Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1029 AM EST Sun Dec 09 2018 Valid 12Z Wed Dec 12 2018 - 12Z Sun Dec 16 2018 ...Heavy rain threat for parts of the east Fri-Sun... ...Overview and Guidance Evaluation/Preferences... The weather pattern over the next week is expected to remain quite active across the Northwest and become increasingly wet for much of the eastern third of the nation later in the week. The models continue to waver (sometimes significantly) with the details in an otherwise rather predictable longwave upper pattern that is well-represented in the latest ensemble means and ongoing WPC continuity. Accordingly, the WPC medium range product suite was primarily derived from a blend of the latest GEFS/ECMWF ensemble means and continuity with minority weighting applied to the 00Z ECMWF/UKMET early in the period (favoring the slower solutions) but the 06Z GFS later in the period (favoring the best ensemble clustering well away from the much farther south ECMWF in the Southeast). Confidence is no better than average with the track of the system next Saturday in the east, but the signal for modest to locally heavy rain remains focused on the southern Appalachians. ...Weather Highlights and Hazards... Precipitation/wind periodically increases into the Northwest midweek and again into the weekend as heights fall with a succession of approaching 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. Precipitation may be enhanced with stronger-than-forecast surface lows that may track near/over northwestern Washington Friday but these waves have little predictability at this time range. By Thursday, a Pacific shortwave will dive from the Rockies into the southern Plains to reinforce upper troughing while spawning surface cyclogenesis over Oklahoma. Gulf moisture/warm advection increasing ahead of the deepening system will help to expand the precipitation shield to the east and then wrap around to the north of the surface low. Relatively slow movement of the system will support a heavy rain threat and some cold sector (marginally cold at best) snows through the Midwest/Great Lakes and central/northern Appalachians to the northwest of the uncertain low track. Rainfall may be heavy from the east side of the Appalachians to the coastal plain with the infusion of Atlantic moisture as well, which may fall over areas currently receiving appreciable snowfall (especially over the southern Appalachians). Temperatures will trend milder in the east (supporting a mostly rain event next weekend) but the track of the surface low will determine the areal extent of wintry weather (most uncertainty in the Northeast). Fracasso/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