Extended Forecast Discussion NWS Weather Prediction Center College Park MD 142 AM EDT Fri Sep 10 2021 Valid 12Z Mon Sep 13 2021 - 12Z Fri Sep 17 2021 ...Potential Heavy Rain Threat for Western Gulf Region early next week... ...Overview and Guidance Evaluation and Preferences... Fast, quasi-zonal flow across the northern tier of the U.S. will keep fronts progressive and the majority of the CONUS in a drier weather pattern. The latest suite of 12z/18z guidance shows good enough agreement days 3-4 (Mon-Tues) to warrant a purely deterministic model blend (favored more heavily towards the ECMWF/GFS). After mid-week though, some timing and intensity differences begin to arise, especially with a pair of shortwaves entering the West Coast on Wednesday and again Friday. The 00z CMC is more amplified than the ECMWF and GFS and thus, slower with the first shortwave, but is flatter and faster with the second. So by late period, it quickly gets out of phase from the better consensus. The ECMWF and GFS are more consistent with the ensemble means, but also continue to show some run to run variability. As a result, the WPC forecast leans more towards the ensemble means the second half of the period with minor contributions from the ECMWF and GFS just for a little added system definition. This maintains good WPC continuity for days 3-6, with some slightly faster frontal speed. In the western Gulf of Mexico, a broad tropical plume of moisture associated with a weak disturbance will gradually lift north-northwest toward the TX/LA coasts late this weekend and into early/middle of next week. There remains considerable uncertainty in the potential strength, development, and track of this feature but the latest guidance is in reasonable agreement for the potential of heavy rainfall along the immediate coasts that could reach several inches or more over the multi-day period. The National Hurricane Center is currently monitoring this disturbance for possible development over the coming days. ...Weather/Hazard Highlights... The eventual track and evolution of a tropical disturbance in the western Gulf late this weekend into the early/middle portions of next week may bring multiple rounds of heavy rainfall along the immediate Texas and southwest Louisiana coasts. Several inches of rainfall is possible over multiple days and this could lead to some flooding concerns, especially in areas that have already seen above normal rainfall over the past couple of weeks. Otherwise, mostly lighter rainfall will follow frontal passages through the central and northeastern parts of the CONUS with some potential for isolated/localized pockets of heavier amounts. Temperatures across the Western and northern tiers should be near or below normal, while periods of above normal temperatures shift across the Southwest/Central U.S. into the East ahead of a couple frontal passages. Expect the western Gulf Coast to be below normal through much of next week as well associated with clouds and ample tropical moisture. Santorelli/Taylor Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php 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