EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1100 AM EST WED JAN 07 2015 VALID 12Z SAT JAN 10 2015 - 12Z WED JAN 14 2015 ...PATTERN/SENSIBLE WEATHER HIGHLIGHTS AND GUIDANCE/UNCERTAINTY ASSESSMENT... WPC MEDIUM RANGE SURFACE FRONTS/PRESSURES AND 500 MB PROGS ALONG WITH HPCGUIDE GRIDS HAVE BEEN PRIMARILY DERIVED FROM A COMPOSITE BLEND OF REASONABLY COMPATABLE GUIDANCE FROM THE 06 UTC GFS PARELLEL/GEFS MEAN AND THE 00 UTC ECMWF/ECMWF ENSEMBLE MEAN DAYS 3-5 FRI-SUN ALONG WITH WPC CONTINUITY. INCREASING DETERMINISTIC MODEL FORECAST SPREAD PROMPTED A QUICK TRANSITION TO AN ENSEMBLE MEAN SOLUTION TUE/NEXT WED USING A 50-50 BLEND OF THE 06 UTC GEFS AND 00 UTC ECMWF ENSEMBLE MEANS. THE LARGER SCALE PATTERN FEATURES A LESS DEFINED SRN STREAM RUNNING UNDERNEATH AND INTERACTING WITH A DOMINANT AND AMPLIFIED NRN STREAM FLOW WITH A MEAN RIDGE BUILDING ALOFT NEAR THE NW US/WRN CANADIAN COASTS AND A COLD DOWNSTREAM MID-UPPER LEVEL TROUGH DUG SOUTHWARD INTO THE US FROM EAST-CENTRAL CANADA...ALL OVERTOP A MEAN SERN US RIDGE ALOFT. FORECAST PREDICTABILLITY SEEMS TO HOLD AT NEAR AVERAGE TO ABOVE AVERAGE LEVELS FOR THE NEXT WEEK IN THIS OVERALL PATTERN WITH MOST UNCERTAINTY WITH UNRESOLVED AMPLITUDE/TIMING ISSUES WITH EMBEDDED SMALLER SCALE IMPULSES AND ASSOCIATED SURFACE SYSTEM REFLECTIONS AND SENSIBLE WEATHER FOCUS. WHILE EMBEDDED IMPULSES WILL LEAD TO SEVERAL SWATHS OF LIGHT-SIDE PCPN IN BOTH STREAMS ACROSS THE LOWER 48 IN LIMITED MOISTURE...FORECAST SPREAD REMAINS MOST PROBLEMATIC WITH POTENTIAL SRN AND NRN STREAM IMPULSE INTERACTION WITH A LOWER ATMOSPHERIC COLD DOME SUNK FAR SOUTHWARD DOWN THROUGH THE CENTRAL AND ERN US. THIS WOULD INCLUDE A THREAT FOR ENHANCED LEAD MOISTURE INFLOW/QPF AND OVERRUNNING WINTER WEATHER POTENTIAL FOCUSING ALONG AND TO THE NORTH OF THE TRACK OF ANY ORGANIZED FRONTAL/COASTAL WAVES. HEAVIEST RAINFALL POTENTIAL SEEMS ACROSS THE GULF COAST STATES/SERN US. THE NORTHWARD EXTENT OF THE OVERALL PCPN SHIELD IS A TOUGH CALL. THE STRENGTH OF THE HIGH MAY FAVOR RECENT GFS/GEFS RUNS THAT HAVE BEEN MORE SOUTHWARD SUPPRESSED WITH QPF THAN RECENT ECMWF/ECMWF ENSEMBLE MEANS THAT MAY BE FAVORED BY UPSTREAM FLOW AMPLITUDE...LEADING TO THE 50-50 WPC BLEND. EITHER WOULD PLACE A POTENTIALLY SIGNIFICANT BAND OF SLEET/ICE THEN SNOW OVERTOP ON THE MORE NRN EDGES OF THE OVERALL PRECIPITATION SHIELD...PERHAPS IN THIS PERIOD NORTHWARD FROM THE LOW-MID MS/TN VALLEY THROUGH THE APPALACHIANS/MID-ATLANTIC. SCHICHTEL