Extended Forecast Discussion NWS Weather Prediction Center College Park MD 142 AM EDT Sat Sep 11 2021 Valid 12Z Tue Sep 14 2021 - 12Z Sat Sep 18 2021 ...Heavy Rain Likely Across Coastal Texas and Louisiana into mid next week... ...Overview and Guidance Evaluation and Preferences... A fast, quasi-zonal flow will set up across the northern tier of the U.S. during the medium range period with frontal systems quickly progressing from the West/central U.S. into the East. For days 3 and 4, model guidance shows good enough agreement for a general model compromise between the latest deterministic solutions. For the second half of the period, models continue to struggle both with the timing and amplification of troughing reaching the Western U.S. next Friday-Saturday. A blend of majority ECENS/GEFS mean with minority contributions from the ECMWF/GFS provided a good starting point and was most consistent with previous WPC continuity as well. There remains quite a bit of uncertainty surrounding potential tropical development in the western Gulf of Mexico late this weekend and the resulting heavy rainfall threat along the Texas/Louisiana coasts through the middle of next week. Most of the guidance agrees that there will be heavy rainfall associated with this system (regardless of development into a tropical cyclone) but the bigger question is exactly where that will occur. Both the ECMWF and GFS bring some kind of surface low along the Texas coast Monday-Tuesday and eventually inland into either southeast Texas (ECMWF) or Louisiana (GFS). The CMC remains well south of the consensus bringing the low closer to Brownsville region. The WPC QPF for this system was based largely on a blend of the ECMWF and GFS with our in-house bias corrected model and the NBM. Refer to the National Hurricane Centers tropical weather outlook for additional details on development potential with this system. ...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 will likely bring multiple rounds of heavy rainfall along the immediate Texas coast and into parts of Louisiana. Models continue to suggest some isolated storm total amounts near or above 10 inches are possible but it is too early to determine the areas at greatest risk. These kinds of amounts would lead to at least some flash and urban flooding, particularly for more sensitive locations... urban regions and those 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. Moderate to locally heavy rainfall may also reach the Pacific Northwest late this week with some snow possible in the highest elevations of the Washington Cascades. Periods of above normal temperatures will shift across the Southwest/Central U.S. into the East ahead of a couple frontal passages, with below to much below normal temperatures probable by late week across the Pacific Northwest. Elsewhere, expect the western Gulf Coast to be below normal through much of next week associated with clouds and ample tropical moisture. Santorelli 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