Extended Forecast Discussion NWS Weather Prediction Center College Park MD 217 AM EST Mon Dec 28 2020 Valid 12Z Thu Dec 31 2020 - 12Z Mon Jan 4 2021 ***More bad weather expected for the Pacific Northwest and the Eastern U.S. to end the year 2020*** ...Weather Pattern Overview... The weather pattern will remain active going into the end of the week, with a strong southern stream disturbance across the Gulf Coast region spurring surface cyclogenesis over the western Gulf Coast on Thursday. The surface low develops further as it tracks towards the Great Lakes and Northeast states by Saturday morning, with a strong cold front following in its wake. A quasi-zonal mid-upper level flow pattern develops by the beginning of next week as the eastern U.S. upper trough departs. Multiple northeast Pacific storm systems will impact the Pacific Northeast and northern Rockies, leading to a rather unsettled start of the new year. ...Guidance/Predictability Assessment... The 00Z guidance suite, along with the 12Z ECENS, is in relatively good agreement through Friday on the overall synoptic scale pattern and the evolution of the major eastern U.S. storm system. By Saturday night, the GFS becomes faster with the shortwave crossing the Northeast, and the CMC becomes more out of phase with the other guidance near the Pacific Northwest with the shortwaves approaching the coast. There has also been a stronger model signal for a trailing, positively tilted shortwave disturbance crossing the Ohio Valley Saturday night and into Sunday, and will continue to be monitored. The WPC medium range fronts and pressures were primarily derived from a blend of the 12Z ECMWF/EC mean, the 18Z GFS/GEFS, and some of the 12Z CMC for the first half of the forecast period, followed by less of the GFS and more of the EC and GEFS means for the weekend and into next Monday. ...Weather/Threats Highlights... Another impactful weather system is expected across the central and eastern U.S. to close out the year. A corridor of moderate to locally heavy snow is becoming more likely from the central Plains to the Great Lakes region as the surface low lifts northeastward, and depending on the eventual strength of the low and the resulting pressure gradient, blizzard conditions may once again be possible for some portions of the Midwest states. Some lake effect snow is likely once the surface cyclone exits the Northeast by the weekend. In the warm sector of the low, more unwelcome rain is expected from the Gulf Coast and extending northward across the Mid-Atlantic, Appalachians, and then New England, with widespread 1-3 inch rainfall totals within the realm of possibility. Ground conditions are already saturated across many of these areas, and this additional rainfall may result in some instances of flooding and rising river levels. Given some of the similarities from the Christmas storm, there could also be some severe thunderstorms to end the year across portions of the Deep South and the Southeast. Meanwhile, areas from the Pacific Northwest to northern California will continue to see a return of rain and mountain snow late this week in episodes of enhanced onshore flow with the approach of multiple Pacific systems in progressive flow aloft. The highest precipitation totals are expected to occur over the favored coastal terrain and also the Cascades and Sierra. Additional snow showers will track inland over the Northern Rockies. 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 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