Extended Forecast Discussion NWS Weather Prediction Center College Park MD 152 PM EST Sun Jan 02 2022 Valid 12Z Wed Jan 05 2022 - 12Z Sun Jan 09 2022 ...Arctic air sets stage for multiple heavy snow threats from the Great Lakes and OH/TN Valleys/Appalachians to the Mid-Atlantic/Northeast... ...Additional rounds of heavy coastal rain and mountain focusing snow from the Northwest/Northern California through the north-central Great Basin/Rockies/High Plains... ...Guidance assessment and uncertainty... Model and ensemble forecast spread and uncertainty remains lower than normal for the most part through medium range time scales. A composite of reasonably well clustered mass field guidance from the 06 UTC GFS/GEFS mean, the 00 UTC ECMWF/ECMWF ensemble mean/UKMET/Canadian along with the 13 UTC National Blend of Models seems to offer a good basis for the WPC product suite for days 3/4 (Wed/Thu). Opted to increase WPC blend weight emphasis on the ensemble means into days 5-7 (Fri/next weekend) to maintain max WPC continuity. The composite tends to mitigate lingering embedded system timing/strength differences and run to run variable lower atmospheric temperature profiles. It should be noted that by Thu/Fri with the Southern to Eastern U.S./coastal system, the plausible but less likely 00/06/12 UTC GFS and a minority amount of GEFS/ECMWF ensemble members are more suppressed by the cold airmass in the wake of a midweek deep Great Lakes low than the 00/12 UTC ECMWF/UKMET/Canadian runs, with a farther offshore track of the southern based coastal low/storm. ...Pattern Overview and Weather/Hazard Highlights... The upper level flow pattern over the nation will be rather amplified through the end of the week with a large scale trough over the east-central U.S. and a broad ridge over the West Coast, before transitioning to a more quasi-zonal flow by next weekend. A deepening storm will track into the Great Lakes and eastern Canada Wed/Thu. Arctic air will dig across the central then eastern U.S. behind the windy and precipitation focusing low to include a threat of plowable snow from the Great Lakes/Ohio Valley to the Northeast. Another quickly developing heavy snow/ice threat with some aforementioned uncertainty is on tap underneath Thu into Fri from the TN/OH Valleys through the Appalachians/Mid-Atlantic to the Northeast by frontal low/coastal storm genesis. Meanwhile, multiple Pacific systems with moderate atmospheric river blueprints will meanwhile take aim at the Pacific Northwest/northern CA this week, that will fuel multi-day coastal rainfall and terrain/mountain enhancing snows. Energies will work inland across a snowy north-central Great Basin/Rockies/High Plains where lift will be further enhanced by post-frontal upslope. Downstream by next weekend, return flow from the Gulf of Mexico is also expected to fuel an emerging enhanced rainfall area inland over the Gulf Coast states/TN Valley in advance of yet another Arctic frontal surge down through the central to east-central U.S.. Temperatures are expected to be frigid across the central/northern Plains to the Upper Midwest through the end of the week with a large surface high of arctic origin governing the weather pattern for this region. Highs in the double digits below zero are once again likely for portions of eastern Montana and into North Dakota, and highs in the teens settling as far south as southern Kansas on Thursday. Some abatement in the severe cold may come to pass across this region ahead of the next arctic surge next weekend. Elsewhere, seasonably cold conditions are expected west of the Continental Divide with readings generally within 10 degrees of climatological averages, and cold next weekend over the East with highs 10+ degrees below normal. Hamrick/Schichtel 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