Extended Forecast Discussion NWS Weather Prediction Center College Park MD 252 PM EST Mon Dec 06 2021 Valid 12Z Thu Dec 09 2021 - 12Z Mon Dec 13 2021 ...Back-to-back heavy precipitation expected across the Pacific Northwest... ...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 under increasingly amplified upper-level pattern. This scenario will be monitored in the future model guidance for early next week. Over the Pacific Northwest, heavy mountain snows late this week will likely be followed by the next surge of moisture ahead of the next Pacific front. Models have generally shown a progressively faster/earlier arrival of this round of moisture by the weekend. The WPC medium-range forecasts were based on 40% from the 00Z ECMWF and EC mean, 40% from the 00Z/06Z GFS and the 06Z GEFS, as well as 20% from the 00Z CMC/CMC mean. More of the ensemble means were included for Days 6 and 7. ...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. Highest 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 a low pressure system develops over the central Plains on Friday, snow moving through the central Rockies will exit into the the northern Plains toward the upper Midwest. By Friday night, the snow and mixed precipitation should spread into the Great Lakes with moderate to locally heavy amounts possible. Some wintry mix is also possible across interior New England Friday night/Saturday morning. Farther south, rain with embedded thunderstorms are expected to extend southwestward into the Deep South, with heavy rainfall possible over the Tennessee Valley on Saturday. While the cold front is currently forecast to clear the East Coast by early next week, there is some potential for an upper low to form over the Deep South which would significantly change the weather outlook. Out West, the first wave of heavy mountain snows late this week will likely be followed by the arrival of the next wave of moisture by the weekend. It appears that this renewed atmospheric river has the potential to deliver heavier snowfall amounts for the higher elevations across the Pacific Northwest. 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 from portions of Texas to 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 assumed to build 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 Hazards: - Heavy rain across portions of the Lower Mississippi Valley, the Central Appalachians, the Tennessee Valley, the Southern Appalachians, the Great Lakes, and the Ohio Valley, Fri-Sat, Dec 10-Dec 11. - Heavy snow across portions of the Pacific Northwest and the Northern Great Basin, Thu-Sun, Dec 9-Dec 12. - Heavy snow across portions of the Central Plains, the Central Rockies, the Central Great Basin, the Southern Rockies, California, and the Southwest, Thu, Dec 9. - Heavy snow across portions of the Northern Rockies and the Northern Great Basin, Sat-Sun, Dec 11-Dec 12. - Heavy snow across portions of the Great Lakes, the Upper Mississippi Valley, and the Northern Plains, Fri, Dec 10. - High winds across portions of the Southern Rockies and the Southern Plains, Fri, Dec 10. 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