Extended Forecast Discussion NWS Weather Prediction Center College Park MD 120 PM EST Sat Dec 07 2019 Valid 12Z Tue Dec 10 2019 - 12Z Sat Dec 14 2019 ...Much below normal temperatures for the Northern Plains/Midwest/Great Lakes... 18 UTC Update... No major changes were made to the ongoing forecast in this update. The GFS/ECMWF were heavily used in the forecast for days 3-5 (Tue-Thu). For days 6-7 (Fri-Sat), ECENS/GEFS ensemble means comprised a majority of the forecast, with some continued use of the deterministic ECMWF as well. Models continue to show support for the development of a significant wave of low pressure along a surface front in the Gulf of Mexico by late next week, potentially tracking northeastward across the Southeast U.S. by early next weekend. Spread is considerable with respect to the timing and the eventual track of this feature, however, which supported a solution close to the ensemble means. Updated winter weather outlooks this morning showed an expanded region of 10-30 percent probabilities for days 6-7 across portions of the Mid-Atlantic and Northeast, associated with this system. Ryan Previous Discussion (issued at 0721 UTC)... ...Pattern Overview and Guidance/Predictability Assessment... A southern stream flow associated with an amplifying Southwest upper trough early week still offers some uncertainty in terms of magnitude and ejection timing for the U.S. southern tier Tuesday into Thursday, but guidance clustering has improved to bolster forecast confidence. Overtop, a cold Hudson Bay upper vortex lingers next week. A notable upper trough rounding underneath amplifies to the central then eastern lower 48 states Tuesday/Wednesday, with less amplified troughing/cyclonic flow for much of the rest of week. An upstream ridge amplifies up the North American West Coast early week before being weakening midweek by eastern Pacific shortwaves that cut inland from the Northwest to the snowy Rockies. A favored and reasonably well clustered GFS/ECMWF and GEFS/ECMWF ensemble solution suggests this energy may subsequently act to amplify central to southern/eastern U.S. upper troughing. This may lead to organized cyclogenesis to focus moisture and lift to produce a risk of heavy rainfall across the Southeast. Subsequent system deepening, northward lifting, and interaction with receding lower atmospheric cold air damming over the East heading into next weekend may offer an emerging wintry snow/ice threat to the north to monitor. ...Weather Highlights/Threats... There remains a strong signal for a swath of moderate rainfall ahead of a main cold front from the lower MS Valley to the Appalachians/Mid-Atlantic and southern/eastern New England Tuesday. Activity will shift offshore midweek. Frontal waves and a dynamic upper trough/jet also supports some enhanced snows on the cooled northwest periphery of the precipitation shield from Appalachians to the Northeast. Lake effect snow should also persist across the Great Lakes in the wake of the cold front which is particularly cold. Polar plunges are expected behind the cold front offer temperature anomalies of 15 to 30 degrees below normal centered over the Upper Midwest with reinforcing shots of cold air. Over the West...a building West Coast upper ridge should keep most of the West dry through early next week beyond some Rockies snows. The pattern should become more wet and elevation snowy for especially the Pacific Northwest, but also inland to the Rockies as shortwave energies round the northern part of the ridge later week. Schichtel Additional 3-7 Day Hazards information can be found on the WPC medium range hazards 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