Extended Forecast Discussion NWS Weather Prediction Center College Park MD 104 AM EST Thu Dec 31 2020 Valid 12Z Sun Jan 3 2021 - 12Z Thu Jan 7 2021 ...Weather Pattern Overview... A progressive flow pattern aloft is expected to remain in place for Sunday and going through the middle of next week, and a very active storm track across the northeast Pacific is likely to continue with deep surface lows across the Gulf of Alaska supporting multiple cold fronts moving inland across the western U.S. through the period. A rather impressive 500 mb height positive anomaly becomes established over the North Atlantic, whilst the opposite is true across the eastern Pacific with negative height anomalies. There will likely be an organized storm system across the Plains and then the Ohio Valley region by next Thursday. ...Guidance/Predictability Assessment... The timing and progression of multiple waves across the continental U.S. in the fast zonal flow aloft contributed to the main model differences. There is a strong model consensus with the nor'easter passing southeast of New England for the beginning of the week, although the 00Z UKMET becomes slower and the EC/CMC closer to the coast than the more offshore track of the GFS/GEFS. The 00Z CMC is slower with the next deep trough reaching the West Coast Monday night, and that trend continues through mid-week as the disturbance reaches the Plains, and the EC not as amplified as the GFS/CMC and well to the northeast with the developing surface low. The GFS represents a good middle ground solution for the Wednesday-Thursday storm system over the Midwest and Ohio Valley. For the Pacific Northwest, model agreement has improved compared to yesterday's solutions. The WPC fronts and pressures forecast was primarily derived from an initial blend of the GFS/ECMWF/UKMET through Day 4, and then maintaining some deterministic EC/GFS through Day 7 whilst gradually increasing contributions from the means. ...Weather/Threats Highlights... There is still some uncertainty across the Northeast Sunday and into Monday with the potential nor'easter. A corridor of moderate snow or mixed precipitation is possible from Upstate New York to Maine, and rain closer to the coast. 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 result in several inches of melted equivalent over the course of the period. The highest precipitation totals are expected to occur over the favored coastal terrain and also the Cascades and Sierra, and several feet of snow is likely for the highest elevations. Temperatures across the continental U.S. during the period are expected to be near to above normal for most areas. The greatest departures from normal are forecast to be across the northern Plains for the Sunday to Tuesday time period (15-25F above normal) before moderating to 5-15F above normal for the remainder of the period. No significant intrusions of Arctic air are expected during this forecast period. 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