Extended Forecast Discussion NWS Weather Prediction Center College Park MD 151 AM EST Wed Dec 15 2021 Valid 12Z Sat Dec 18 2021 - 12Z Wed Dec 22 2021 ...Guidance/Predictability Assessment... Models and ensembles remain in good agreement this weekend into Mon in a pattern with above average forecast confidence and continuity. Forecast spread and continuity issues become increasingly evident with the timing of embedded weather features and stream interactions next week. The WPC medium range product suite was primarily comprised of a blend of best clustered solutions from the 18 UTC GFS/GEFS mean and the 12 UTC ECWMF/ECMWF ensemble mean. Blend weighting favors the models this weekend before transition to mainly the compatible ensemble means next week amid growing system uncertainty and run to run variances. WPC preferred guidance and latest 00 UTC runs overall have suppressed southward to off the Southeast a threat for coastal low genesis early-mid next week. ...Weather/Hazards Highlights... A developing frontal low has potential to bring a swath of plowable snow as enhanced by terrain from the upper OH Valley through especially the interior Northeast this weekend. An enhanced rainfall focus with the trailing front will meanwhile sink across the Gulf Coast and Southeast as post-frontal cooling high pressure builds across the Plains and to the East. Guidance has been inconsistent with the prospect of subsequent frontal wave developments and coastal low genesis early next week up the East Coast in part from northern stream energies and/or with latest guidance trends now favoring ejection of a separated southern stream trough that leads to southward suppressed wet lows from the Gulf to offshore the Southeast. Well upstream, dynamic but progressive upper trough and surface system passage will build moisture to support moderate to terrain enhanced heavy precipitation over the Northwest this weekend. Amplified upper trough digging over the eastern Pacific early-mid next week in the wake of this system offers potential to focus ingredients for moderate to heavy precipitation into north-central CA with slow system approach. Schichtel Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook 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