Extended Forecast Discussion NWS Weather Prediction Center College Park MD 144 AM EST Mon Dec 10 2018 Valid 12Z Thu Dec 13 2018 - 12Z Mon Dec 17 2018 ...Heavy Precipitation Threat for the Northwest... ...Heavy Rain Threat for the Lower MS Valley through the East... ...Heavy Snow Threat Mid-MS/OH Valley through the Interior Northeast... ...Overview and Guidance Evaluation/Preferences... The weather pattern for the next week will remain quite active over the Northwest into CA and become increasingly wet for much of the eastern third of the nation later in the week. There is a strong guidance signal heralding these hazards. However, deterministic models continue to waver increasingly over time (often significantly) with the details in an otherwise rather predictable longwave upper pattern that is well represented for much of the upcoming week by recent ensemble means and WPC continuity. Accordingly, the WPC medium range product suite was primarily derived from a composite blend of the latest GEFS/ECMWF ensemble means and to a lesser extent over time the latest GFS/ECMWF. ...Weather Highlights and Hazards... Precipitation/wind will substantially increase into the Northwest into CA Fri/Sat and again by Mon/Tue as heights fall with a succession of approaching energetic Pacific shortwaves/frontal systems. Higher Pacific Northwest terrain will observe the heaviest rain/mountain snow (with varying snow levels as upper temperatures rise and fall between each system), with activity then working to the north-central Great Basin/Rockies. Lead shortwave energy digs from the Rockies to the southern Plains Thu to reinforce upper troughing in an emerging southern stream to spawn surface cyclogenesis/frontogenesis. Ample Gulf moisture/warm advection increasing ahead of the deepening system will help to expand the precipitation shield to the east and also wrap to the north of the surface low. Relatively slow movement of the organized system and favorable dynamics support a heavy rain threat for the lower MS Valley/Mid-South to Southeast. Heavy rainfall spreads to the east side of the Appalachians and coastal plain with added infusion of Atlantic moisture. Runoff problems will be enhanced for areas with recent heavy snowfall in a lead-in pattern with moderating temperatures. New heavy cold sector snows are meanwhile likely to wrap back into the Midwest/Great Lakes/Appalachians to the northwest of the uncertain low track. 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