Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1229 PM EST Wed Dec 30 2020 Valid 12Z Sat Jan 02 2021 - 12Z Wed Jan 06 2021 ...Weather Pattern Overview... A fast quasi-zonal flow pattern is expected across much of the continental U.S. during the period (Jan. 2-6) that will feature several storm systems impacting the West Coast with several episodes of heavy precipitation. In the wake of the East Coast storm system, a secondary area of low pressure is forecast to develop off the Mid-Atlantic and potentially brush the Northeast and New England with precipitation. The middle of the CONUS looks to be relatively dry while temperature anomalies are above normal for much of the CONUS. ...Guidance/Predictability Assessment... The timing and progression of multiple waves across the CONUS in the fast zonal flow was the primary model differences noted this cycle. The 00Z UKMET was less favored to discounted as it appeared too fast with shortwaves from the Pacific Northwest to central U.S. while the 00Z ECMWF, 06Z GFS, and 00Z CMC offered reasonable utility through day 5/6. In the wake of the low pressure and East Coast storm system, a secondary wave of low pressure is now expected to form off Cape Hatteras then track northeast. This cycle of model guidance has trended to the northwest from continuity and brings the low closer to Cape Cod on Day 5. This bears watching for any possible winter precipitation on the northwest side but its speed and lack of deep forcing may limit a heavy precipitation signal. By day 6 and 7, the messy and more uncertain forecast led to a higher weight of the 00Z ECENS and 06Z GEFS means. ...Weather/Threats Highlights... Impacts from the secondary wave of low pressure off the Northeast and New England are still uncertain with the recent guidance trending back to the northwest. An area of snow or mixed precipitation is possible but heavy amounts do not look likely at this point. Elsewhere, the active weather is expected to be mainly confined to the West Coast where multiple rounds of heavy precipitation (mountain snows, valley rains) could total several inches over the course of the period. he 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 (nearing 12 inches) of rainfall will be possible for some of these areas. Temperatures across the CONUS during the period are expected to be mostly above normal. The largest departures from normal are forecast to be across the Eastern U.S. and northern Plains on Day 3 (10-20F above normal) before moderating to 5-10F above normal for the remainder of the period. No significant intrusions of Arctic air are expected during the period at this time. Taylor 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