EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 151 PM EST WED JAN 04 2017 VALID 12Z SAT JAN 07 2017 - 12Z WED JAN 11 2017 ...ANOTHER HEAVY RAINFALL EVENT EXPECTED TO IMPACT LOCATIONS ACROSS NORTHERN AND CENTRAL CALIFORNIA LATE THIS WEEKEND INTO EARLY NEXT WEEK... PATTERN OVERVIEW AND MODEL ASSESSMENT/PREFERENCES... MODELS ARE IN GOOD AGREEMENT WITH THE LARGE SCALE FLOW PATTERN OVER THE LOWER 48 THIS WEEKEND...WHICH WILL FEATURE A DEEP VORTEX IN THE NORTHEAST PACIFIC SLOWLY DROPPING SOUTHWARD TOWARDS NORTHWEST COAST...DOWNSTREAM UPPER RIDGING BUILDING IN FROM THE SOUTHERN ROCKIES/PLAINS TO THE UPPER MIDWEST...AND A LONGWAVE TROUGH SETTING UP OVER THE EASTERN U.S.. ENSEMBLE SPREAD AND FORECAST UNCERTAINTY BEGINS TO INCREASE BY EARLY NEXT WEEK AS THE LEADING EDGE OF HEIGHT FALLS FROM THE DEEP VORTEX OFF THE NORTHWEST COAST PROGRESS INLAND OVER THE ROCKIES AND GREAT PLAINS...AND A SECOND VORTEX POTENTIALLY DEEPENS IN THE NORTHEAST PACIFIC BY THE MIDDLE OF NEXT WEEK. ALSO...THERE REMAINS SOME SMALL...BUT IMPORTANT...MODEL SPREAD WITH CYCLOGENESIS OFF THE SOUTHEAST COAST EARLY SATURDAY...AND SOME UNCERTAINTY WITH THE DETAILS OF INDIVIDUAL PIECES OF SHORTWAVE ENERGY ROTATING AROUND THE DEEP VORTEX OFF THE NORTHWEST COAST. THE WPC DAY 3-7 FORECAST RELIED HEAVILY ON THE 00Z ECMWF ENSEMBLE MEAN...WHICH OFFERED A GOOD COMPROMISE BETWEEN A WEAKER/FARTHER EAST GFS AND STRONGER/CLOSER TO THE COAST ECMWF FOR THE CYCLONE DEEPENING OFF THE SOUTHEAST COAST EARLY IN THE FORECAST PERIOD...AND THE ECMWF ENSEMBLE MEAN SMOOTHED OUT SOME OF THE DETAIL DIFFERENCES WITH SHORTWAVES ROTATING WITHIN THE DEEP VORTEX OFF THE NORTHWEST COAST. ALSO...THE PAST FEW RUNS OF DETERMINISTIC GUIDANCE HAVE BEEN FAIRLY CONSISTENT WITH ADVERTISING A STRENGTHENING SURFACE LOW TRACKING FROM THE CENTRAL PLAINS TO THE UPPER MIDWEST EARLY-MID NEXT WEEK. ALTHOUGH THE ECMWF ENSEMBLE MEAN IS WEAKER THAN DETERMINISTIC GUIDANCE...IT IS IN THE MIDDLE OF MODEL SPREAD WITH THE LOW POSITION AND ALLOWS FOR FLEXIBILITY SINCE IT IS STILL A DAY 6-7 FORECAST. WEATHER HIGHLIGHTS/THREATS... THE FOCUS EARLY THIS WEEKEND WILL BE WITH CYCLOGENESIS OFF THE SOUTHEAST COAST ON SATURDAY...WHICH COULD CLIP PORTIONS OF THE MID-ATLANTIC STATES AND COASTAL AREAS OF THE NORTHEAST WITH ACCUMULATING SNOW BEFORE THE SYSTEM LIFTS INTO THE CANADIAN MARITIMES EARLY NEXT WEEK. ATTENTION WILL QUICKLY SHIFT TO THE WESTERN U.S. TOWARDS THE LATER HALF OF THE WEEKEND...WHERE AN ANOMALOUS SURGE OF MOISTURE AHEAD OF A DEEP VORTEX OFF THE NORTHWEST COAST WILL BRING WIDESPREAD PRECIPITATION TO THE WESTERN U.S. SATURDAY NIGHT THROUGH MONDAY. THE MOIST FLOW INTO THE TERRAIN SHOULD RESULT IN CONSECUTIVE DAYS OF SIGNIFICANT PRECIPITATION TOTALS ALONG THE COASTAL RANGES OF NORTHERN/CENTRAL CALIFORNIA AND THE SIERRA NEVADA RANGE. SUCH INTENSE AND PROLONGED RAINFALL MAY LEAD TO HYDROLOGIC ISSUES WHICH MAY INCLUDE MUDSLIDES...DEBRIS FLOWS...ETC. ALTHOUGH CONDITIONS WILL BECOME INCREASINGLY MILD ON SUNDAY BEFORE HEIGHT FALLS EDGE INLAND SUNDAY NIGHT INTO MONDAY...SNOWFALL WILL STILL BE MEASURED IN FEET OVER THE HIGHER ELEVATIONS. ALSO...MODELS HAVE BEEN SIGNALING A CHANCE FOR FREEZING RAIN FOR PORTIONS OF THE PACIFIC NORTHWEST AT THE START OF THE EVENT. EARLY NEXT WEEK...CONDITIONS SHOULD BE RELATIVELY DRY AND COOL WITH HIGH PRESSURE SPRAWLING OVER THE EASTERN U.S....WHILE THE CHANCE FOR PRECIPITATION...INCLUDING WINTER PRECIPITATION...WILL INCREASE WITH A POSSIBLE SURFACE CYCLONE TRACKING FROM THE PLAINS TO THE UPPER MIDWEST NEXT TUESDAY THROUGH WEDNESDAY. GERHARDT