Extended Forecast Discussion NWS Weather Prediction Center College Park MD 130 AM EDT Sun Oct 4 2020 Valid 12Z Wed Oct 7 2020 - 12Z Sun Oct 11 2020 ***Heavy rain possible for portions of the Gulf Coast region, and Tropics are becoming more active*** ***Major pattern change for the western U.S. with cooler temperatures and increasing precipitation*** ...Overview and Guidance/Predictability Assessment... The synoptic scale upper level trough over the Great Lakes and Northeast that will be present through the beginning of the medium range period is forecast to evolve into a more zonal flow pattern by the end of the week, with the polar vortex that had been in place across Hudson Bay weakening and lifting to the north. A rather significant pattern change is expected to evolve across the western half of the nation beginning on Friday as the recent upper level ridge that had been anchored across this region will give way to an amplifying eastern Pacific trough. This trough is forecast to move inland across the Intermountain West by next weekend and lead to the next noteworthy storm system crossing the Rockies and eventually the western High Plains. It remains the case that the deterministic guidance is in very good agreement on the handling of the Northeast U.S. trough and the surface cold front reaching the East Coast Wednesday night, and here a general model compromise was incorporated through Friday. There is also good agreement on the position and magnitude of the surface high settling in behind the front. Forecast confidence is a little below average regarding the actual evolution of the eastern Pacific trough and the individual shortwave perturbations within it, and also across southwestern Canada, mainly Friday and into Saturday, before the trough becomes more consolidated by Sunday with some increase in confidence by that time. The ensemble means served as a good starting point here given the model spread, with the CMC too strong with a shortwave crossing southern Canada, the GFS much stronger with a lead shortwave approaching the Pacific Northwest, and the ECMWF initially more zonal with the flow compared to the model consensus. ...Sensible Weather... The tropics have become active again with Tropical Storm Gamma expected to track very slowly westward over the Bay of Campeche by the middle to end of the week, and another tropical wave could develop further as it exits the western Caribbean and then over the Gulf of Mexico, but significant uncertainty remains on its potential evolution and eventual strength (see the National Hurricane Center for updates). Tropical moisture from these systems is forecast to continue tracking northward across portions of Florida, and a surface front could help focus the rainfall to create potentially heavy amounts there. The central and eastern Gulf Coast could also see noteworthy rainfall by the end of the week depending on how the possible tropical system evolves, with changes expected in future forecast updates. Elsewhere across the continental U.S., temperatures should generally be within 10 degrees of climatological averages for the middle of week, followed by a significant warm-up across the central and northern Plains by Friday and Saturday. Highs are likely to be well into the 80s for many areas, which is 15-20 degrees above normal. By next weekend, the strong cold front crossing the western U.S. in response to the building trough will bring a welcomed relief to the persistent hot temperatures lately, with highs running 5 to 15 degrees below normal across the Great Basin and portions of the West Coast by Sunday. Widespread rainfall along with some high mountain snow is also appearing increasingly likely, and this will also provide a welcomed relief from ongoing wildfire concerns. Hamrick 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