EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1056 AM EST TUE NOV 18 2014 VALID 12Z FRI NOV 21 2014 - 12Z TUE NOV 25 2014 ...OVERVIEW... A VERY BRIEF PERIOD OF FLATTER MEAN FLOW DURING THE FIRST HALF OF THE PERIOD... BRINGING A MODERATING TREND IN TEMPS TO THE ERN STATES... WILL BE REPLACED BY AN AMPLIFIED MEAN TROUGH THAT SHOULD REACH A GRTLKS TO SRN PLAINS ORIENTATION AS OF DAY 7 NEXT TUE. AT THAT TIME A STRENGTHENING UPSTREAM RIDGE SHOULD BE POISED TO ENTER THE WRN CONUS. THIS PATTERN/EVOLUTION WILL FAVOR HEAVIEST PCPN OVER THE NORTHWEST AND FROM THE GULF COAST/SRN PLAINS NEWD. MOST ASPECTS OF THE LARGE SCALE FCST DISPLAY AVG TO ABOVE AVG AGREEMENT BUT SOME SIGNIFICANT DETAIL UNCERTAINTIES EXIST. ...MODEL EVALUATION/WPC PREFERENCES... THE DOMINANT ITEM OF INTEREST DURING THIS FCST PERIOD WILL BE EXACTLY HOW NRN PAC ENERGY/LEADING SFC SYSTEM ENTERING THE NORTHWEST FRI-SAT ULTIMATELY INTERACTS WITH A LEADING SYSTEM TRACKING FROM THE GRTBASIN/SOUTHWEST INTO THE PLAINS/GRTLKS. AT THE MOMENT THERE IS A SPLIT AMONG OPERATIONAL MODELS REGARDING WHETHER THE LEADING SYSTEM EJECTS NEWD AWAY FROM THE INCOMING AMPLIFYING PAC ENERGY OR HOLDS BACK OVER THE GRTLKS AND SEES GREATER INFLUENCE FROM THE UPSTREAM ENERGY. RECENT GFS/GFS PARALLEL RUNS LEAN MORE TOWARD THE FORMER SCENARIO... YIELDING A WEAKER AND SOMEWHAT MORE DISJOINTED OVERALL SFC EVOLUTION THAN ECMWF RUNS/00Z UKMET WHICH FOLLOW THE LATTER IDEA. SUCH MEDIUM TO SMALLER SCALE DETAIL ISSUES CAN SOMETIMES TAKE INTO THE SHORT RANGE TIME FRAME TO BE RESOLVED SO IT IS DIFFICULT TO COMMIT FULLY TO ONE CLUSTER YET. HOWEVER PREFER TO LEAN SOMEWHAT TOWARD A MORE CONSOLIDATED SFC SYSTEM AS INDICATED BY THE 00Z GEFS/ECMWF/CMC ENSEMBLE MEANS... WITH AMPLIFICATION OF THE LARGE SCALE PATTERN OFFERING A DEGREE OF SUPPORT FOR SLOWER TIMING OF INITIAL SWRN CONUS ENERGY. SLOWER TRENDS OF 06Z GFS/GFS PARALLEL RUNS VS THEIR 00Z COUNTERPARTS GO ALONG WITH THIS IDEA. IF THE TWO STREAMS INTERACT JUST RIGHT THERE IS CERTAINLY POTENTIAL FOR A VERY DEEP STORM SO WOULD FAVOR DEPTH A FEW MB DEEPER THAN THE MEANS AS A STARTING POINT. WILL AWAIT BETTER OPERATIONAL AGREEMENT BEFORE FURTHER DEEPENING THE SYSTEM. BY LATE IN THE PERIOD THERE MAY BE A PERIOD OF MINIMAL HGT CHANGE ALOFT OVER THE SOUTHEAST WITH ONE OR MORE RIPPLES OF SHRTWV ENERGY PSBLY SUPPORTING WAVINESS ALONG THE TRAILING FRONT THAT MAY BE SLOW MOVING FOR A TIME. THE 00Z ECMWF APPEARS TO BE ON THE MORE AGGRESSIVE SIDE OF THE SPREAD WITH ITS LOWER HGTS ALOFT LEADING TO GREATER CLEARING OF ERN CONUS MSTR THAN MOST OTHER SOLNS. THE GREATEST ENSEMBLE SPREAD OF THE FCST COMES INTO THE PICTURE FROM THE NRN PAC INTO THE NWRN STATES ON DAY 7 TUE. INTERESTINGLY... IN SPITE OF THE SPREAD SEEN IN THE SPAGHETTI PLOTS THE RESULTING MEANS AS WELL AS LATEST OPERATIONAL RUNS ARE FAIRLY SIMILAR WITH A RIDGE NEAR THE WEST COAST THOUGH WITH SOME GREATER DIFFS IMMEDIATELY UPSTREAM. ...SENSIBLE WEATHER HIGHLIGHTS... THE START OF THE PERIOD WILL BE QUITE CHILLY OVER THE GRTLKS/NORTHEAST/MID ATLC WITH HIGHS 10-20F BELOW NORMAL AT MOST LOCATIONS. ERN CONUS TEMPS SHOULD RECOVER TO ABOVE NORMAL LVLS BY LATE WEEKEND/EARLY NEXT WEEK AHEAD OF APPROACHING PLAINS-GRTLKS SFC DEVELOPMENT. TRAILING COLDER AIR SPREADING ACROSS THE WRN-CNTRL STATES MAY CONTAIN SOME POCKETS OF 10-20F BELOW NORMAL READINGS BUT IN GENERAL SHOULD BE LESS EXTREME THAN THE CURRENT/RECENT COLD EPISODE DUE TO MORE OF A PACIFIC ORIGIN. EXPECT ONE OR MORE PERIODS OF TERRAIN ENHANCED AND LOCALLY HVY RNFL/HIGHER ELEV SNOW FROM THE NRN HALF OF THE WEST COAST INTO THE NRN ROCKIES... WITH HIGHEST CONFIDENCE IN THE EARLY TO MIDDLE PART OF THE FCST PERIOD. MEANWHILE THE COMBINATION OF VIGOROUS DYNAMICS ALOFT AND LEADING GULF INFLOW WILL LIKELY SUPPORT AN EPISODE OF HVY RNFL OVER PARTS OF THE SRN PLAINS AND ALONG THE GULF COAST. SOME CONVECTION MAY ALSO BE ON THE STRONG SIDE. CONSULT SPC OUTLOOKS FOR FURTHER DETAILS REGARDING SEVERE THREATS. A PORTION OF THIS MSTR WILL LIKELY STREAM NEWD AS PLAINS/GRTLKS LOW PRESSURE EVOLVES... WITH SIGNIFICANT SNOW/WIND PSBL ON THE COLD SIDE OF THE SYSTEM. ALSO POTENTIAL EXISTS FOR ONE OR MORE BANDS OF ENHANCED RNFL OVER THE SOUTHEAST EARLY NEXT WEEK DEPENDING ON PROGRESSION/WAVINESS OF THE TRAILING FRONT. THIS LATTER ASPECT OF THE FCST HAS AVG OR LOWER CONFIDENCE AT THIS TIME THOUGH. RAUSCH