Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 PM EST Wed Feb 03 2021 Valid 12Z Sat Feb 06 2021 - 12Z Wed Feb 10 2021 ...Major Arctic Outbreak for the Central and Eastern U.S... ...Pattern Overview and Guidance/Predictability Assessment... Models and ensembles continue to agree upon broad mean troughing aloft across the lower 48, to the south of a deep/elongated upper low likely positioned between the Great Lakes and Hudson Bay over multiple days--in turn forced by an upper ridge that builds from Baffin Bay into northern Canada. This pattern will support persistence of Arctic air to the east of the Rockies and a gradual southward/eastward expansion of much below normal temperatures with time. Confidence is somewhat lower for embedded details that will affect precipitation coverage/amounts/type over some regions. For example, Eastern Pacific mean ridging should be weak enough to allow one or more impulses through but there is a lot of spread for shortwave/precip specifics over the West. Pacific shortwave uncertainties will combine with question marks over the details of northern stream flow to lower confidence further for precise coverage of moisture over the eastern half of the country. Over recent days there had been a signal for potential East Coast development around Sun-Mon, involving a wave along the southeastern coast and an approaching wave/front. However recent trends have been toward a farther offshore track in response to flow aloft becoming broader and less amplified. Through 00Z/06Z cycles there was still a broad range with the UKMET on the western side of the spread and the CMC on the eastern side, with an intermediate solution favored. New 12Z runs add more support for the recent offshore trend. Then there are indications that another front with embedded waviness may evolve over parts of the southern/eastern U.S. This part of the forecast will depend on specifics of northern stream troughing/southern Canada upper low position and orientation. Models have not yet stabilized on this aspect of the forecast, favoring a conservative approach to its depiction for the time being. Farther west, a model/mean blend approach appears best for resolving low confidence aspects of shortwave energy filtering through the East Pacific ridge (with fairly wide ensemble spread). In particular the 00Z/06Z GFS runs and the 00Z CMC could be overdone with the depth of their features, with the new 12Z runs looking more reasonable. The desire to reflect the most agreeable aspects of guidance while yielding an intermediate or less-defined solution where conflict existed led to a mostly operational model emphasis during the first half of the period (somewhat more 00Z ECMWF/00Z-06Z GFS relative to the UKMET/CMC, along with modest input of the 06Z GEFS/00Z ECMWF means) followed by a trend toward 40-50 percent ensemble weight. Late in the forecast the GFS/ECMWF runs offset to maintain the desired mean-like flow over the eastern Pacific. ...Weather/Threats Highlights... Cold cyclonic flow on the south side of energetic low pressure reaching eastern Canada on Saturday will promote lingering enhanced snows, including lake effect, along with blustery/windy conditions. Meanwhile upstream, expect upslope snows over the northern Rockies as shortwaves cross over the region and a potent Arctic blast surges southward. This energy and subsequent systems will dive into an amplified central U.S. upper trough position this weekend into next week and produce periodic snow swaths in the reinforced Arctic air. A lingering lead front over the Southeast/Florida should focus weekend showers/thunderstorms. A Southeast coast wave developing along the front should lift northeastward Sunday/Monday with precipitation extending to the northwest of the wave. Recent trends for a more offshore track are lowering the snow/ice threat across Mid-Atlantic and Northeast, but a fairly broad model/ensemble spread up to this point (related to uncertainty over phasing of this wave and mid/upper level energy) and the lingering possibility of a trend reversal at some point require continued monitoring of this feature. Impulses running through the base of the upper trough early-mid next week offer potential for swaths of overrunning precipitation including a snow/ice threat deeper into the cold post-frontal airmass over much of the south-central states and Southeast where modest amounts have an enhanced impact. Areas to the west of the Rockies should see fairly light and scattered precipitation, with uncertainty in specifics due to low confidence for details of one or more weak supporting shortwaves. To start the weekend expect the core of coldest air to extend from the northern half of the Plains into the Midwest and western Great Lakes with readings generally 15-30F below normal. Through the rest of the weekend and into next week expect this core of most extreme anomalies to persist while the cold air also pushes farther south and east--bringing temperatures down to 10-20F below normal over areas from the southern Plains through Ohio Valley and Great Lakes/Northeast. Rausch/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