS STILL MODEL GUIDANCE THAT SUGGESTS THE POSSIBILITY FOR DEVELOPMENT AS THE SYSTEM TRACKS WEST/NORTH-WESTWARD THROUGH THE WESTERN ATLANTIC AND TOWARDS THE SOUTHEAST COAST THIS WEEKEND (THE 00Z ECMWF AND ESPECIALLY 00Z CMC), BUT GEFS MEMBERS HAVE YET TO SHOW A STRONG SIGNAL FOR DEVELOPMENT. PLEASE CONTINUE TO REFER TO THE NATIONAL HURRICANE CENTER FOR THE LATEST INFORMATION ON THIS SYSTEM. GERHARDT ...PATTERN OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES... THE MEAN UPPER-LEVEL PATTERN WILL BE IN TRANSITION DURING THE MEDIUM RANGE PERIOD. A CLOSED LOW/NEG ANOMALY OVER THE NERN PAC WILL DIG AND BECOME AN AMPLIFIED UPPER TROUGH OVER THE WEST, UPPER RIDGE EXTENDING FROM THE INTERIOR NW/NORTHERN ROCKIES TO THE CANADIAN NORTHWEST TERRITORIES WILL PROGRESS DOWNSTREAM TO BE BASED OVER CENTRAL CANADA UP TO GREENLAND AND A BROAD UPPER TROUGH WITH MULTIPLE SHORT WAVES EVENTUALLY BECOMES AN AMPLIFIED UPPER TROUGH OVER THE EAST WITH SLIGHT HEIGHT RISES OVER THE MIDWEST. THOUGH THE MEANS DO SHOW MORE SPREAD AND UNCERTAINTY THROUGHOUT THE FCST PERIOD THAN THE PAST SEVERAL NIGHTS BUT OFFER A SOLID MIDDLE OF THE ROAD SOLUTION. THE GREATEST UNCERTAINTY IS THE DIGGING UPPER TROUGH INTO THE WEST. THE 12Z/08 ECMWF REMAINS STEADY WITH THE IDEA OF AN EXTREMELY DEEP 500MB LOW CLOSING OFF OVER THE PAC NW. THE 12Z/08 CMC STILL CLOSES OFF A LOW BUT TRANSITIONS THE FEATURE QUICKER DOWNSTREAM TO THE NORTHERN PLAINS. THE 18Z/08 GFS HAS BACKED OFF SOME COMPARED TO ITS 12Z/08 VERSION, IN STILL CARRYING OVERALL A PROGRESSIVE UPPER TROUGH BUT SLOWING ITS TRANSITION TO JUST THE NORTHERN PLAINS. A SECONDARY CONCERN WITH UNCERTAINTY IS WITH A TROPICAL DISTURBANCE EAST OF THE LEEWARD ISLANDS. A LARGE DOME OF UPPER HIGH PRESSURE OVER THE ATLANTIC SHOULD STEER THE TROPICAL WAVE WITH AN AREA OF LOW PRESSURE WNW OVER TIME. THE OPERATIONAL ECMWF AND ECMWF ENSEMBLES REMAIN EXTREMELY CONSISTENT IN SHOWING THIS TROPICAL ENTITY DEVELOPING, WHILE SLIDING OFF TO THE NORTH AND EAST OF THE BAHAMAS. THE OPERATIONAL CMC HAS A SIMILAR SCENARIO. THE LAST FEW OPERATIONAL RUNS OF THE GFS AND ALL ITS ENSEMBLES CAPTURE MID-LEVEL VORTICITY SLIDING OFF TO THE WNW BUT LITTLE IN THE WAY OF SURFACE DEVELOPMENT. THIS SYSTEM DEFINITELY BEARS WATCHING THIS WEEKEND INTO EARLY NEXT WEEK, ESPECIALLY THE SERN STATES. PLEASE REFER TO THE NHC ON ALL CONCERNING THIS SYSTEM. WPC STARTED OFF WITH ABOUT 60 TO 80 PERCENT OF THE BLEND BEING OPERATIONAL GUIDANCE THROUGH D4/SUN. THEN FROM D5/MON THROUGH D7/WED, THE BLEND TRANSITIONS TO MOSTLY MEANS (50 TO 80 PERCENT) TO FILTER OUT THE DETAILS AND ATTEMPT TO CAPTURE THE MEAN PATTERN. ...WEATHER HIGHLIGHTS... THE WPC 3-7 DAY MEAN MAX TEMP ANOMALY HIGHLIGHTS BELOW AVG HIGHS FOR A LARGE PORTION OF THE COUNTRY. THE LARGEST POCKET OF NEG ANOMALIES WITH HIGHS AOA 10 DEG COOLER THAN AVG REMAINS THE DAKOTAS TO PARTS OF KS/MO/OK/AR IN A POST-FRONTAL COLD ADVECTION REGIME. THE HEAVIEST RAIN CORRIDOR OVER THE FIVE DAYS APPEARS LIKELY FROM THE SOUTHEAST/MID-ATL THROUGH THE SOUTH INTO MOST OF THE PLAINS AND FRONT RANGE OF THE CENTRAL/SOUTHERN ROCKIES, WHERE MOISTURE CONTENT AND INSTABILITY SHOULD BE ELEVATED. DEPENDING ON OUTCOME OF THE TROUGH OVER THE WEST, PRECIPITATION COULD RETURN FOR THE NW INTO THE NORTHERN ROCKIES IF THE MOST AMPLIFIED AND SLOWER FORECAST SCENARIO COMES TRUE. MUSHER