Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 AM EST Thu Dec 06 2018 Valid 12Z Sun Dec 09 2018 - 12Z Thu Dec 13 2018 ...Significant winter weather event possible for portions of the southern Appalachians and foothills this weekend... ...Overview... Interaction between northern and southern stream shortwaves advertised for days should be ongoing on day 3 (Sun) as surface low pressure crosses the Florida Panhandle and moves to off the Outer Banks by Mon morning, producing a widespread precipitation (rain and snow) event across portions of the Southeast and southern Mid-Atlantic states. A potential second wave of low pressure (in response to the amplifying northern stream wave across the Southeast) may linger off the Eastern Seaboard through Mon before ejecting quickly out to sea on Tue. In terms of the big picture, ensemble guidance suggests perhaps a significant pattern change across the northern hemisphere beginning early next week and continuing beyond the forecast period. Strong positive height anomalies which have been persistent across Siberia for a couple weeks should begin to break down, with downstream flow across the North Pacific becoming much more progressive than seen recently. Developing negative height anomalies across Alaska and into western North America as a result of this pattern change will favor an upper-level ridging centered in the vicinity of Hudson Bay, which would significantly reduce the flow of polar/arctic air southward into the CONUS, while also resulting in period of much wetter conditions for the Pacific Northwest than seen recently. Strong height falls are expected to reach the West coast on Mon, as the first in a rapid series of systems to affect the Pacific Northwest in particular. The tendency may continue for energy to split into northern and southern stream components, with the northern stream waves generally more progressive relative to the southern stream. Height falls should reach the central U.S. Tue night into Wed, reinforced by the rapid succession of northern stream energy. A resultant low pressure system will likely develop across the Plains by the middle of next week. ...Guidance Evaluation/Preferences... Models continue to show variability with respect to both timing and amplitude of northern and southern stream waves across the central/eastern U.S. Sun-Mon, but the degree of volatility in solutions seems to be slowly decreasing. Models seem to show improved agreement on the initial surface low crossing Florida and moving off the Outer Banks Sun-Mon, with a second surface low developing off the Carolinas late Mon in response to the northern stream wave (which never fully phases with the leading wave). The critical northern stream wave remained over the Northeast Pacific Ocean as of the 00Z model initialization, and should move into British Columbia later today, hopefully contributing to a reduction in spread in the guidance. A blend of deterministic guidance (ECMWF/GFS/FV3/UKMET) was used as a starting point for the forecast on days 3-4 (Sun-Mon), excluding the 12Z CMC which was overly amplified with the northern stream energy, resulting in a much farther north track of the surface low along the Southeast U.S. coast. The resulting forecast shows a surface low track close to/along the Gulf Stream, a track which is fairly climatological. Spread begins to increase across the western U.S. by day 5 (Tue) as the incoming trough separates into the two streams, with models showing the most spread in the progressive northern stream. Differences only grow through time in the northern stream, with even run-to-run inconsistencies in the ECMWF and the GFS by the middle of next week. As a result, weighting of ECENS and GEFS ensemble means was boosted starting on day 5 with continued increases through day 7 (Thu). ...Weather Highlights/Hazards... The low pressure system across the Southeast Sun-Mon is expected to produce a potentially significant winter weather event (beginning late in the short range forecast period) for portions of the southern Appalachians and potentially the foothills, with multi-inch snowfall accumulations possible. See the WPC Day 4-7 Winter Weather Outlooks for further probabilistic details on the winter weather threat associated with this system. Along the West Coast, expect an increase in precipitation from Sun onward as heights begin to fall and a rapid succession of shortwaves/frontal systems approach. Current indications are that the higher terrain of western Oregon and Washington will see the potential for the heaviest precipitation amounts (rain and mountain snow) through the middle of next week. Below average temperatures (negative anomalies of 5 to 15 deg F for high temperatures) will prevail across much of the central and eastern U.S. Sun-Mon, with a slow moderation of temperatures possible through early next week beginning across the northern High Plains and gradually spreading eastward as the aforementioned upper-level pattern change begins to restrict the flow of polar air southward into the CONUS. Ryan 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