Extended Forecast Discussion NWS Weather Prediction Center College Park MD 418 PM EDT Mon Oct 19 2020 Valid 12Z Thu Oct 22 2020 - 12Z Mon Oct 26 2020 ...Guidance Evaluation/Preferences... Guidance is agreeable and consistent with the large scale forecast of broad troughing aloft over North America, evolving toward a somewhat sharper and more amplified trough over the western U.S. during the weekend. Within this pattern an operational model consensus provides a good depiction of significant features in the days 3-5 Thu-Sat period--initial energy ejecting from the Northwest and corresponding Plains to Upper Great Lakes wave Thu into Fri, followed by strong shortwave energy dropping into the Northwest along with an arriving Northeast Pacific surface wave (12Z CMC somewhat fast with this wave). By late weekend and early next week models/ensemble members diverge for the details of the amplifying energy and its eventual ejection along with the associated surface evolution (00Z/06Z GFS runs straying to the fast side of the spread). As a result the forecast rapidly increases weight of the similar 06Z GEFS/00Z ECMWF means during days 6-7 Sun-Mon, yielding a wave that reaches the western Great Lakes by next Mon. The new blend provides good continuity in principle relative to the previous cycle. ...Weather/Hazard Highlights... The shortwave initially ejecting from the West from Thu onward and associated low pressure will produce periods of moderate precipitation from the northern Rockies/Plains into the Upper Mississippi Valley and Upper Great Lakes regions Thu-Fri. Highest totals are most likely over parts of the Upper Mississippi Valley and Upper Great Lakes, while locations from the northern Rockies through the Dakotas and northern Minnesota will see the best potential for meaningful snowfall. At least over northern areas the precip should trend lighter as the front trailing from the surface low continues into the eastern U.S. The Northeast Pacific wave reaching the Northwest late this week will bring more moisture across the northern third of the West, with the rain/higher elevation snow extending southward with time as upper troughing amplifies/sharpens. Toward the start of next week another Plains/Great Lakes wave may develop, spreading potential for some snow from the Rockies across parts of the northern half of the Plains into the Upper Midwest along with rainfall farther southeast along the front trailing from the surface low. Overall expect best potential for heavy snow to be over the northern Rockies while details farther east will take additional time to come into focus. Persistence of cold Canadian air will keep the core of most extreme below normal temperature anomalies centered over the northern Rockies/High Plains during the period. Parts of Montana may see readings up to 25-35F below normal for multiple days while only moderately less extreme anomalies push southward over the Plains in response to the individual shortwaves/surface systems expected early and late in the period. Expect some of this cold air to extend into the West as well, with temperatures 5-20F below normal over the Interior Northwest/northern Great Basin from the start of the period Thu, followed by expansion/southward push of minus 10-20F anomalies over the West as upper troughing amplifies. Farther east, the warm sector ahead of the first front will contain well above normal temperatures (up to plus 10-20F anomalies). Frontal passage will bring highs close to normal over the East but morning lows will remain above normal over the southeast third of the lower 48 as the boundary lingers over the Florida Peninsula and adjoining waters. Meanwhile expect moisture and rainfall of varying intensity currently over and near Florida to reach the eastern Gulf of Mexico by the start of the period. This rain may lift northward across the Southeast in advance of the leading front that reaches the East. There is still fairly high uncertainty regarding the amount of moisture that ultimately reaches the Southeast and how it may interact with the front. In addition there has been a signal for additional moisture that could reach Florida and the east-central Gulf states in association with low pressure that could emerge from the western Caribbean Sea. However guidance has been diverse and inconsistent with this aspect of the forecast. Rausch/Schichtel Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php Hazards: - Heavy rain across portions of the Central Plains, the Lower Mississippi Valley, the Tennessee Valley, the Great Lakes, the Middle Mississippi Valley, the Southern Plains, and the Ohio Valley, Sun-Mon, Oct 25-Oct 26. - Heavy rain across portions of the Southeast, the Great Lakes, and the Upper Mississippi Valley, Thu-Fri, Oct 22-Oct 23. - Heavy rain across portions of the Northeast and the Great Lakes, Mon, Oct 26. - Heavy snow across portions of the Northern Plains, the Northern Rockies, and the Northern Great Basin, Fri-Sat, Oct 23-Oct 24. - Heavy snow across portions of the Central Rockies, the Central Plains, the Central Great Basin, the Northern Plains, and the Northern Rockies, Sat-Sun, Oct 24-Oct 25. - Heavy snow across portions of the Upper Mississippi Valley and the Northern Plains, Thu, Oct 22. - Flooding occurring or imminent across portions of the Lower Mississippi Valley. - Much below normal temperatures across portions of the Central Plains, the Central Great Basin, the Northern Rockies, the Central Rockies, the Northern Great Basin, and the Pacific Northwest, Sun-Mon, Oct 25-Oct 26. - Much below normal temperatures across portions of the Northern Plains, the Northern Rockies, and the Northern Great Basin, Thu-Mon, Oct 22-Oct 26. - Much below normal temperatures across portions of the Central Plains, the Northern Plains, the Northern Rockies, the Central Rockies, the Middle Mississippi Valley, and the Upper Mississippi Valley, Fri-Mon, Oct 23-Oct 26. - Heavy precipitation across portions of mainland Alaska and the Aleutians, Thu-Fri, Oct 22-Oct 23. - Heavy precipitation across portions of the Alaska Panhandle and mainland Alaska, Sun-Mon, Oct 25-Oct 26. - Heavy snow across portions of mainland Alaska, Thu-Fri, Oct 22-Oct 23. - High winds across portions of mainland Alaska and the Aleutians, Thu-Fri, Oct 22-Oct 23. 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