EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 153 AM EST WED JAN 04 2017 VALID 12Z SAT JAN 07 2017 - 12Z WED JAN 11 2017 ...MULTI-DAY HEAVY PRECIPITATION EVENT EXPECTED ACROSS NORTHERN AND CENTRAL CALIFORNIA LATE THIS WEEKEND INTO EARLY NEXT WEEK... PATTERN OVERVIEW AND MODEL ASSESSMENT/PREFERENCES... THE MODELS HAVE STARTED TO COME INTO MUCH BETTER AGREEMENT WITH THE HEMISPHERIC PATTERN RELATIVE TO PREVIOUS DAYS. HIGH LATITUDE BLOCKING REMAINS IN PLACE OVER AK WHICH WILL FAVOR THE DEVELOPMENT OF CLOSED LOWS TO THE SOUTH ACROSS THE NORTHEASTERN PACIFIC. THESE FEATURES SHOULD SLIDE SOUTHWESTWARD HELPING BUILD HEIGHTS DOWNSTREAM OVER THE INTERMOUNTAIN WEST/ROCKY MOUNTAIN REGION. PERSISTENT MID-LEVEL SOUTHWESTERLY FLOW ACROSS THE WEST COAST WILL SET THE STAGES FOR A MULTI-DAY HEAVY PRECIPITATION EVENT...PARTICULARLY AFFECTING CENTRAL/NORTHERN CA. GRADUALLY THE CORE OF HEIGHT FALLS WILL MOVE THROUGH THE WESTERN U.S. AND TOWARD THE HIGH PLAINS BY MONDAY/TUESDAY WHICH INDUCES A PRETTY STRONG AREA OF LOW PRESSURE OVER THE CENTRAL U.S. GIVEN LITTLE CHANGE IN THE SYNOPTIC PATTERN OVER THE PACIFIC...THERE IS THE POTENTIAL FOR ADDITIONAL LONGWAVE TROUGHING TO TAKE SHAPE ACROSS THE EASTERN PACIFIC BY THE MIDDLE OF NEXT WEEK. LOOKING TO THE EASTERN U.S...A DEEP UPPER TROUGH WILL INHABIT THE REGION WITH A FAIRLY ENHANCED SHORTWAVE SWINGING THROUGH THE MID-SOUTH ON SATURDAY MORNING. AS IT ROUNDS THE BASE OF THE UPPER TROUGH...IT WILL INDUCE A SIGNIFICANT AREA OF LOW PRESSURE OVER THE WESTERN ATLANTIC. WHILE DETERMINISTIC MODEL RUNS CONTINUE TO WAVER BETWEEN FAVORING COASTAL IMPACTS VERSUS HARMLESSLY OUT TO SEA...THERE STILL REMAINS SOME UNCERTAINTY WITH THE LOW TRACK. EXAMINATION OF ALL OF THE CMC/GEFS/ECMWF ENSEMBLE MEMBERS...THERE ARE ONLY A FEW WHICH TRY TO MOVE WEST OF THE 40N/70W BENCHMARK. REGARDLESS...EXPECT A STEADY PERIOD OF COLD ADVECTION INTO THE EASTERN U.S. KEEPING CONDITIONS CHILLY INTO EARLY NEXT WEEK. GIVEN MUCH STRONGER MODEL AGREEMENT...WAS ABLE TO INCORPORATE A SMALL FRACTION OF DETERMINISTIC SOLUTIONS ALL THE WAY THROUGH DAY 7. THE FIRST FEW DAYS OF THE FORECAST CONSISTED APPROXIMATELY 60 TO 80 PERCENT OF THE 18Z GFS/12Z ECMWF BEFORE REVERTING TO A MAJORITY ENSEMBLE MEANS BY DAYS 6 AND 7. FELT COMPELLED TO KEEP A MINOR FRACTION OF THE 18Z GFS/12Z ECMWF ALL THE WAY INTO JANUARY 10-11 TO ACCOUNT FOR THE DECENT SIGNAL OF CYCLOGENESIS ACROSS THE GREAT PLAINS. ENSEMBLE LOWS WERE IN THE BALLPARK AND ATTEMPTED TO GET A DEEPER SURFACE SOLUTION THAN THE 18Z GEFS AND 12Z ECMWF/NAEFS ENSEMBLE MEANS OFFERED. WEATHER HIGHLIGHTS/THREATS... THE START OF THE WEEKEND WILL BE QUITE CHILLY ACROSS MUCH OF THE COUNTRY WITH HIGH TEMPERATURES ROUGHLY 15 TO 25 DEGREES BELOW NORMAL ACROSS THE NORTHERN PLAINS/UPPER MIDWEST. SLIGHTLY LOWER ANOMALIES DO EXTEND OVER THE REMAINDER OF THE U.S. AS THE MID-LEVEL RIDGE BEGINS TO EXPAND EASTWARD INTO THE ROCKIES/GREAT PLAINS BY EARLY NEXT WEEK...A CONSIDERABLE WARMING TREND WILL TAKE PLACE. BY TUESDAY...MUCH OF TX SHOULD SOAR WELL INTO THE 70S WHICH EQUATES TO ANOMALIES OF 15 TO 20 DEGREES ABOVE NORMAL. THIS WARMTH WILL CONTINUE TO EXPAND EASTWARD ALTHOUGH SOME COOLING IS EXPECTED ACROSS THE NORTHERN TIER BY DAY 7/JANUARY 11 WITH THE NEXT SHOT OF ARCTIC AIR INTO THE REGION. AS MENTIONED IN THE HEADLINE...AN IMPRESSIVE ATMOSPHERIC RIVER SET UP IS IN PLACE ACROSS THE WEST COAST WITH A PARTICULAR FOCUS ACROSS CENTRAL/NORTHERN CA. INTEGRATED WATER VAPOR TRANSPORT VALUES EXCEED 1000 KG/M/S DURING THE HEIGHT OF THE EVENT WITH STRONG SOUTHWESTERLY FLOW POOLING VERY DEEP MOISTURE CONTENT FROM THE SUBTROPICAL PACIFIC. RECENT MODEL GUIDANCE SUGGESTS THE POTENTIAL FOR OVER 10 TO 15 INCHES OF PRECIPITATION ACROSS THE SIERRA NEVADA RANGE WITH A SEPARATE MAXIMA ACROSS THE COASTAL RANGES. SUCH INTENSE AND PROLONGED RAINFALL MAY LEAD TO HYDROLOGIC ISSUES WHICH MAY INCLUDE MUDSLIDES...DEBRIS FLOWS...ETC. GIVEN THE MILD ORIGIN OF THE INTRUDING AIR...SNOW LEVELS SHOULD BE RATHER HIGH EARLY ON BEFORE DROPPING AS LOWER HEIGHTS MIGRATE INLAND. ELSEWHERE...EXPECT A THREAT FOR WINTRY PRECIPITATION SOMEWHERE EAST OF THE I-95 CORRIDOR WHICH WILL OF COURSE BE DEPENDENT ON THE EVENTUAL LOW TRACK. AND FINALLY...A SHIELD OF PRECIPITATION SHOULD BREAK OUT IN RESPONSE TO INCREASING WARM ADVECTION OVER THE GREAT PLAINS EARLY NEXT WEEK. THERE SHOULD BE ENOUGH COLD AIR IN PLACE TO BRING SOME SNOW TO THE NORTHERN TIER. RUBIN-OSTER