Extended Forecast Discussion NWS Weather Prediction Center College Park MD 417 PM EDT Sun Oct 04 2020 Valid 12Z Wed Oct 07 2020 - 12Z Sun Oct 11 2020 ...Moisture with a potential tropical cyclone could spread heavy rain into the Gulf Coast states... ...Major pattern change expected for the western U.S. with cooler temperatures and increasing precipitation... ...Overview... After persistent upper ridging across the western U.S. recently, a pattern change is forecast late week there, as precipitation and cooler temperatures accompany an upper trough coming into the region. Meanwhile, upper troughing remains over the eastern U.S. early in the period, transitioning to more zonal by the end of the week, while some degree of ridging could occur over the central part of the country ahead of the western trough, causing warm temperatures. A potential tropical cyclone could spread rain to the Gulf Coast states especially Fri into the weekend, but the intensity and exact track of this system and its rainfall and wind remain uncertain. ...Guidance/Predictability Assessment... With continuing good agreement regarding the midweek eastern U.S. trough, cold front, and surface high pressure system coming in behind the front, a deterministic multi-model blend was able to be used there. Model differences arise with the troughing moving across the eastern Pacific late week and into the western U.S. by the weekend. Beginning Thu, the 00Z ECMWF was one of the faster models to propagate the trough eastward, while the 00Z/06Z GFS and 00Z UKMET runs were slower. The 00Z CMC seemed out of tolerance with other guidance as it sent a shortwave quickly east while its main upper low was significantly slower. Given these differences, model and ensemble guidance was not too agreeable with the degree of ridging or zonal flow downstream across the central U.S. either. Once again the 00Z CMC seemed unlikely as energy in central Canada led to troughing there, but ensemble members do indicate a good amount of spread. Thus, shifted to a focus on the ensemble means in the later part of the period rather than trust any particular deterministic model (particularly the 00Z CMC). Low positions for the potential tropical cyclone were taken from the National Hurricane Center. ...Weather/Hazard Highlights... The tropics have become active again with Tropical Storm Gamma expected to track very slowly westward over the Bay of Campeche through late week, while a potential tropical cyclone is developing to its northeast. This second system is currently forecast to move across the western Caribbean and into the Gulf of Mexico, but its evolution and strength remain uncertain (see the National Hurricane Center for updates). Tropical moisture from these systems is forecast to continue spreading northward across much of Florida, and a series of surface fronts could help focus the rainfall to create potentially heavy amounts there. Heavy rainfall may also be a threat over the central Gulf Coast and the Southeast depending on the evolution of the possible tropical system, and changes to this forecast are expected. 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 Sun, after warm weather through Thu. Widespread precipitation is also likely across much of the West by the weekend, and high elevations could see snow. The precipitation and cooler temperatures should provide relief from ongoing fire weather concerns. Farther east, much above normal temperatures are forecast across the central U.S., with highs well into the 80s as far north as South Dakota Fri. Tate 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