Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 AM EST Wed Dec 30 2020 Valid 12Z Sat Jan 2 2021 - 12Z Wed Jan 6 2021 ...Weather Pattern Overview... A split flow pattern aloft is expected to be in place across the continental U.S. by this weekend in the wake of the departing storm system late this week. The closed low that will initially be over the central Plains is expected to track towards the East Coast by Monday morning, followed by another trough in northern stream flow across the north-central U.S. early next week. The storm track will remain active across the northeast Pacific, and this will lead to multiple frontal passages for the Pacific Northwest region. ...Guidance/Predictability Assessment... Noteworthy model differences begin to emerge as early as Sunday morning regarding the timing of the upper low across the Ohio Valley and the progressive shortwaves in the northern stream flow. The GFS is generally a little faster, and the CMC becomes stronger off the northeast Coast by Monday. Predictability becomes below average for early-middle part of next week regarding both amplitude and timing, along with potential phasing, of individual shortwave impulses, with the CMC being more out of phase with the Pacific disturbances. The WPC forecast was primarily based on a GFS/ECMWF/CMC blend and some ensemble means through Monday, and then primarily GEFS and EC means as a starting point for Tuesday and Wednesday. ...Weather/Threats Highlights... The second wave of low pressure developing along the front is not expected to be as impactful as the earlier storm system in the short range forecast period. However, there will likely be a corridor of enhanced rainfall for portions of the Southeast U.S. on Saturday, with some potential for two inch rainfall amounts from the Florida Panhandle to eastern North Carolina, and lighter totals farther north. Portions of northern New England may have moderate snow on the backside of the first departing storm system. Mainly dry conditions are expected for the majority of the central U.S., although changes to that forecast are certainly possible given the current model differences. Meanwhile, areas from the Pacific Northwest to central California will continue to see additional rounds of rain and mountain snow this weekend and through early-middle of next week with the approach of multiple Pacific systems in progressive flow aloft. The highest precipitation totals are expected to occur over the favored coastal terrain and also the Cascades and Sierra, with an atmospheric river event possible over the weekend for western Oregon and Washington. Several inches of rainfall will be possible for some of these areas. Temperatures in the West will be near to slightly above average through the period. Hamrick Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indices are at: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml