Extended Forecast Discussion NWS Weather Prediction Center College Park MD 346 PM EST Tue Nov 24 2020 Valid 12Z Fri Nov 27 2020 - 12Z Tue Dec 01 2020 ...Widespread heavy rainfall threat across the South Friday-Monday... ...Overview... As the medium range period begins Fri, troughing across the western and central CONUS may have separated into northern and southern streams. As the northern stream troughing shifts quickly toward the Northeast by the weekend, the potentially closed upper low will linger over the south-central CONUS through the weekend, causing possibly heavy rain centered over eastern Texas to the Lower Mississippi Valley and the Southeast. The separate streams may combine again around Mon/Tue as additional northern stream energy enters the north-central CONUS. The trough axis is likely to shift east toward the Midwest Mon, then into the Great Lakes/Ohio Valley Tue. This pattern may also support some wintry weather from the Great Lakes through the Appalachians and interior Northeast as we usher in December. 'Tis the season. Additional troughing and energy is forecast to drop southeastward through the Northwest and Northern Plains Mon/Tue. ...Guidance Evaluation/Preferences... Guidance was in generally good agreement with the pattern described above through the weekend. The main differences that develop have to do with the degree of separation that develops in the northern and southern streams over the weekend and the timing for joining the streams back up, as well as regarding the potency of the southern stream upper low. Like in previous days/cycles, recent GFS and GEFS runs have remained on the weaker and progressive side of the guidance envelope with the southern stream energy. The WPC forecast for fronts/pressures continued leaning toward the stronger/slower solutions, led by the 00Z deterministic ECMWF and its ensemble mean, which maintained good continuity with previous forecasts. The 00Z CMC initially clustered well with the EC solutions and was able to be used for the first part of the period, before it became much slower/deeper with the next round of energy coming through the eastern Pacific early next week and took a different track into the Northwest. While the 00Z UKMET had the same idea as the 00Z EC/CMC with a closed upper low in the southern U.S., it appeared to be on the slow side Fri but rushed to catch up and combine the streams Sun, so it was not utilized today. Transitioned more toward the EC ensemble mean by the end of the forecast period. ...Sensible Weather and Hazards... The energetic trough/closing low tracking across the West should lead to snow chances in the Central/Southern Rockies, as well as cooler than normal temperatures by a few degrees for the Great Basin to Southern Plains through the end of the week. However, the likely bigger threat with this potent upper trough/low will be multiple days of rainfall as moisture from the Gulf of Mexico streams in ahead of the low. Areas from the eastern half of Texas to the Lower Mississippi Valley can expect persistent enhanced rainfall/convection Fri into the weekend. A surface low/frontal system is forecast to develop and strengthen in the Mississippi Valley over the weekend as the upper low pivots eastward. Precipitation should then continue along and ahead of this system, with heavy rains spreading across the Southeast and the Southern Appalachians/Carolinas early next week. Moderate precipitation could meanwhile spread northward through the Ohio Valley, Appalachians, and Mid-Atlantic/Northeast, but amounts and placement of the heaviest totals remain uncertain given the model spread. Wintry weather may occur along the northwestern part of this precipitation shield--the Great Lakes, Appalachians, and particularly higher elevations of the interior Northeast--by Mon/Tue. Cooling temperatures are forecast for the central and eastern U.S. next week behind the cold front associated with the surface low. Elsewhere, generally light precipitation is expected for the Northwest for this time of year. A quick round of precipitation is possible there around Mon as renewed troughing passes through behind an amplifying/warming ridge slated to shift from the Rockies to the Plains. Tate 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 precipitation across portions of the Northeast, Mon, Nov 30. - Heavy rain across portions of the Lower Mississippi Valley and the Southern Plains, Fri-Sat, Nov 27-Nov 28. - Heavy rain across portions of the Lower Mississippi Valley, the Central/Southern Appalachians, the Tennessee/Ohio Valleys, the Mid-Atlantic, and the Southeast, Sun, Nov 29. - Heavy rain across portions of the Mid-Atlantic and the Northeast, Mon, Nov 30. - Flooding occurring or imminent across portions of the Southeast. - Heavy precipitation across portions of the Alaska Panhandle and mainland Alaska, Sat-Mon, Nov 28-Nov 30. - Heavy precipitation across portions of the Alaska Panhandle, Tue, Dec 1. - High winds across portions of mainland Alaska, Fri-Mon, Nov 27-Nov 30. - High winds across portions of mainland Alaska and the Aleutians, Fri-Tue, Nov 27-Dec 1. 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