Extended Forecast Discussion NWS Weather Prediction Center College Park MD 250 PM EST Mon Dec 07 2020 Valid 12Z Thu Dec 10 2020 - 12Z Mon Dec 14 2020 ...Overview... An upper low exiting the Southwest and an incoming Pacific system into Washington/Oregon will promote troughing over the central states by the weekend. Progressive flow will help lift that system to the northeast as another Pacific system comes into the Northwest by early next week. Overall pattern is devoid of arctic air as much of the lower 48 will see near to above normal temperatures over the period but with a couple below normal days behind departing cold fronts. Rainfall will be on the modest side overall with some snow on the northern side of the systems. Coastal/mountainous areas of WA/OR will see the heaviest amounts of precipitation. ...Model Guidance Evaluation and Preferences... Ensembles were in good agreement through the period with the longwave pattern and evolution. The deterministic models showed varying degrees of clustering with the means as the UKMET, then Canadian, then GFS departed from the overall consensus through the period. 00Z ECMWF showed the best agreement overall with the preference. Lead system will track northeastward through the central Plains to the Great Lakes and into Canada Fri-Sun with an advancing warm front into the Northeast and trailing cold front to the Gulf of Mexico. Models have varied on timing/track/depth of the sfc low with some north-south spread seen in the recent guidance. Ensembles have not settled on a solution so a middle-ground/blended solution was used to start. Thereafter, more spread in the Pacific flow precluded the more aggressive options for now, but expect a system offshore this weekend to work inland late Sun to early Mon per the consensus. ...Weather/Threats Summary... Temperatures will be above normal by about 5-20 degrees ahead of the lead Southwest system, initially over the Plains Thursday then to the Great Lakes and Midwest Friday and into the Northeast this weekend. Cooler air behind the system will only yield temperatures about 5 degrees below normal. Rainfall ahead of the lead system will expand across the Plains/Midwest/Upper Midwest but limited Gulf moisture feed may keep amounts light to modest. Some snow is possible on the northern side from the northern Plains to the upper Great Lakes. The Northwest will see more appreciable precipitation with continued onshore flow and especially as a warm front lifts along the coast late Fri to early Sat. Expect another increase in low elevation rain and mountain snow Sun-Mon as the cold front moves in. Amounts will be modest to perhaps locally heavy overall as multi-day totals could exceed 1-3 inches areal average (more in the mountains, less in the valleys). Fracasso 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 California, the Pacific Northwest, and the Northern Great Basin, Sat-Sun, Dec 12-Dec 13. - Heavy snow across portions of the Central/Northern Great Basin and the Central/Northern Rockies, Sun-Mon, Dec 13-Dec 14. 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