Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1256 AM EST Thu Dec 13 2018 Valid 12Z Sun Dec 16 2018 - 12Z Thu Dec 20 2018 ...Significant precipitation for the Northwest... ...Guidance Evaluation/Preferences... Models and ensembles continue to differ on the timing/track of the embedded systems across the lower 48, but forecast clustering has improved over the past few runs. Accordingly, the WPC medium range product suite was primarily derived from a composite blend of models and ensembles, the National Blend of Models, and WPC continuity, with more weighting applied to ensemble means over time consistent with gradually growing forecast spread. ...Pattern Overview and Weather Highlights/Hazards... A lead main Southeast/Mid-Atlantic system will shift offshore Sunday. Lingering wrapback rains will spread from the Mid-Atlantic to southern New England by early next week as the coastal storm/maritime threat lifts to the Canadian Maritimes. Also expect modest wintry weather back over northern New England with infusion of uncertain northern stream energy. A heavy precipitation and high wind threat for the Pacific Northwest/northern CA resurges next week as a succession of energetic Pacific systems approach the region. Some deeper lower latitude moisture tap and flow suggests favored/higher terrain will focus heavy rainfall/elevation snows, with snow working to the northern Great Basin/Rockies within a pronounced northern stream flow. Meanwhile, a split/separated southern stream trough will carve out over the Southwest U.S./northern Mexico by early next week. Uncertain southern stream system energies will then eject out across the U.S. southern tier and could increasingly tap Gulf of Mexico moisture mid-later next week to enhance rainfall potential. 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