Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1232 PM EST Sun Dec 08 2019 Valid 12Z Wed Dec 11 2019 - 12Z Sun Dec 15 2019 ...Much below normal temperatures linger for the Upper Midwest/Great Lakes Midweek... ...Emerging wet flow over the Northwest... ...Cold air damming lends a wintry risk over the East with potential low developments... 18 UTC Update... Model consensus was relatively good through at least the first half of the forecast period, and a majority 00Z ECMWF/06Z GFS-based blend was used during days 3-4 (Wed-Thu). Shortwave energy crossing the southern tier Thu-Fri along with additional northern stream shortwave energy moving into the north central U.S. at the same time will be prominent features in the latter half of the forecast period, particularly with respect to the eventual development and track of a low pressure system across the eastern U.S. As is often the case, the degree of phasing/interaction between these two initially separate features will be a determining factor in the eventual evolution. The last couple runs of the GFS have phased the shortwaves relatively quickly, resulting in a Gulf of Mexico surface low that tracks much farther west (across the Appalachians or Ohio Valley) relative to the less phased ECMWF/CMC/UKMET, which track much closer to the Eastern Seaboard. At this time, the latter idea is the preferred one, with the 00Z GFS being the most recent run that looked more like this. Thus, the forecast during days 5-7 (Fri-Sun) was comprised of gradually increasing weight on ensemble means (ECENS and GEFS), with some continued use of the 00Z ECMWF/00Z GFS. Overall, no major changes were made to the ongoing forecast. Ryan Previous Discussion (issued at 0514 UTC)... ...Pattern Overview and Guidance/Predictability Assessment... A cold Hudson Bay upper vortex lingers next week. A notable upper trough rounding underneath shifts over the Midwest/Great Lakes midweek then ejects over the Northwest, with less amplified but still active troughing/cyclonic flow for much of the rest of week over the lower 48. This occurs as an early week North American West Coast ridge is rounded by eastern Pacific shortwaves that cut inland from the Northwest to the north-central Great Basin/Rockies. This is also as an active southern stream provides multiple trough energy passages underneath over the U.S. southern tier to further focus inflow/activity, culminating with potential QPF/wintry enhancement later week from the Southeast to up the Eastern Seaboard. The WPC medium range product suite was primarily derived from a composite blend of the GFS/ECMWF and GEFS/ECMWF ensemble means, lending increasing weighting in this blend from the deterministic models to ensembles through the period amid growing forecast spread over time. ...Weather Highlights/Threats... Great Lakes effect snows will persist midweek. Temperature anomalies of 15 to 30 degrees below normal will be over the Upper Midwest and Upper Lakes with reinforcing Arctic shots. Out West, the pattern should become quite wet across the Pacific Northwest by mid-later this week as Pacific shortwave energies punch through rounding West Coast flow. Dynamic support for activity will extend inland with impulse progressions, with moderate to heavier snows to continue on favored terrain of the north central Great Basin/Rockies into next weekend. Models continue to show support for the development of a significant waves of low pressure along a surface front in the Gulf of Mexico by late next week, tracking northeastward across the Southeast U.S. and up the Eastern Seaboard into next weekend. Spread is considerable with the timing and the eventual tracks of these features, but the pattern seems to favor 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 a slow to recede lower atmospheric cold air damming over the East may offer an emerging snow/ice threat to monitor. 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