Extended Forecast Discussion NWS Weather Prediction Center College Park MD 130 AM EST Fri Dec 10 2021 Valid 12Z Mon Dec 13 2021 - 12Z Fri Dec 17 2021 ...Atmospheric river will spread southward along the West Coast by early next week to bring 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 over 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, wintry precipitation will spread along the north side of the low track. Downstream, 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 generally modest rainfall focusiung frontal passage. ...Guidance/Predictability Assessment... Model and ensemble forecast clustering and spread through medium range time scale is 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/Canadian and GEFS/ECMWF ensemble means days 3-7 along with WPC continuity and the National Blend of Models. ...Weather/Hazards Highlights... Moist onshore flow with an atmospheric river will cause widespread precipitation in the western U.S. early next week. Heaviest precipitation will spread southward across central and southern California Mon-Tue and eastward over the Great Basin and 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, even down into Southern CA. 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. 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