EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 139 AM EST THU JAN 15 2015 VALID 12Z SUN JAN 18 2015 - 12Z THU JAN 22 2015 ...OVERVIEW... FLOW OF LOW AMPLITUDE FROM THE MULTI-DAY MEAN PERSPECTIVE... ALBEIT CONTAINING A STRONG TROUGH FCST TO AFFECT THE EAST DURING THE FIRST HALF OF THE PERIOD... SHOULD BECOME MORE AMPLIFIED TOWARD THE MIDDLE OF NEXT WEEK AS GUIDANCE MAINTAINS CONTINUITY WITH THE IDEA OF A RIDGE STEADILY BUILDING JUST OFF THE WEST COAST. TELECONNECTIONS RELATIVE TO THE STRONG POSITIVE HGT ANOMALY CENTER JUST OFF THE BC COAST BY D+8 SUPPORT THE POSITIVELY TILTED CONUS TROUGH WHICH MOST MODEL/ENSEMBLE MEAN SOLNS SHOW BY NEXT WED-THU. WITH LARGE SCALE FLOW WELL AGREED UPON... THE LATEST UPDATE TO THE FCST PRIMARILY REFLECTS A REFINEMENT OF FEATURES OR MODEST ADJUSTMENTS TO REFLECT LATEST GUIDANCE TRENDS. ...GUIDANCE EVALUATION AND PREFERENCES... SUCCESSIVE CYCLES OF GUIDANCE ARE GRADUALLY INCREASING CONFIDENCE IN THE EXISTENCE OF A SIGNIFICANT STORM THAT SHOULD DEEPEN AS IT TRACKS NWD NEAR THE EAST COAST SUN-MON. HOWEVER CRUCIAL DETAILS IN TRACK/STRENGTH WILL TAKE ADDED TIME TO WORK OUT SINCE THE SOURCE ENERGY ALOFT IS STILL OVER THE ERN PAC AND NOT SCHEDULED TO REACH THE WEST COAST UNTIL AROUND EARLY FRI. STEADILY DEEPER TREND OF THE SFC LOW IN ECMWF MEANS OVER THE PAST 24-36 HRS SEEMS TO SUPPORT PARTIAL WEIGHT OF OPERATIONAL SOLNS THAT ARE STRONGER THAN THE MEANS. WITH A FAIR CLUSTER OF MODEL SOLNS AND EVENTUALLY THE ECMWF MEAN CLOSING OFF AN UPR LOW OVER THE NORTHEAST... AS WELL AS A WWD TREND IN THE 00Z GFS TOWARD THE 12Z ECMWF... THE WRN PORTION OF THE SFC LOW ENVELOPE MAY BE PLAUSIBLE. HOWEVER THUS FAR THERE HAVE BEEN ENOUGH RUN TO RUN MODEL CHANGES AND OVERALL SOLN SPREAD TO FAVOR A MORE INTERMEDIATE SOLN JUST A LITTLE WWD OF THE CENTER OF THE ENSEMBLE LOW SPREAD. REGARDING ERN PAC FLOW THAT ULTIMATELY SHAPES THE POSITIVELY TILTED MEAN TROUGH OVER THE CONUS BY MIDWEEK... THE PRIMARY TREND OVER THE PAST DAY OR TWO IS TOWARD GREATER SEPARATION BETWEEN LEADING ENERGY THAT SHOULD CROSS THE NRN HALF OF THE CONUS AND IMMEDIATELY TRAILING FLOW AMPLIFYING OVER THE WEST. THIS ADJUSTMENT HAS LED TO A GRADUALLY FASTER TREND WITH THE WEAK SFC LOW/FRONT EXPECTED TO CROSS THE CNTRL-ERN STATES MON ONWARD. THE 12Z ECMWF BECOMES MORE SUPPRESSED WITH THIS SYSTEM VERSUS MOST OTHER GUIDANCE BUT THAT SCENARIO WILL HAVE TO BE WATCHED IF THE DEPARTING NERN STORM TRENDS ANY SLOWER. THE MAIN QUESTION BY DAY 7 THU IS HOW UPSTREAM PAC ENERGY AFFECTS THE SHAPE OF THE RIDGE NEAR THE WEST COAST. RELATIVELY SMALL SCALE OF THIS DETAIL LEADS TO LOW PREDICTABILITY AT THAT TIME FRAME. FOR MOST ASPECTS OF THE FCST THROUGH DAY 6 WED A COMPROMISE AMONG THE 18Z GFS/12Z ECMWF AND THEIR ENSEMBLE MEANS... SLIGHTLY MORE TO THE MODELS SUN-MON AND TOWARD THE MEANS TUE-WED... PROVIDED A REASONABLE STARTING POINT FOR DEPICTING INTERMEDIATE PREFS ALONG THE EAST COAST AND REPRESENTING THE MOST COMMON ELEMENTS OF GUIDANCE UPSTREAM. DAY 7 THU STARTED WITH 2/3 WEIGHTING OF THE 12Z ECMWF MEAN RELATIVE TO THE 18Z GEFS MEAN GIVEN A SLIGHT MAJORITY OF GUIDANCE LEANING TOWARD A MORE SUPPRESSED PATTERN OVER THE EAST AT THAT TIME. ...SENSIBLE WEATHER HIGHLIGHTS... LOW PRES FCST TO DEVELOP ALONG THE EAST COAST SUN-MON HAS POTENTIAL TO GENERATE STRENGTHENING WINDS AND AN AXIS OF HVY RNFL FROM THE MID ATLC COAST INTO THE NORTHEAST... WITH COLD AIR ARRIVING FROM THE WEST/CLOSURE OF AN UPR LOW LEADING SNOW IN THE WRN PART OF THE PCPN SHIELD. BASED ON THE SOURCE OF SUPPORTING ENERGY IT WILL LIKELY TAKE ANOTHER DAY OR SO TO GAIN SOME CONFIDENCE IN THE STRENGTH/TRACK OF THE SFC LOW. MEANWHILE THE NORTHWEST WILL TREND DRIER WITH TIME AS THE PATTERN ALOFT AMPLIFIES. WEAK SFC SYSTEM CROSSING THE CNTRL/ERN STATES MON-WED SHOULD BE ACCOMPANIED BY MOSTLY LGT PCPN WHILE THE COMBINATION OF PSBL EJECTING SWRN CONUS ENERGY AND SOME DEGREE OF GULF INFLOW INTERACTING WITH A FRONT REACHING THE SRN TIER MAY SUPPORT INCREASED COVERAGE/INTENSITY OF RNFL OVER THE SRN PLAINS/MS VLY BY NEXT THU. SYSTEM AFFECTING THE EAST WILL BRING A BRIEF PERIOD OF BELOW NORMAL TEMPS. ELSEWHERE ABOVE TO WELL ABOVE NORMAL TEMPS EARLY IN THE PERIOD... ESPECIALLY FROM THE INTERIOR WEST THROUGH THE PLAINS/UPR MIDWEST... SHOULD TREND COOLER WITH TIME AS LATE PERIOD MEAN FLOW ALOFT FAVORS GREATER COVERAGE OF NEAR TO BELOW NORMAL READINGS. RAUSCH