Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Wed Dec 11 2019 Valid 12Z Sat Dec 14 2019 - 12Z Wed Dec 18 2019 ...Eastern U.S. heavy rain and interior wintry weather threat into Saturday and again Monday/Tuesday-... ...Heavy mountain snows possible this this weekend from Northwest to Rockies... ...Pattern Overview and Guidance/Predictability Assessment... An active and complex pattern should persist through much of the upcoming medium range period (Saturday-Wednesday) as northern/southern stream shortwaves interact. As one main lead shortwave/trough exits the Northeast this weekend, another dives south and eastward from the eastern Pacific into the West, moving into the South-Central U.S. by early next week, then to the East. Through the period, there is fairly good agreement in the large scale pattern, though forecast confidence remains somewhat tempered due to run to run inconsistencies with the more complex details. The WPC medium range product suite was primarily derived from a composite blend of reasonably well clustered model and ensemble forecasts and the National Blend of Models Saturday into Monday. Increased weighting significantly to the more run to run consistent GEFS/ECENS ensemble means into Days 6/7 amid growing forecast spread. Even so, the overall active pattern seems to offer above normal predictability and decent continuity. ...Weather Highlights/Threats... Southern stream shortwave progression and subsequent northern stream flow interaction will support low development and track up the East Coast into Saturday. This will bring heavy rain from the Appalachians/Mid-Atlantic through the Northeast. System interaction with lead/receding lower atmospheric cold air damming and wrap-back/post-frontal flow offers an interior heavy snow/ice threat and Lake effect snows. The degree of phasing between two initially separate streams and baroclinic zone/coastal details will be determining factors of eventual evolution and potential coastal system threat. The pattern will also become wet upstream over the Pacific Northwest/California into Saturday as Pacific shortwave energies punch onshore. Moderate to heavy snows will develop as the system penetrates inland, especially over favored north-central Great Basin/Rockies terrain through the weekend. By next Monday into Tuesday, the exiting shortwave will eject and then interact with Gulf moisture to spawn a rainfall focusing low pressure/frontal system forecast to move over the Southern Plains and Deep South, then with some added uncertainty up the East Coast. This scenario also sets the stage for a threat of heavy snow/ice on the northern periphery of the precipitation shield from the S-Central Plains to the Mid-MS/OH Valleys then Northeast. 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