Extended Forecast Discussion NWS Weather Prediction Center College Park MD 339 PM EST Fri Feb 05 2021 Valid 12Z Mon Feb 08 2021 - 12Z Fri Feb 12 2021 ...Northern Plains to Midwest Arctic outbreak gradually expanding to the south and east... ...Pattern Overview and Guidance/Predictability Assessment... Models and ensembles still advertise a pattern favorable for extremely cold conditions persisting over the central U.S. through next week with gradual expansion of the cold air farther south/east with time. The West will see more moderate temperatures but with a general cooling trend. During the forecast period the eastern half of the lower 48 should see multiple precipitation areas of various types, with a general tendency for the rain-snow line/possible icy mix transition zone to push farther south by late in the week. Consensus continues to show a blocky Canada pattern holding a deep upper low just north of the U.S.-Canada border through at least midweek and possibly beyond, while mean ridging persists over the eastern Pacific. These features will help to maintain cyclonic mean flow over the continental U.S. next week. Teleconnection relationships seem to provide some background on the developing guidance spread that becomes significant over some areas by late in the week. Multi-day mean charts at the D+8 time frame generally highlight two positive height anomaly centers, one over the eastern Pacific and another just northeast of Hudson Bay. The Pacific center favors positively tilted troughing over the West and moderately above normal heights over about the southeastern third of the CONUS (along with near/moderately above normal temperatures in the East). In contrast the core of positive anomalies near Hudson Bay would favor moderately below normal height anomalies over the East (and below normal temperatures over the southern two-thirds of the region), though with agreement on some degree of troughing over the West. Questions include how quickly the southern Canada upper low begins to depart/shear out as initial northern Canada ridging weakens and flow tries to adjust toward some composite of the telecon relationships (GFS/GEFS runs slowest, 00Z ECMWF fastest but prior run slower), along with the relative influence of the Pacific/Canadian features. For the late week forecast, current uncertainty favors a multi-guidance/multi-run approach to yield a deterministic solution that maximizes flexibility depending on how future runs adjust/cluster. As for other details, guidance offers some significant differences in northern tier U.S. upper flow and associated surface evolution over and near New England by midweek. Lack of agreement among fast GFS runs/12Z CMC, slow UKMET runs, and the deep 00Z ECMWF--and low predictability for specifics of the fast flow--recommend an intermediate solution for the time being. Meanwhile there is reasonable agreement with weak shortwave energy filtering through the East Pacific ridge aloft early-mid week but with a general consensus for it to produce lighter precipitation than a number of GFS runs have been showing over parts of the West. The late period blend reflects some degree of west-central U.S. troughing that would expand moisture over the southern/eastern U.S while keeping the forecast between the GFS and 00Z ECMWF extremes. ...Weather/Threats Highlights... Terrain/upslope snow over the northern Rockies and vicinity should begin to slowly taper off on Monday as the main shortwave energy slides eastward to support frontal wave development over the Northeast mostly on Tuesday with some effects possibly lasting into Wednesday. This should lead to a band of potentially meaningful snow from the Midwest into the Northeast from Monday into Tuesday. Over the Midwest/Lower Great Lakes the guidance has decreased totals somewhat over the past day, while models have been conflicted over how the Northeast wave will evolve, so precise effects remain uncertain. Meanwhile, a stationary front across southern Florida as of early Monday will lift northward and likely provide a period of enhanced rainfall from the eastern Gulf Coast region to the southeastern U.S. coast. By later next week, there is an increasing chance for Gulf moisture to return and expand in coverage across the southern tier states and possibly the East in an overrunning pattern north of a stationary front across the northern Gulf. Depending on details of upper flow there is also potential for this front/embedded wave(s) to lift farther northward over the East. With the cold air in place farther inland, a swath of wintry weather is possible from portions of the southern tier into the Northeast later next week but with lower confidence on the specifics. The forecast is fairly consistent in showing coldest anomalies of at least 20-30F below normal early in the week from the northern-central Plains into the Midwest, with some readings possibly as low as 30-45F below normal over parts of Montana. During the rest of the week this core of cold air will remain in place and steadily push to the south and east. Temperatures reaching 20-35F below normal may extend into the southern Plains and much of the Mississippi Valley by later in the week but uncertainty in the eastern U.S. pattern lowers confidence in exactly how cold the East will be at that time. Most locations west of the Rockies will see a general cooling trend. Northern areas should start out near to moderately below normal and trend a bit colder, while the Great Basin/Southwest start the weak moderately above normal and then decline toward normal (or slightly below in parts of the Great Basin). Rausch/Kong 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 Hazards: - Heavy precipitation across portions of the Lower Mississippi Valley, the Tennessee Valley, the Ohio Valley, the Southern/Central Appalachians, the Southeast, and the Mid-Atlantic, Thu-Fri, Feb 11-Feb 12. - Heavy rain across portions of the Southern Plains, the Lower Mississippi Valley, the Tennessee Valley, the Southern Appalachians, the Southeast, and the Mid-Atlantic, Thu-Fri, Feb 11-Feb 12. - Heavy snow across portions of the Middle/Upper Mississippi Valley, the Great Lakes, and the Ohio Valley, Mon, Feb 8. - Heavy snow across portions of the Northeast and the Great Lakes, Tue, Feb 9. - Much below normal temperatures across portions of the Northern Rockies, the Northern/Central Plains, the Upper/Middle Mississippi Valley, the Great Lakes, and the Ohio Valley, Mon-Fri, Feb 8-Feb 12. - Much below normal temperatures across portions of the Central/Southern Plains, the Middle/Lower Mississippi Valley, the Great Lakes, and the Ohio Valley, Wed-Fri, Feb 10-Feb 12. - Much below normal temperatures across portions of the Southwest, the Southern Rockies, the Central/Southern Plains, the Middle/Lower Mississippi Valley, the Tennessee Valley, the Ohio Valley, the Great Lakes, the Central/Southern Appalachians, the Mid-Atlantic, and the Northeast, Fri, Feb 12. 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