Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Mon Dec 06 2021 Valid 12Z Thu Dec 09 2021 - 12Z Mon Dec 13 2021 ...Back-to-back heavy mountain snows expected across the Cascades, Sierra Nevada, Intermountain West, and Rockies... ...Overview... A generally progressive pattern is expected to persist through the medium range period with a tendency for a ridge to become more dominant across the South by early next week. A couple of synoptic scale systems moving through this progressive pattern will be of concern. A low pressure system forecast to track through the central Plains to the Great Lakes will likely bring a swath of wintry weather across the interior western U.S. late week to the Great Lakes by the weekend, while heavy rain is possible on Saturday across the Tennessee Valley ahead of a cold front with timing uncertainties. Over the Pacific Northwest, the first wave of heavy mountain snows late this week will likely be followed by another wave of moisture which has the potential to deliver heavier snowfall amounts for the higher elevations. ...Guidance/Predictability Assessment... The latest model guidance this morning exhibits reasonably good agreement with the overall large-scale pattern described above, with increasing uncertainties across the South by early next week. For the low pressure system forecast to develop over the central Plains on Friday, models have generally increased the amplitude of the shortwave dipping into the western U.S., resulting in a stronger/deeper low to develop with a slightly faster forward motion. There has been a tendency to push the associated cold front faster toward the east through the eastern U.S. during the weekend as well. Both the GFS and the ECMWF have attempted to develop a frontal wave over the southern Appalachians during the weekend in some previous runs although their ensemble means do not support such development. By early next week, ensemble means generally support the establishment of a ridge across the South. However, the latest (12Z) ECMWF develops an anomalous upper low over the Deep South amid significant amplification of the frontal wave over increasing amplitude of the upper-level pattern. ...Weather/Hazards Highlights... As the trough deepens in the West, precipitation will spread from the Pacific Northwest and Northern Rockies southward to California into the Great Basin to Central Rockies Thursday and Friday. Favored terrain in Arizona could also see some enhanced activity. Generally expect higher elevation snow and lower elevation rain, with snow levels likely somewhat low as upper troughing amplifies. Parts of Southern California may see the first rainfall in over a month. Then as the western upper trough approaches/pushes into the Plains toward the end of the week, precipitation should track ahead of a low pressure system and a strong cold front. Some wintry weather could spread into the Northern Plains and Midwest/Great Lakes Friday in the cold air north and west of the low track. Then rainfall may become moderate to heavy over parts of the east-central U.S. especially by Saturday with the aid of strengthening Gulf inflow ahead of the strong cold front. Additionally, heavy precipitation chances are forecast to increase in the Pacific Northwest starting around Friday night or Saturday and continuing through the weekend as a potential atmospheric river event sets up, though uncertainty remains with the progression/southward extent of moisture, and will continue to be monitored. Ahead of the trough and cold front, mild to warm temperatures are forecast to be in place in the central U.S. Thursday and Friday, with highs currently forecast to rise into the 80s in portions of Texas and the Lower Mississippi Valley, which could approach or set daily records as the temperatures should be 15-30F above normal. These warm temperatures will push into the Ohio/Tennessee Valleys and Eastern Seaboard on Saturday, while temperatures moderate to near normal behind. The western U.S. can expect below average temperatures especially Friday and Saturday, with the Central Great Basin in particular seeing temperatures 10-20F below normal. Then, as ridging builds across the lower 48 by early next week, temperatures should return to 10-20F above normal across the northern two-thirds of the Plains by next Sunday and Monday. Kong/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