Extended Forecast Discussion NWS Weather Prediction Center College Park MD 224 PM EST Tue Dec 01 2020 Valid 12Z Fri Dec 04 2020 - 12Z Tue Dec 08 2020 ...Guidance Evaluation/Preferences... Latest guidance continues to agree on the large scale upper pattern featuring a mean ridge from parts of the eastern Pacific and the northern two-thirds of the western U.S. into western Canada, while an elongated weakness (possibly containing an upper low) settles over the southwestern U.S./northwestern Mexico and mean troughing prevails over eastern North America. There is much greater uncertainty for some embedded specifics though. The most significant involves potential surface development over the eastern U.S. during late week/weekend in response to short-range energy that drops down into the Plains/Mississippi Valley, with some eventual influence from northern stream flow as well. There are also meaningful differences with upstream energy expected to reinforce the eastern mean trough aloft along with the ultimate evolution of Pacific shortwave energy that heads into the mean ridge by next Sun. The updated forecast used a blend of the 00Z NAEFS/ECMWF means along with some 00Z UKMET/CMC for the first half of the period followed by trending toward the means exclusively. This model/mean blend provided a good intermediate solution over the East late this week through the weekend while downplaying decreasing-confidence shortwave specifics within the eastern U.S. upper trough and for energy heading into the western ridge from the weekend into early next week. From early in the period there is a dramatic contrast in possible solutions over the East, arising from a combination of differences in handling of central U.S. energy as well as amplitude of Great Lakes/Northeast flow. Recent ECMWF runs have tended to pull the most energy to the south of the western ridge and be the most amplified with the Northeast stream, resulting in weaker/farther east development associated with Mississippi Valley energy. 00Z ECMWF ensemble members show some deference to the operational scenario but enough members in the 00Z cycle differ to suggest more East Coast development potential than in previous ECMWF mean runs. In contrast, latest GFS and to a fair degree GEFS runs have been more consolidated with the MS Valley upper feature leading to slower/westward development. The 00Z UKMET/CMC/CMCens all suggest a scenario closer to the 00Z-06Z GFS in principle but not as slow. Given that historically the ECMWF has occasionally had a tendency to be overdone with how much energy gets pulled to the south of mid-latitude ridges, and remaining non-GFS guidance tilts toward a compromise but closer to the GFS in principle, preferences adjusted closest to the UKMET/CMC and NAEFS/ECens. The 12Z GFS has adjusted faster to compare better to the preferred solution. The 12Z CMC/UKMET are also fairly close in principle though with the UKMET a bit slow near the East Coast. The new ECMWF has changed considerably from its 00Z run--favorably trending slower with the Mississippi Valley upper feature but becoming much deeper than other solutions with the initial western Great Lakes upper trough, leading to a very deep storm by Sat. In spite of the Fri-Sun differences, as the eastern upper trough reloads there seems to be a little more consistency in the means for low pressure near the Canadian Maritimes by next Mon-Tue. Meanwhile solutions are all over the place for the Pacific shortwave energy coming into the West, as can sometimes be the case for such features heading into a mean ridge. Some energy may become embedded within the ridge and could eventually find its way into the mean weakness over the Southwest versus the rest rounding the ridge. There are some local QPF differences associated with this energy's evolution but overall they are not as extreme as for the eastern system. ...Sensible Weather and Hazards... The primary precipitation focus will be in the East and associated with the late-week into weekend system forecast to track from near the Gulf Coast northeastward close to the East Coast. Guidance is still in the process of resolving how this system will evolve but at the moment the best signal for heavier amounts extends from over or just west of the central-southern Appalachians through the Mid-Atlantic and into New England. Most snow should be confined to the northwest periphery of the moisture shield. Whether from persistence of moisture/flow around this system or upstream energy/new surface low development near the Canadian Maritimes, periods of snow could continue over parts of New England and vicinity into early next week. The cold front trailing from the Gulf/East Coast system will also bring rain of varying intensity to the Southeast/Florida. A weakening cold front and associated shortwave energy nearing the Northwest this weekend may bring mostly light precipitation to parts of the region, with low confidence for coverage. Another front approaching the Northwest next Tue could also bring some precip. Coverage is also uncertain for possible Southwest/southern Rockies precipitation that should be mostly light and will depend on the exact depth of an upper low/trough over the region. The upper feature will also have an influence on how much if any rain falls near the western Gulf Coast. Warmest temperature anomalies will likely extend from Montana into the Upper Midwest with multiple days of readings 10-20F above normal at some locations. Best potential for highest anomalies may come toward the end of the period next Tue. Less extreme above normal temperatures should extend into a majority of the West. On the other hand the southern tier will be most consistently below normal, with flow behind the East Coast system also drawing in some below normal air over more of the East. Most of these cool anomalies should be in the single digits though some highs may be 10-12F below normal over southern/eastern Texas and into the Lower Mississippi Valley on Fri. The southern Rockies and vicinity may moderate from below normal highs Fri if the upper low/trough over the region is sufficiently weak. Rausch Additional 3-7 Day Hazard information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php Hazards: - Heavy precipitation across portions of the Appalachians, the Ohio Valley, the Great Lakes, the Mid-Atlantic, and the Northeast, Sat-Sun, Dec 5-Dec 6. - Heavy rain across portions of the Appalachians, the Southeast, the Mid-Atlantic, and the Northeast, Fri-Sun, Dec 4-Dec 6. 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