Extended Forecast Discussion NWS Weather Prediction Center College Park MD 327 PM EST Wed Nov 25 2020 Valid 12Z Sat Nov 28 2020 - 12Z Wed Dec 02 2020 ...Widespread heavy rainfall threat across the South through the weekend, with precipitation spreading across the East for the first half of next week... ...Overview... A reasonably complex upper pattern is forecast during the medium range. At the start of the period Sat, split flow is expected, with a progressive northern stream while a southern stream upper low moves more slowly across the south-central CONUS. The streams may combine again around Mon as northern stream troughing comes into the north-central U.S., which will likely create another closed low centered over the Ohio Valley Tue. Given this upper pattern, heavy rain will be a threat across the South, shifting from eastern Texas and the Lower Mississippi Valley Sat toward the Southeast/Southern Appalachians Sun, and confidence is increasing that notable precipitation should spread into the Eastern Seaboard Mon/Tue. Wintry weather is possible across portions of the Ohio Valley/Great Lakes, Appalachians and the interior Northeast on the cold backside of the system. Meanwhile, ridging is generally expected for the West until a shortwave enters from the eastern Pacific around Mon and moves through the Intermountain West, ahead of renewed ridging for the West Coast by Wed. ...Guidance Evaluation/Preferences... Model and ensemble guidance in now good agreement with the initially split upper flow Sat described above, after many runs of the GFS/GEFS were weaker and more progressive with the southern stream energy in the past few days. Thus was able to incorporate the various deterministic models including the 00Z/06Z GFS runs into the WPC forecast today for the beginning of the forecast period over the weekend. There are some differences in timing and position of the combination of streams around Mon and with the associated surface lows, but a GFS/GEFS mean/ECMWF/EC mean blend seemed to suffice there in the East. This combination also worked with the trough coming into the West Mon-Wed, as the 00Z UKMET was on the too strong and slow side of the guidance envelope and the 00Z CMC was on the too weak and progressive side. Did transition to using the 00Z rather than the 06Z deterministic GFS by Wed, as the 06Z had a trough/ridge orientation in the West that was unsupported by other guidance. ...Sensible Weather and Hazards... Gulf of Mexico moisture streaming in ahead of the southern stream closed low should lead to enhanced rainfall/convection for the Gulf Coast states through the weekend, transitioning north and eastward early next week. A widespread couple of inches of rain is forecast for the Eastern Seaboard Sun-Tue, lingering in New England Wed. Locally higher amounts may cause flooding/flash flooding issues. On the western side of the precipitation shield, precipitation could fall as snow across parts of the Great Lakes region, Ohio Valley, favored higher elevations of the Appalachians, and interior portions of the Northeast Tue/Wed. Placement and amounts of notable snowfall remain uncertain at this time, however. Below normal temperatures can be expected across southern portions of the Plains and Mississippi Valley toward the Tennessee Valley/Southeast for the first half of next week given anomalously low upper-level heights and behind a couple of cold fronts. Meanwhile, instances of above average temperatures are possible across parts of the northern tier. While the Northwest will see pretty quiet weather for late Nov/early Dec, a round of light precipitation is possible Sun/Mon ahead of the trough and its associated surface front. 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, the Central Appalachians, the Tennessee/Ohio Valley, the Mid-Atlantic, the Great Lakes, and the Ohio Valley, Mon-Tue, Nov 30-Dec 1. - Heavy rain across portions of the Lower Mississippi Valley and the Southern Plains, Sat, Nov 28. - Heavy rain across portions of the Lower Mississippi Valley, the Tennessee/Ohio Valleys, the Mid-Atlantic, the Southern Appalachians, and the Southeast, Sun, Nov 29. - Heavy rain across portions of the Southeast, the Southern Appalachians, the Mid-Atlantic, and the Central Appalachians, Mon, Nov 30. - Heavy rain across portions of the Mid-Atlantic and the Northeast, Tue, Dec 1. - Heavy snow across portions of the Great Lakes, Mon, Nov 30. - Flooding occurring or imminent across portions of the Southeast. - Heavy precipitation across portions of the Alaska Panhandle and mainland Alaska, Sat-Tue, Nov 28-Dec 1. - High winds across portions of the Alaska Panhandle and mainland Alaska, Sat, Nov 28 and Tue, Dec 1. - High winds across portions of mainland Alaska and the Aleutians, Sat-Mon, Nov 28-Nov 30. 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