Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1257 AM EST Sat Dec 11 2021 Valid 12Z Tue Dec 14 2021 - 12Z Sat Dec 18 2021 ...Atmospheric river will spread southward along the West Coast by early next week bringing a threat of heavy rain and heavy mountain snow... ...Record warmth to spread from the central U.S. through the Mid-Atlantic/Northeast Tuesday-Thursday... ...Overview... Amplified upper troughing will dig through the West early next week. This will promote increasing and widespread rains with local runoff issues and terrain enhancing heavy snows over much of the West/Rockies, first along the coast then progressing southward and eastward, as an atmospheric river event ensues. As the main system lifts northeastward through the Plains next Wed-Thu and spawns cyclogenesis and an associated large enhanced wind field, wintry precipitation will spread along the north side of the low track across the northern Plains and Upper Midwest/Upper Great Lakes and eroding high pressure dammed northern New England. Meanwhile underneath, upper ridging will build from the south-central U.S. to the East. Warm sector record warmth will include temperatures upwards to 20-35F above normal Tue-Thu spreading from the central U.S. to the Mid-Atlantic/Northeast prior to the arrival of generally modest rainfall focused along a trailing cold front. ...Guidance/Predictability Assessment... Model and ensemble forecast clustering and spread through most of the medium range remains much better than normal, bolstering forecast confidence. The WPC medium range product suite was primailry derived from a seemingly reasonable and consistent composite blend of the GFS/ECMWF/UKMET/Canadian and GEFS/ECMWF ensemble means days 3-6 along with WPC continuity and the National Blend of Models (NBM). Forecast spread increases a bit more rapidily into Day 7, but the GEFS/ECMWF ensemble means remain a still compatible forecast option along with the NBM. ...Weather/Hazards Highlights... Moist onshore flow with an atmospheric river will fuel widespread precipitation in the western U.S. early next week. Heaviest precipitation will spread southward across central to southern California Tue and out across over the Great Basin, Four Corners states and Rockies through midweek. Enhanced totals are likely along favored terrain, with heavy mountain snow especially in the Sierra Nevada and then the central Great Basin and Rockies. Heavy coastal to interior rains are expected in California. Some locally higher rainfall rates may cause some flooding issues, especially for sensitive burn scar areas. Temperatures will become below normal in the West underneath the upper trough, with temperatures 10-15F below average for parts of California and Nevada. Downstream, deep midweek north-central U.S. cyclogenesis and subsequent notheastward track into Canada will spread snow across the northern Plains and Upper Midwest/Upper Great Lakes. Snows will combine with enhanced winds around the large circulation and the heaviest wrap-around focus may setup over the Upper Midwest. 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