Extended Forecast Discussion NWS Weather Prediction Center College Park MD 152 AM EST Wed Dec 08 2021 Valid 12Z Sat Dec 11 2021 - 12Z Wed Dec 15 2021 ...Locally heavy rain is possible particularly in the southern Appalachians Saturday with record-breaking warmth along the Eastern Seaboard ahead of a strong cold front... ...An atmospheric river will spread southward along the West Coast this weekend into early next week bringing coastal rain and heavy mountain snow... ...Overview... A generally progressive and active weather pattern is forecast to occur through the weekend, as a mid-upper trough pushes from the central U.S. Saturday toward the East by Sunday, while a surface low and potent cold front track ahead of it. Record-setting warm temperatures and moderate to locally heavy precipitation are likely in the East ahead of the cold front. Meanwhile, a slow-moving and reloading trough in the eastern Pacific will direct a persistent and moist inflow (atmospheric river) to the West Coast, with heavy coastal rain and mountain snows likely that should spread southward with time. The overall pattern is expected to stagnate Monday and Tuesday with upper ridging building into much of the lower 48 to the east of the Pacific trough, before the trough begins to make its way eastward around midweek. ...Guidance/Predictability Assessment... Model guidance for the 12/18Z yesterday cycle continues to show very good agreement through much of the period with the pattern described above. After some previous waffling, guidance has been consistent for a couple of cycles now showing a more phased (no northern/southern stream separation) trough in the East over the weekend, causing a faster track of the trough and associated cold front, so this trend was maintained in the WPC forecast. Farther west, there remain some model differences with the eastern Pacific trough regarding the distribution of energy and the potential for a closed low to develop (especially with GFS runs) within it. But overall guidance agrees in a slow track of the trough axis southward bringing persistent moist southwesterly flow into the West Coast. There are increasing timing differences among deterministic and ensemble guidance with the trough eventually pushing eastward Tuesday and Wednesday, along with distribution of shortwaves within it. The WPC forecast utilized a multi-model deterministic blend of the 12/18Z guidance early in the period, and was able to hold onto a majority deterministic models blend through much of the period, but with increasing incorporation of the ensemble means especially by day 7 to mitigate individual model differences especially with the Western trough differences. ...Weather/Hazards Highlights... A strong cold front will track through the eastern third of the U.S. on Saturday into Sunday and spread widespread rain with embedded thunderstorms along and ahead of it. There is potential for heavy rain amounts with quick-hitting high rainfall rates for portions of the Southeast, Tennessee and Ohio Valleys, and Appalachians, which could cause flooding/flash flooding issues especially in the more sensitive terrain of the southern Appalachians. With a quicker upper trough and cold frontal track, moisture will likely clear out of the Eastern Seaboard by Sunday, and with high pressure in its wake, dry conditions are likely in the central and eastern U.S. for the first part of next week other than some possibility of increasing showers in the south-central U.S. and Great Lakes by Monday and Tuesday. Meanwhile, persistent moist onshore flow with a weak atmospheric river is likely initially in the Pacific Northwest and northern Rockies Saturday, and then will track south into California by early next week. Enhanced totals are likely along favored terrain, with heavy mountain snow in the Cascades, Sierra Nevada, and northern Rockies, and spreading to the central Great Basin and Arizona by Tuesday. Ahead of the trough and cold front, mild to warm temperatures are forecast for the eastern third of the U.S. on Saturday. Dozens of daily record high maximum and minimum temperatures could be set from the central Gulf Coast to Northeast with temperatures 20-30F above normal. Temperatures should moderate to near normal behind, but by early next week as ridging builds, temperatures are expected to warm to 10-20F above normal on Sunday in the central U.S., with warmth expanding into the eastern U.S. as the workweek begins, and warming further to possibly 30+F above average in central parts of the Plains and Mississippi Valley by next Wednesday. 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