Extended Forecast Discussion NWS Weather Prediction Center College Park MD 139 PM EST Thu Dec 09 2021 Valid 12Z Sun Dec 12 2021 - 12Z Thu Dec 16 2021 ...An atmospheric river will spread southward along the West Coast early next week bringing coastal rain and heavy mountain snow... ...Record warmth likely for much of the central portion of the Lower 48... ...Overview... Upper troughing in the East on Sunday will be replaced by burgeoning upper ridging around midweek as lead and successive troughing digs through the West. This will promote increasing and widespread precipitation through much of the West, first along the coast then progressing southward and eastward, as a modest atmospheric river event ensues. To the east, strong upper ridging and southerly flow ahead of the western system will bring in well above normal temperatures to the central states with daily record highs likely for many locations. As the western system moves northeastward through the northern Plains next Wed-Thu, wintry precipitation may spread across the High Plains along the north side of the surface low track. ...Guidance/Predictability Assessment... Through the 00Z/06Z cycle, the deterministic models showed very good agreement overall with the pattern evolution of the western trough and downstream ridge, with expected detail differences. By next Tue-Thu, upstream flow from the Gulf of Alaska may spawn a trailing upper trough that could influence where another round of precipitation may impact the West (lots of north-south spread), and opted to trend toward the GEFS mean and ECMWF ensemble mean while favoring the ECMWF and Canadian for some added details. The GFS was on the southern end of the spread but remained a plausible solution. Trend has been a bit slower by next Wed-Thu with the trough and resultant area of low pressure moving through the Plains to the Upper Midwest, dependent on the sharpness of the upper pattern. This may support a modest area of low pressure with snow on the northern side of the low but rain for many areas even into the Great Lakes with mild temperatures. ...Weather/Hazards Highlights... Moist onshore flow with a weak atmospheric river will cause widespread precipitation in the western U.S. through the first half of next week. Precipitation is forecast for the Pacific Northwest, northern Rockies, and especially northern California on Sunday, which will spread southward into southern California Monday and eastward into the Great Basin and Four Corners states on Tuesday. Enhanced totals are likely along favored terrain, with heavy mountain snow especially in the Sierra Nevada and portions of the Rockies. Coastal rain is expected in California, even down into Southern CA, and some locally higher rainfall rates may cause some flooding issues, especially for sensitive burn scar areas. Appreciable rainfall and mountain snow is also likely into the Great Basin and central Rockies as the system moves eastward. Temperatures will be near to somewhat below normal in the West underneath the upper trough, with temperatures 10-20F below average for parts of California and Nevada. Farther east, increasing warmth in the central and eastern U.S. via strong ridging aloft will lead to temperatures about 10-20F above average on Sunday but increasing to 20-35F above normal by Wednesday. Parts of the Plains/Mississippi Valley could see highs into the 70s which will likely break daily records for multiple locations. Record temperatures will be possible/likely from the Southern Plains to Great Lakes region ahead of the front, with temperatures 10-20F above average forecast to expand into the East. Any widespread precipitation is not likely in the central/eastern U.S. until next Wednesday as the western system lifts toward/into Canada. Fracasso/Tate 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