Extended Forecast Discussion NWS Weather Prediction Center College Park MD 141 AM EST Thu Feb 04 2021 Valid 12Z Sun Feb 07 2021 - 12Z Thu Feb 11 2021 ...Major Arctic Outbreak for the Central and Eastern U.S... ...Pattern Overview and Guidance/Predictability Assessment... Medium range guidance strongly supports 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. This pattern will support persistence of Arctic air, mainly east of the Rockies and a gradual southward/eastward expansion of much below normal temperatures with time. Confidence is low for local details that will affect precipitation coverage/amounts/type due to significant embedded shortwave uncertainties and focus within the larger scale flow. Accordingly, the WPC medium range product suite was primarily derived from a composite of reasonably well clustered mid-larger scale guidance from the 12 UTC GFS/ECMWF/Canadian and GEFS/ECMWF ensemble means along with the 01 UTC National Blend of Models. This blend tends to mitigate the less predictable smaller scale features and maintained decent WPC product continuity. Latest 18 UTC and 00 UTC models and ensembles offer varied trends with these harder to predict systems and stream interactions, lowering forecast confidence to a degree. ...Weather/Threats Highlights... Terrain/upslope snows are expected for the northern Rockies as shortwaves cross the region and potent Arctic blasts surge southward. These systems will dive into an amplified central U.S. upper trough position to produce periodic, but uncertain snow swaths in the reinforced Arctic air. The best bets may be from the Midwest/Great Lakes to the Northeast. Well south, a lingering lead front and secondary surges over the Southeast will pool moisture for showers/thunderstorms through this period. A Southeast coast wave developing along the front will lift northeastward Sunday with precipitation extending to the northwest of the wave, but now on a more offshore path. Impulses running through the base of the upper trough next week offer modest moisture/precipitation, but any swaths of overrunning snow/ice deeper into the cold post-frontal airmass from the south-central states to the Mid-South/TN Valley and Appalachians would be locally impactful. These systems then offer potential up the east coast considering how far west the main upper trough may settle. Areas to the west of the Rockies should see fairly light and scattered precipitation, with with uncertainty in specifics due to low confidence for supporting shortwave details. 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 this weekend through next midweek. Cold temperatures will also spread robustly far south and east, bringing temperatures down to 10-20F below normal over areas from the southern Plains through the TN/OH Valleys and Great Lakes. 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