Extended Forecast Discussion NWS Weather Prediction Center College Park MD 302 PM EST Wed Dec 15 2021 Valid 12Z Sat Dec 18 2021 - 12Z Wed Dec 22 2021 18Z Update: The 00/06Z deterministic guidance and ensembles are in very good synoptic scale agreement on Saturday across the continental U.S., although the 00/12Z UKMET diverges from the model consensus by Sunday across southern Canada with a much faster shortwave which becomes out of phase by Monday across the northern Great Lakes and the Northeast. This leads to a surface low well to the east of the other guidance, and thus not favored. Besides that, model agreement remains decent through Tuesday before more noteworthy differences become apparent across the northeast Pacific, with the 06/12Z GFS much stronger with an upper low dropping south from the Gulf of Alaska by next Wednesday. One of the main changes from the overnight forecast issuance is higher QPF totals across central/northern California for the storm system early next week, with the potential for an atmospheric river event. The WPC fronts and pressures forecast was primarily derived with a GFS/ECMWF/CMC blend through Monday, followed by increased use of the GEFS/ECENS means for Tuesday and Wednesday. The previous discussion is appended below for reference. /Hamrick ...Guidance/Predictability Assessment... Models and ensembles remain in good agreement this weekend into Monday 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 the potential to bring a swath of plow-able 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 a slow system approaching. Schichtel/Hamrick 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 Hazards: - Heavy precipitation across portions of the Pacific Northwest and the Northern Great Basin, Sat, Dec 18. - Heavy rain across portions of California, the Central Great Basin, and the Pacific Northwest, Mon-Tue, Dec 20-Dec 21. - Heavy rain across portions of the Lower Mississippi Valley and the Southern Plains, Sat, Dec 18. - Heavy snow across portions of the Central Great Basin, California, and the Southwest, Mon-Tue, Dec 20-Dec 21. - Heavy snow across portions of the Northeast, the Northern Rockies, and the Northern Great Basin, Sat, Dec 18. 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