EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1159 AM EDT THU SEP 15 2016 VALID 12Z SUN SEP 18 2016 - 12Z THU SEP 22 2016 ...PATTERN OVERVIEW/UNCERTAINTY ASSESSMENT... MODEL GUIDANCE IS IN GOOD AGREEMENT WITH KEEPING FAST ZONAL FLOW OVER THE NORTHERN TIER OF THE NATION THROUGH THE WEEKEND AND INTO EARLY NEXT WEEK...BUT THEN MODEL SOLUTIONS BEGIN TO DIVERGE TOWARDS THE MIDDLE OF NEXT WEEK AS AN UPPER LOW DROPPING THROUGH THE GULF OF ALASKA PROGRESSES INLAND OVER THE NORTHWEST AND A DOWNSTREAM RIDGE BUILDS IN OVER THE MIDWEST...OHIO VALLEY...AND MID-ATLANTIC STATES. SLIGHTLY MORE WEIGHT WAS GIVEN TO A MORE AMPLIFIED GFS/GEFS SOLUTION RATHER THAN A FLATTER ECMWF/ECMWF ENSEMBLE MEAN SOLUTION BY THE LATER HALF OF THE FORECAST PERIOD SINCE THE MORE AMPLIFIED SOLUTION HAD SUPPORT FROM THE LATEST NAEFS AND CMC ENSEMBLE MEANS...CONTINUITY...AND A GENERAL TREND IN THE GUIDANCE TOWARDS A MORE AMPLIFIED PATTERN BY THE MIDDLE OF NEXT WEEK. ANOTHER AREA OF FORECAST UNCERTAINTY IS WITH AN AREA OF LOW PRESSURE SOUTH OF MEXICO CURRENTLY BEING MONITORED BY THE NATIONAL HURRICANE CENTER FOR CYCLONE FORMATION. THE ECMWF/UKMET CONTINUE TO ADVERTISE THE POTENTIAL FOR A SYSTEM TO TRACK ACROSS BAJA CALIFORNIA AND INTO THE SOUTHWEST EARLY NEXT WEEK...BUT THESE SOLUTIONS HAVE LITTLE SUPPORT FROM OTHER DETERMINISTIC GUIDANCE AND THERE IS STILL A LOT OF SPREAD AMONG INDIVIDUAL ECMWF ENSEMBLE MEMBERS. PLEASE REFER TO THE NATIONAL HURRICANE CENTER (WWW.NHC.NOAA.GOV) FOR THE LATEST INFORMATION SINCE THERE COULD BE BIG IMPACTS TO THE SOUTHWEST IF SOMETHING SIMILAR TO THE ECMWF/UKMET WERE TO VERIFY. ...WEATHER PATTERN HIGHLIGHTS/THREATS... SHOWERS AND THUNDERSTORMS WILL BE POSSIBLE WITH A COLD FRONT PRESSING THROUGH THE OHIO VALLEY...NORTHEAST..AND NORTHERN MID-ATLANTIC STATES SUNDAY INTO MONDAY. SCATTERED CONVECTION WILL ALSO BE POSSIBLE ALONG THE TAIL END OF THE BOUNDARY WHILE IT STALLS ACROSS THE SOUTHERN TIER OF THE NATION EARLY NEXT...BUT THEN THE CHANCE FOR PRECIPITATION SHOULD DIMINISH AS THE BOUNDARY WEAKENS AND A RIDGE BUILDS IN ALOFT. MOIST PACIFIC FLOW INTO THE TERRAIN SHOULD LEAD TO PERIODS OF HEAVY PRECIPITATION ALONG THE FAVORED SLOPES OF THE PACIFIC NORTHWEST AND NORTHERN ROCKIES SUNDAY AND MONDAY. A HEAVY RAIN EVENT WILL ALSO BE POSSIBLE ACROSS THE SOUTHWEST EARLY-MID NEXT WEEK. GERHARDT