Extended Forecast Discussion NWS Weather Prediction Center College Park MD 123 AM EDT Mon Oct 05 2020 Valid 12Z Thu Oct 08 2020 - 12Z Mon Oct 12 2020 ...Heavy rainfall from Tropical Depression #26 and the remains of Gamma headed for the Southeast... ...Major pattern change expected for the western U.S. with cooler temperatures and increasing precipitation... ...Overview... A progressive flow pattern is expected across the Lower 48 as a trough in the East is slowly replaced by ridging aloft, which a trough in the eastern Pacific moves through the West and into the Plains while amplifying. The 12z UKMET departed from the remainder of the guidance suite early, with aspects of the 12z ECMWF, 12z Canadian, and 18z GFS reasonably similar for use during the period. The 12z Canadian was most problematic with T.D. #26 (too slow), and the 18z GFS was a tad quick, but they all are reasonably close to the 12z ECMWF and NAEFS ensemble means. The pressures, 500 hPa heights, and wind grids started with a deterministic combination of the three early before increasing the use of the 12z NAEFS mean and 12z ECMWF ensemble mean. This led to slightly more progression with the trough moving through the West into the Plains. For more information on Gamma and T.D. #26, see the latest advisories and discussions from the National Hurricane Center. ...Weather/Hazard Highlights... T.D. #26 is expected to become a hurricane over the next few days and accelerate between the subtropical ridge and T.S. Gamma before recurving northward while either absorbing Gamma or flinging its remains around its southern and southeast side. Some combination of the two should move across the north-central and northeast Gulf coast and west of the Appalachians while occluding this weekend. Tropical moisture from these systems is forecast to create potentially heavy amounts near and east of its track across portions of the Southeast, Tennessee Valley, and southern Mid-Atlantic states. With the potent trough coming into the West, a cold front should bring cooler than average temperatures to the West Coast late week and into the Great Basin by Sunday. Widespread, needed precipitation is likely across much of the West by the weekend, and higher elevations could see snow. The precipitation and cooler temperatures should provide relief from ongoing fire weather concerns. Ahead of the upper trough and cold front, the area of possible record high temperatures shifts from the southern Rockies into the southern Plains with time. Roth 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