Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1234 AM EST Tue Dec 10 2019 Valid 12Z Fri Dec 13 2019 - 12Z Tue Dec 17 2019 ...Eastern Seaboard heavy rain threat Friday/Saturday and lead wintry precipitation risk for the interior Mid-Atlantic/Appalachians & Northeast... ...Wet Pacific Northwest to snowy N-Central Great Basin/Rockies flow into the weekend leads to Deep South low/rainfall development early next week... ...Pattern Overview and Guidance/Predictability Assessment... A cold upper vortex will reform and persist over Hudson Bay this period. Over the lower 48, an active and complex pattern remains as several streams laden with a series of impulses interact. Model and ensemble solutions have become better clustered, but forecast confidence remains somewhat tempered by recent run to run continuity issues and flow complexity. Opted for a composite GFS/ECMWF and GEFS/ECMWF ensemble mean solution blended with the National Blend of Models as a basis for the WPC medium range product suite. ...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. 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