Extended Forecast Discussion NWS Weather Prediction Center College Park MD 313 PM EST Tue Dec 10 2019 Valid 12Z Fri Dec 13 2019 - 12Z Tue Dec 17 2019 ...Eastern Seaboard heavy rain and interior wintry weather threat Friday/Saturday and again Monday/Tuesday-... ...Heavy mountain snows possible Friday-Saturday from Northwest to Rockies... ...Pattern Overview and Guidance/Predictability Assessment... An active and complex pattern should persist through the medium range period (Friday-Tuesday) as northern/southern stream shortwaves interact. As one shortwave/trough exits the Northeast this weekend, another dives south and eastward into the West, moving into the Central U.S. by early next week. 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. Notable differences is with the initial northern stream shortwave moving into the Great Lakes/Northeast on Saturday, the GFS remains a bit more amplified and results in a deeper surface low, but a blend with other solutions provides a reasonable forecast. Late in the period, troughing moving into the Central U.S. shows some slight timing differences with the CMC and its mean a bit slower/stronger than the rest of the guidance. The model choices for this cycle of the medium range progs features a majority deterministic GFS/ECMWF blend days 3-5, with increased weighting of the ECENS/GEFS means to come to a 50/50 blend between deterministic and ensembles by day 7. This maintains very good continuity with previous WPC continuity. ...Weather Highlights/Threats... Southern stream shortwave progression and subsequent northern stream flow interaction should support low development and track from the Gulf of Mexico/Gulf Coast to the Southeast then up the East Coast Friday/Saturday. This will bring heavy rain up the East Coast mainly from the Southeast to the Appalachians, Mid-Atlantic then Northeast. System interaction with lower atmospheric cold air damming offers an interior snow/ice threat to monitor. 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 into late week 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 wave will eject and then interact with Gulf moisture to spawn a rainfall focusing low pressure/frontal system forecast to move over the Deep South, then potentially again up the East Coast. Santorelli/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 Hazards: - Heavy precipitation across portions of the Northeast, the Central Appalachians, the Tennessee Valley, the Mid-Atlantic, the Southern Appalachians, the Great Lakes, and the Ohio Valley, Mon-Tue, Dec 16-Dec 17. - Heavy precipitation across portions of the Northeast and the Great Lakes, Sat-Sun, Dec 14-Dec 15. - Heavy rain across portions of the Lower Mississippi Valley, the Tennessee Valley, the Mid-Atlantic, the Southern Appalachians, the Southeast, the Southern Plains, and the Ohio Valley, Sun-Tue, Dec 15-Dec 17. - Heavy rain across portions of the Northeast, the Central Appalachians, the Tennessee Valley, the Mid-Atlantic, the Southern Appalachians, the Southeast, and the Ohio Valley, Fri-Sat, Dec 13-Dec 14. - Heavy snow across portions of the Central Rockies, the Northern Plains, the Northern Rockies, and the Northern Great Basin, Fri, Dec 13. - Heavy snow across portions of the Central Great Basin, Fri-Sat, Dec 13-Dec 14. - Heavy snow across portions of the Central Rockies, the Central Plains, and the Central Great Basin, Fri-Sun, Dec 13-Dec 15. - Flooding possible across portions of the Northern Plains. - Flooding occurring or imminent across portions of the Northern Plains. - High winds across portions of the Northeast, Sat, Dec 14. 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