Extended Forecast Discussion NWS Weather Prediction Center College Park MD 225 AM EST Thu Dec 16 2021 Valid 12Z Sun Dec 19 2021 - 12Z Thu Dec 23 2021 ...Guidance/Predictability Assessment... Guidance is in good agreement into early next week in a pattern with above average forecast confidence. Forecast spread and embedded system continuity issues become more of an issue into next midweek both with eastern Pacific system interactions and potential Southeast U.S. coastal low development and proximity. The WPC medium range product suite was derived from a composite of well clustered solutions from the 18 UTC GFS, the 12 UTC ECWMF/UKMET/Canadian and the 01 UTC National Blend of Models Days 3-5 (Sun-Tue). This solution offers good WPC continuity and has favorable ensemble support. Switched focus to the still compatible ensemble means into days 6/7 amid growing system uncertainty and run to run variances. Latest 00 UTC guidance overall remains in line with this forecast plan. ...Weather/Hazards Highlights... Northern stream upper trough passage and an exiting frontal low may support some lingering snows Sun over northern New England and off the lower Great Lakes. Locally enhanced rain should focus with waves along a trailing/stalling front sunk through the Gulf Coast and Southeast into early next week. Guidance has become better clustered with approach of a separted/amplified southern stream shortwave, but still offers less than stellar run to run continuity into next midweek with the depth and coastal proximity of a potentially well organized Southeast U.S. coastal low and inland extent for enhanced wrapped rains. Manually adjusted WPC progs show a decent coastal low offshore given favorable but steadily progressive upper trough support and improving guidance signal. Upper trough and surface system passage will support lingering terrain enhanced precipitation over the Northwest Sun. Details remain less certain upstream over the eastern Pacific, but there remains a good guicance signal that amplified upper trough digging early next week would offers potential to focus ingredients for moderate to heavy precipitation for north-central CA considering slow system approach, developing moisture plume approach and favored southern facing terrain enhancement. Some heavier model qpf solutions within the full envelope of solutions would better portend local runoff issues. Main precipitation focus, including heavy Shasta and Sierra snows, is set to spread down across central-southern CA through through next midweek, with additional activity spreading across Great Basin terrain to the Rockies with inland shortwave system progression. There is also then an emerging guidance signal to dig upper trough energy from Alaska to renew precipitation into the Pacific Northwest by next Wed/Thu. 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