Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Wed Dec 12 2018 Valid 12Z Sat Dec 15 2018 - 12Z Wed Dec 19 2018 ...Heavy Precipitation Threat for the Northwest... ...Heavy Rain Threat for the East... ...Overview and Guidance Evaluation/Preferences... Models and ensembles agree to show a threat for periods of heavy precipitation across the Northwest/northern CA this period as well as a lingering heavy rainfall threat into this weekend from the Southeast/Appalachians/Mid-Atlantic to southern New England. However, significant timing and depth differences amongst guidance portends below average forecast confidence for daily forecasts specifics in both areas. Accordingly, the WPC medium range product suite was primarily derived from the latest ECMWF ensemble mean that offers a compromise guidance timing/strength composite solution for both areas. ...Weather Highlights and Hazards... A lead deep closed low will eject this weekend from the Southeast/southern Mid-Atlantic to the western Atlantic and ample Gulf moisture will fuel lingering heavy precipitation to the east and wrapping to the north of main/re-developing surface lows. Heavy rainfall will robustly spread through the Appalachians and Eastern Seaboard with added infusion of Atlantic moisture where runoff problems will be enhanced in areas with recent heavy snowfall and ongoing river flooding. Some heavy snow may also occur in a marginally cooled airmass from the north-central Appalachians to the interior Northeast this weekend on the northern periphery of the broad precipitation shield. Upstream, precipitation/wind substantially increases from coastal Washington to northern CA into the weekend, with resurgence early-mid next week as a succession of energetic Pacific shortwaves/frontal systems pass through the region. Higher Pacific Northwest terrain will see the heaviest rain/mountain snow (with varying snow levels as temperatures rise and fall between each system, with snows then working to the north-central Great Basin/Rockies. While this activity remains concentrated within a pronounced northern stream flow, the concurrent development of a separated southern stream will meanwhile carve out a Southwest U.S. to Mexico closed trough aloft and associated surface system that offers heavy rain potential for the southern Plains, but only the deterministic Canadian model is progressive enough to develop deep activity before the end of Day 7. Schichtel WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes are found 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