Extended Forecast Discussion NWS Weather Prediction Center College Park MD 143 PM EDT Thu Sep 09 2021 Valid 12Z Sun Sep 12 2021 - 12Z Thu Sep 16 2021 ...Potential Heavy Rain Threat for Western Gulf Region Late This Weekend and Early Next Week... ...Overview... Fast, quasi-zonal flow across the northern tier of the U.S. will keep the majority of the CONUS in a drier weather pattern. A tropical disturbance that may emerge into the western Gulf late this weekend into early next week does pose a potential heavy rain threat for portions of the Texas and southern Louisiana coasts. Temperatures will generally be near to below normal across the northern tier north of the front and near the Gulf, but warmer than normal in between from the Southwest to the Mid-Atlantic. ...Guidance Evaluation and Preferences... Faster, quasi zonal flow that is expected to setup across the CONUS over the course of the medium range period and the latest model guidance continues to struggle with that evolution. Particularly noted as an outlier and therefore not included in the WPC model blend preferences was the 00Z/06Z runs of the GFS where were initially too fast with the trough progression on day 3/4 and then become nearly out of phase with other deterministic models and associated ensemble means with another approaching shortwave energy late in the period. Once that timing difference starts, it expands through the period rendering an all-encompassing blend ineffectual. For now, a non-GFS blend was preferred which transitions well from continuity and favors more of the ensemble support. For the western Gulf, a broad tropical plume associated with a weak disturbance will gradually lift northwest toward the TX/LA coasts late Sunday into early/middle of next week. There is 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. Interests in these areas should continue to monitor the latest forecasts. ...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 Gulf coasts. Several inches appear possible and this could lead to some flooding concerns. Otherwise, frontal boundaries slipping through the flow across the central and northern U.S. will bring mostly lighter rainfall amounts to portions of the CONUS with some potential for isolated/localized pockets of heavier amounts. Temperatures will largely be below normal across the northern tier with swings above normal ahead of frontal passages across the central/eastern U.S.. Readings on Sunday and Monday could top 10-15F above normal across the Central Plains to lower Ohio Valley and lower Great Lakes. 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