EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1100 AM EST THU JAN 08 2015 VALID 12Z SUN JAN 11 2015 - 12Z THU JAN 15 2015 ...OVERVIEW... PACIFIC AIR ENTERING NORTH AMERICA IN A SPLIT-FLOW WILL MAKE INROADS TOWARDS ERADICATING THE ARCTIC AIRMASS IN PLACE EAST OF THE CONTINENTAL DIVIDE. HOWEVER...EROSION OF THIS AIRMASS ALONG ITS SOUTHWESTERN AND SOUTHERN FLANKS FROM THIS SOUTHERN STREAM ENERGY WILL GENERATE PERIODS OF OVER-RUNNING PRECIPITATION AND THE POSSIBILITY OF WINTERY PCPN INCLUDING AN AMPLE TRANSITION ZONE ICE THREAT ALL FROM PORTIONS OF THE SRN PLAINS...OZARKS AND LOWER TO MID-MS VALLEY NEWD THROUGH THE TN/OH VALLEYS/APPALACHIANS THEN MID-ATLANTIC/NERN US. IN THE WEST...A MAJOR THEME WILL BE A GRADUAL EASTWARD MIGRATION OF THE BROAD ALEUTIAN LOW FROM THE WRN ALEUTIAN CHAIN TO THE W-CENTRAL GULF OF ALASKA. THIS WILL BUILD DOWNSTREAM MID-UPPER LEVEL RIDGING AND GRADUALLY BRING MOIST SOUTHWESTERLY FLOW TO THE ALASKA PANHANDLE AND SOUTHWEST CANADA THEN PACIFIC NW. ...UNCERTAINTY ASSESSMENT/MODEL PREFERENCES... THE SYNOPTIC-SCALE PATTERN APPEARS TO BE SETTING UP OVER THE LOWER 48---THAT INCLUDES A MAINLY DRY...BUT COLD NORTHERN STREAM AND A PROGRESSIVE/ACTIVE AND MILDER SOUTHERN STREAM WITH A BROAD FETCH ORIGINATING OUT OF THE GULF OF MEXICO. THE EVOLUTION OF THE SOUTHERN STREAM ALOFT INLAND OVER THE NATION FROM THE WEST COAST...SEEMS TO BE WHERE THE GREATER SPREAD OCCURS IN GUIDANCE. WHAT REMAINS OF THE UPPER-LEVEL CIRCULATION MOVES INTO TX SUN/MON AND OFFERS A P-TYPE FORECAST CHALLENGE FOR THE SOUTHERN PLAINS/OZARKS ALONG ITS NORTHERN AND NORTHEAST PCPN PERIPHERY. THIS LEADS TO THE DOWNSTREAM TIMING DIFFERENCES LATER WITH SURFACE WAVE DEVELOPMENT AND MIGRATION OVER THE GULF STREAM IN THE FAR WESTERN ATLANTIC. MEANWHILE OVER TIME...THE SPREAD ALOFT INCREASES OVER CENTRAL CANADA---WITH THE NORTHERN STREAM DETAILS MIGRATING ACROSS THE UPPER GREAT LAKES THAT EFFECT ANY STREAM INTERACTIONS. IN MOST OF THESE SCENARIOS THOUGH...THE SURFACE RIDGE DAMMED SOUTHWARD FROM THE NERN US REMAINS A SEMI-PERMANENT FEATURE OF THIS MEDIUM RANGE FORECAST AND FLOW PATTERN. 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. 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...ALL PERHAPS MAINLY IN THIS PERIOD WITH A THREAT NORTHWARD FROM THE S-CENTRAL PLAINS AND LOWER-MID MS/TN VALLEYS ENEWD THROUGH OH VALLEY AND THE APPALACHIANS/MID-ATLANTIC. SCHICHTEL