Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Mon Dec 09 2019 Valid 12Z Thu Dec 12 2019 - 12Z Mon Dec 16 2019 ...Heavy Southeast Rains to lift northward over a Wintry/Cold Air Dammed East... ...Wet flow from the Pacific Northwest to the Snowy North-Central Great Basin/Rockies... ...Pattern Overview and Guidance/Predictability Assessment... The cold Hudson Bay mean upper vortex will linger this week. Flow underneath over the lower 48 will become less amplified, but remain quite active and unsettled/stormy this week as a long series of northern and southern stream systems traverse overall mean cyclonic flow. The WPC medium range product suite was mainly derived from a blend of reasonably compatable GEFS/ECMWF ensemble means. This is amid rapidly growing forecast spread that includes significant system and stream phasing uncertainties despite a similar larger scale pattern evolution. This maintains good WPC continuity vs more run-run inconsistent model solutions. ...Weather Highlights/Threats... Models still offer varied low scenarios from the Gulf of Mexico to the Southeast then up the Eastern Seaboard into next weekend. Forecast spread has been considerable, especially with recent outlier GFS runs, but seems to trending a bit better with the 00 UTC guidance with respect to the timing/stream phasing, cyclogensis potential and eventual system tracks. This pattern seems likley to focus moisture and lift to produce a risk of heavy rainfall across the Southeast. Subsequent system lift up through the East Coast and interaction with a slow to recede lower atmospheric cold air damming offers an interior snow/ice threat to monitor. As is often the case, the degree of phasing/interaction between the two initially separate streams and baroclinic zone/coastal setup will be determining factors of eventual evolution and any potential for any deeper coastal storm developments and threats. Out West, the pattern will become wet across the Pacific Northwest into later this week as Pacific shortwave energies/height falls punch onshore. Moderate to heavier snows will develop with system progressions inland, especially for favored terrain of the north-central Great Basin/Rockies later week through the weekend. Schichtel Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indices are 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