Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Tue Dec 29 2020 Valid 12Z Fri Jan 1 2021 - 12Z Tue Jan 5 2021 ***Strong storm system to affect the Eastern U.S. to bring in the New Year, and multiple Pacific systems for the Northwest*** ...Weather Pattern Overview... The weather pattern will remain active to close out the week, with a strong low pressure system developing over the southern Plains by early Friday. The surface low remains well developed 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. Three separate 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 Saturday on the overall synoptic scale pattern and the evolution of the major eastern U.S. storm system. By Sunday, the ECMWF is noticeably slower with the trailing shortwave/upper low that follows behind the major East Coast storm system. The CMC is a bit out of phase with the Pacific storm systems affecting the northwestern states, and by the end of the forecast period, the 00Z GFS becomes quite strong with a northern stream shortwave across south-central Canada compared to the other guidance. 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 UKMET for the first half of the forecast period, followed by slightly less of the GFS and more of the EC and GEFS means for the beginning of next week. ...Weather/Threats Highlights... More active weather is expected across the central and eastern U.S. to bring in the New Year. A corridor of moderate to locally heavy snow is anticipated from eastern Kansas to the northern 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 Iowa and Wisconsin. Some lake effect snow is likely once the surface cyclone exits the Northeast by the weekend. In the warm sector of the low, more unneeded rain is expected from Florida and extending northward across the Mid-Atlantic, Appalachians, and then New England. However, rainfall totals are expected to be a little less than the Christmas storm that hammered the region. Ground conditions are already saturated across many of these areas, and this additional rainfall may result in some instances of increased run-off and rising river levels. Some severe storms are also within the realm of possibility across portions of the Southeast states, along with above normal temperatures. Meanwhile, areas from the Pacific Northwest to central California will continue to see additional rounds of rain and mountain snow late this week and through early next week 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, with an atmospheric river event possible over the weekend for western Oregon and Washington. Several inches of rainfall will be possible for some of these areas. 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