Extended Forecast Discussion NWS Weather Prediction Center College Park MD 204 AM EST Wed Nov 25 2020 Valid 12Z Sat Nov 28 2020 - 12Z Wed Dec 02 2020 ...Widespread heavy rainfall threat across the South Friday-Monday... ...Overview... Troughing will set up over much of the western and central U.S. during the extended period, Friday through Tuesday. The shortwave passing through the north-tier states and the Canadian Prairies is expected to be fairly progressive as it tracks eastward. Meanwhile, the wave/potential closed low over the Great Basin/Rockies will be slower which will cause the flow to separate into two streams. A closed low pressure system associated with the northern stream will lift toward the Northeast while lingering over the south-central portions of the country-- providing a focus for moderate to possibly heavy rainfall from eastern Texas, across the Gulf states to parts of the Southeast. Toward end of the extended period (early next week) the two streams become in phase once more as another wave tracks into the Northern Plains and shifts the frontal boundary toward the Great Lakes and Ohio Valley. Cold air will accompany this front, which will allow for wintry precipitation from the Great Lakes region to the Northeast. ...Guidance Evaluation/Preferences... The overall pattern showing a split flow is supported among the deterministic and ensemble guidance. There are notable differences for the timing, placement and strength of the features however, the CMC and ECWMF persist in having similar solutions and placement of the QPF axis/foot print and the quantity of precipitation. The GFS lagged behind/west and the UKMET stayed on the southern periphery of the cluster over the northern states; while the CMC was very near the EC ensemble and operational ECWMF. As mentioned in the previous discussion, the CMC slows with time and therefore its usability decreases for the later periods. To maintain continuity and the recent trends, the preferred blend comprised of the 00Z CMC/ECWMF/EC ensemble mean. Initially the weighting was 40/40/20 respectively and transitioned to 40% of the mean by the last period. This combination kept maintain progressive flow with merging streams by the end of the forecast. ...Sensible Weather and Hazards... With the closed low/energetic trough trekking across the West a cool airmass will settle in, allowing for below average temperatures for late November/early December. Also, with lowering freezing levels snow will be possible for the Central and Southern Rockies this weekend. The South will be wet for a few days as scattered to widespread rain and thunderstorms accompany the front from eastern Texas, Lower/Middle Mississippi Valley, Tennessee Valley to the Southeast. Intensity will be moderate, at times heavy, and localized areas of rapid runoff and flash flooding may develop. 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, particularly for portions of the Great Lakes, the Appalachians and the favored upslope terrain in the interior Northeast by early next week. This front will usher in cooler temperatures after it passes. Light precipitation can be expected for parts of the Pacific Northwest. 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. Campbell 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