Extended Forecast Discussion NWS Weather Prediction Center College Park MD 136 PM EST Mon Feb 01 2021 Valid 12Z Thu Feb 04 2021 - 12Z Mon Feb 08 2021 ...Midweek Southwest U.S. system to progress to the Plains/Midwest/Northeast later week followed by major arctic outbreak over the central and eastern U.S. by the weekend... ...Guidance/Predictability Assessment... Aside from the 00Z UKMET, the rest of the deterministic guidance showed very good agreement and clustering at the start of the period such that the WPC blend was comprised mainly of the 00Z ECMWF/06Z GFS/00Z CMC for days 3-5. The 00Z UKMET was too deep/progressive with the upper level as it works across the Great Lakes. Other areas of uncertainty were across the Southwest where a potential closed low pinches off as the longwave trough develops over the CONUS. The ECMWF features this solution the most while the GFS and CMC to some degree are a bit more progressive. The WPC blend resulted in a compromise solution with some favoring toward the ECMWF idea. In the big picture sense, all the deterministic models and ensembles show a significant longwave trough over the central CONUS by the end of the period which should allow for a significant intrusion of Arctic air to overspread the northern tier and portions of the east. ...Weather/Threats Highlights... Upper troughing driving through the Southwest/Rockies will produce a swath of terrain enhanced snows into Thursday. System ejection into the Plains still offers additional northern stream trough phasing uncertainties and a complex frontal structure, but suspect that a deepening low will organize and lift to the Great Lakes into Friday. Enhanced snows are expected near and to the north of the main cyclone track from the windy Midwest/Great Lakes to the Northeast. Upstream, expect a swath of heavy terrain snows are forecast from the Northwest to the north-central Rockies late week with a series of potent shortwaves that subsequently dig into an emerging/amplified central U.S. upper trough position this weekend. This will act to drive a frigid arctic airmass fully southward through the central then eastern U.S.. Interaction with a lingering wavy front trailing from the aforementioned lead system over the East may favor Piedmont/coastal rains and inland snows from the OH Valley/Appalachians to the Northeast, though there is much uncertainty on any coastal storm from the Mid-Atlantic to Northeast late in the weekend to early next week. Temperature departures will reach 15-30F over the northern tier and Great Lakes by day 5/6 and overspread the East Coast by day 7. For portions of the northern Plains and Upper Midwest, readings may not break 0. The much below normal temperatures will even spread southward into portions of the Deep South and north Florida where daytime high departures of 5-15F are possible. 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