Extended Forecast Discussion NWS Weather Prediction Center College Park MD 144 AM EST Mon Dec 13 2021 Valid 12Z Thu Dec 16 2021 - 12Z Mon Dec 20 2021 ...Guidance/Predictability Assessment... Model and ensemble forecast clustering remains better than normal for the majority of the systems of concern through medium range time scales. The WPC medium range product suite was primarily derived from a composite of the 18 UTC GFS and 12 UTC ECMWF/UKMET/Canadian along with the 01 UTC National Blend of Models Days 3-5 (Thu-Sat). Added into the mix the compatable 18 UTC GEFS mean and 12 UTC ECMWF ensemble mean Days 6/7 amid gradually growing system variance to maintain max WPC product continuity. 00 UTC guidance seems consistent. ...Weather/Hazards Highlights... A potent upper trough will lift northeastward through the north-central U.S. Thu and cyclogenesis will lead to an expansive area of strong winds together along with a swath of snow on the north side of the low track over the Upper Midwest and upper Great Lakes. Southeast of the deep low track, upper ridging will support some record warm temperatures ahead of the cold front, with positive anomalies upwards to 15-25F shifting from the east-central U.S. to the East Thu-Sat. The trailing portion of the front will meanwhile stall across the South, with an axis of enhanced rainfall and potential of local runoff issues from the southern Plains through the Mid-South/OH Valley with return flow. A developing frontal wave may also bring a swath of wintry precipitation from the OH Valley states through northern New England into the weekend as the enhanced rainfall axis drops southward across the South as cooling high pressure builds across the Plains and to the East. Upstream, a moisture surge with moderate precipitation will work into California and across the Northwest Thu ahead of an approaching Pacific system. This system will gradually weaken into Friday. Moisture from another Pacific system may then modestly impact the Pacific Northwest Fri into the weekend. 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