Extended Forecast Discussion NWS Weather Prediction Center College Park MD 353 PM EST Tue Dec 07 2021 Valid 12Z Fri Dec 10 2021 - 12Z Tue Dec 14 2021 ...Accumulating snowfall likely for the northern Plains to Midwest/Upper Great Lakes with heavy rain across parts of the east central U.S. ahead of the cold front Friday into Saturday... ...Atmospheric River to spread southward along the West Coast late this week into early next week bringing heavy coastal rain and mountain snow... ...Overview... A generally progressive and active weather pattern is forecast to occur through early next week, as a mid-upper trough pushes from the Intermountain West early Friday toward the East by Sunday, while a surface low and potent cold front track ahead of it. A swath of wintry weather is likely to spread from the Central Rockies into northern portions of the Plains to the Midwest/Upper Great Lakes on Friday behind the low, while heavy rain is possible late this week across the east-central U.S. along and 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. ...Guidance/Predictability Assessment... The latest 00z/06z guidance continues to show very good agreement through much of the period and a general deterministic model blend sufficed, especially for the early part of the period. The latest cycle of models seems to have trended towards a more progressive and elongated trough through the East. The main exception to this is the UKMET which continues to show the potential for stream seperation with the trough, leaving behind a compact closed low over the Southeast Sunday-Monday. There are many of the EC ensemble members (and the mean) which also supports this scenario too (though the GEFS, NAEFS, and CMCE means are more progressive). Thus, there remains plenty of uncertainty regarding the late weekend evolution of this trough, which of course would impact how quickly precipitation moves through and overall flood concerns. The WPC forecast for today leans towards the better consensus of a progressive, intact trough from the deterministic GFS, ECMWF, and CMC along with the GEFS and NAEFS means. Out West, the guidance also shows good synoptic agreement regarding the trough dropping south along the West Coast and a potential atmospheric river event for the Pacific Northwest and California. Some questions in the details remain however, and there are some timing differences with the trough axis entering the West Coast by day 7. Incorporating more of the ensemble means by the end of the period helped mitigate these differences. Overall, this update to the WPC medium range forecast remained fairly consistent with the overnight package. The only notable changes were regarding a faster cold front through the East, given the current model trend. ...Weather/Hazards Highlights... The surface low pressure system tracking northeastward into the Midwest/Great Lakes Friday to early Saturday will likely spread snow on its backside from the Central Rockies into northern portions of the Plains and into the Midwest, with moderate to locally heavy snow possible. The trailing cold front associated with this low will track through the eastern half of the U.S. 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 that could cause flooding/flash flooding issues for portions of the Lower Mississippi, Tennessee, and Ohio Valleys on Friday and especially into Saturday. Then, how much rain falls along the Eastern Seaboard will depend on the timing of the upper trough and cold front exiting into the Atlantic, which remains uncertain. Gusty winds, especially across portion of the southern High Plains on Friday, may accompany the low consistent with a tight pressure gradient. Meanwhile, persistent onshore flow is likely initially in the Pacific Northwest and northern Rockies and then could 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. Ahead of the trough and cold front, mild to warm temperatures are forecast to be in place in the central U.S. on 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 and potentially record-setting 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 much of the lower 48 by early next week, temperatures should return to 10-20F above normal across the central U.S. by Sunday and lasting/warming further through Tuesday. Santorelli/Tate Hazards: - Heavy precipitation across portions of the Pacific Northwest, Fri-Sat, Dec 10-Dec 11. - Heavy precipitation across portions of southwestern Oregon into the northern half of California as well as the Sierra Nevada, Sun-Mon, Dec 12-Dec 13. - Heavy precipitation along the Sierra Nevada and into portions of southern California, Tue, Dec 14. - Heavy rain from the interior Deep South to the Tennessee and Ohio Valleys mainly west of the spine of the Appalachians, Fri-Sat, Dec 10-Dec 11. - Heavy snow across portions of the northern Rockies, Sat-Sun, Dec 11-Dec 12. - Heavy snow from the northern/central Plains through the upper Midwest into the upper Great Lakes, Fri-Sat, Dec 10-Dec 11. - High winds across portions of the southern Rockies to the southern High Plains, Fri, Dec 10. - High winds across portions of northwestern Montana, Sat, Dec 11. - High winds across portions of eastern Wyoming, Sat-Sun, Dec 11-Dec 12. 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