Extended Forecast Discussion NWS Weather Prediction Center College Park MD 101 AM EST Sat Jan 09 2021 Valid 12Z Tue Jan 12 2021 - 12Z Sat Jan 16 2021 ...Overview... The latest models and ensembles continue to advertise an upper flow evolution that features a strengthening western ridge and late week deepening of the downstream eastern U.S. trough, while mean troughing from eastern Canada to northern Mexico prevails (with multiple streams) through at least mid-week. By next weekend, the western U.S. ridge will shift eastward as another shortwave enters the Pacific Northwest which should help to erode/push out the Eastern trough. The heaviest precipitation in the medium range should focus in the Northwest and shortwaves/frontal systems head into and around the mean ridge aloft. Elsewhere, precipitation should be exiting the East early in the period with some areas of precipitation (mostly snow) spreads into the Great Lakes and vicinity by late week. ...Guidance/Predictability Assessment... The transition towards a larger scale ridge/trough pattern lends itself to better predictability mid to late in the period, with above average agreement on the overall pattern. Early week smaller-scale features continue to produce lingering uncertainty, especially with a shortwave crossing the Northeast and intensity and timing of energy ejecting across the Southern U.S. from northern Mexico. Out west, there are detail differences needing to still be resolved with a shortwave traversing the U.S./Canada border in the Northwest, eventually amplifying over the Great Lakes by later in the week. The latest run of the CMC continues to be more amplified and a little faster than the better consensus, as represented by the GFS/ECMWF and ensemble means. Another shortwave enters the Pacific Northwest by next weekend, with poor run to run continuity with respect to both strength and timing. The WPC progs were based on a blend of the 12z/Jan 8 ECMWF along with the latest 2 runs (18 and 12z/Jan 8) of the GFS. The CMC was not used tonight given the differences in the west described above. By day 5 and beyond, began to incorporate increasing weighting of the ECENS/GEFS means to help mitigate the smaller scale uncertainties. This approach maintains good WPC continuity with the previous shift. ...Weather/Threats Highlights... Precipitation should be exiting the Eastern U.S. as the medium range period begins, though lingering showers will remain possible across the Florida peninsula. In the Pacific Northwest, precipitation (rain and higher elevation snows) will be heaviest during the first half of the week with the highest totals likely along favored terrain of the coastal ranges and Cascades. As the system moves inland, some moisture should eventually reach the northern Rockies as another shortwave/front brings another episode of at least locally moderate precipitation to the Northwest towards the end of the week. The deepening upper trough and leading frontal system reaching the East by Thursday to Friday should bring a combination of synoptic and lake effect/enhanced snows from the Great Lakes into the northern/central Appalachians. The temperature forecast has been consistent in showing well above normal temperatures across the northern tier through much of the week. From Montana into the Upper Midwest, expect daytime high anomalies 10-30F above normal Tuesday and Wednesday, although much above normal morning lows should persist through the week. Upper ridging will signal a warming trend to the Western U.S. through the period, while a cool start on Tuesday across the South should moderate back to normal by Wednesday and beyond. Santorelli 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