Extended Forecast Discussion NWS Weather Prediction Center College Park MD 519 PM EDT Fri Sep 10 2021 Valid 12Z Mon Sep 13 2021 - 12Z Fri Sep 17 2021 ...Heavy Rain Increasingly Likely Across Coastal Texas and Southwest Louisiana Late Sunday into Early Next Week... ...Overview and Guidance Evaluation and Preferences... The faster, quasi-zonal flow setting up across the northern tier of the U.S. is becoming better resolved by the latest model guidance such that a deterministic model blend yields a solution tied closely to the consensus and trends well with continuity. For days 3-5, a blend of the 00Z ECMWF/UKMET/CMC and 06Z GFS was used while later days used a higher component of the ECENS and GEFS means. The GFS continued to a be a flatter and somewhat more progressive solution but compared to recent cycles, the trend come toward a consensus. The UKMET was slower than a lot of the other guidance and by day 5 onward, the CMC over amplifies the developing Intermountain West trough expected. A ECMWF/GFS weighted blend was preferred overall for the CONUS. There is still considerable uncertainty with the potential development and possible heavy rainfall along the western Gulf coasts late Sunday into early/middle next week. The model signal for multiple inches (possibly 10+" over the multiple days) is still there but spatial spread is quite high. The CMC was on the southern/southwest edge of the spread while the ECMWF has trended toward a slow moving low approaching southwest Louisiana by day 4/5. The GFS and its ensemble members, generally keep the bulk of the higher QPF amounts close to the coast and somewhat lighter amounts. For the WPC QPF, a blend of the 00Z ECMWF, 06Z GFS, and in-house bias corrected ensemble was used primarily with some inclusion of the 13Z NBM. This resulted in a slight northeastward shift of the higher amounts to the upper TX and southwest LA coasts while decreasing amounts along the central TX coast and especially inland areas. Overall, there is higher than normal uncertainty and lower confidence in the locations and amounts, but the potential for several inches of the multi-day period exists. The National Hurricane Center has increased probabilities of tropical development with this disturbance as it emerges into the western Gulf and interests along those areas should monitor the latest forecasts closely. ...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. Heavy rain is becoming increasingly likely with the potential for several inches. Some isolated total amounts near or above 10 inches are possible but it is too early to determine the areas at greatest risk. These amounts could lead to flooding impacts, particularly for the more sensitive locations, urban areas, and those areas that have already seen above normal rainfall over the past couple of weeks. Interests along these areas should monitor the latest rainfall forecasts from WPC and tropical development from the NHC. 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. 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