Extended Forecast Discussion NWS Weather Prediction Center College Park MD 226 PM 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 amplified pattern will continue across the CONUS through the medium range period (Saturday to Wednesday). Beginning with an anomalous shortwave trough over the southeast U.S., that wave will take on a negative tilt as it lifts through the Mid-Atlantic, Northeast, and New England. Out West, another shortwave will drop through the Southwest U.S. early next week with a surface low ejecting out into the south-central Plains early to mid week. There was relatively good model agreement and run to run consistency with the major global deterministic models through Day 5. As such, the WPC medium range products were primarily a blend of the 00Z ECMWF/UKMET/CMC and 06Z GFS. Toward Day 6-7, there remained enough consistency and agreement to include the ECMWF/GFS, but with a near equal inclusion of the ECENS/GEFS means. This approach follows 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/Taylor 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 - Heavy precipitation across portions of the Ohio Valley, Central Appalachians, and the Northeast, Mon-Tue, Dec 16-Dec 17. - Heavy rain from portions of the Lower Mississippi Valley to the Northeast, Dec 16-Dec 17. - Heavy rain across portions of the Northeast, Sat, Dec 14. - Heavy snow across portions of the Central Rockies, Sat-Sun, Dec 14-Dec 15. - Flooding occurring or imminent across portions of the Northern Plains. - Flooding likely across portions of the Northern Plains. - High winds across portions of the Northeast, Sun, Dec 15. - Much below normal temperatures across portions of the Upper Mississippi Valley and the Northern Plains, Sat-Sun, Dec 14-Dec 15. 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