Extended Forecast Discussion NWS Weather Prediction Center College Park MD 141 AM EST Tue Dec 11 2018 Valid 12Z Fri Dec 14 2018 - 12Z Tue Dec 18 2018 ...Heavy Precipitation Threat for the Northwest... ...Heavy Rain Threat for the Lower MS Valley through the East... ...Overview and Guidance Evaluation/Preferences... It remains evident that the weather pattern for the next week will remain quite active from the Northwest into CA and become increasingly wet for much of the eastern half of the nation late week into the weekend. There remains a strong guidance signal heralding these hazards. Deterministic models continue to waver significantly over time with embedded system details in an otherwise rather predictable longwave upper pattern evolution that seems well represented by recent ensemble means. Accordingly, the WPC medium range product suite was primarily derived from a composite blend of the latest GEFS/ECMWF ensemble means. ...Weather Highlights and Hazards... Precipitation/wind will substantially increase from the Northwest into CA Fri into the weekend then again Sun-next Tue with a succession of approaching energetic Pacific shortwaves/frontal systems. 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. Lead shortwave energy digging into the southern Plains/lower MS Valley Thu-Fri is expected to carve out a closed low and spawn surface cyclogenesis/frontogenesis. Ample Gulf moisture/warm advection increasing ahead of the deepening system will expand the precipitation shield to the east and wrapping to the north of the surface low. Relatively slow movement of the well organized system and favorable dynamics support a heavy rain threat for the lower MS Valley/Mid-South to Southeast. Heavy rainfall will robustly spread through the Appalachians and Eastern Seaboard with added infusion of Atlantic moisture. Runoff problems will be enhanced in areas with recent heavy snowfall. Heavy snows may also wrap into a marginally cooled Midwest/Great Lakes to interior Northeast airmass. 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