Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 AM EDT Sun Sep 12 2021 Valid 12Z Wed Sep 15 2021 - 12Z Sun Sep 19 2021 ...Heavy Rain Likely Across Coastal Texas and Louisiana into the middle of next week... ...Overview and Guidance Evaluation/Preferences... Main system of interest continues to be a tropical disturbance emerging into the southwestern Gulf of Mexico this weekend and track north towards the western Gulf Coast by the start of the medium range period on Wednesday. Although most of the guidance now agrees on a heavy rainfall threat along the Texas coast and into Louisiana, there remains quite a bit of uncertainty regarding the evolution of this system and exactly how much moisture will reach the Gulf Coast region and where. The GFS and ECMWF models remain more defined and farther north, while the CMC/UKMET are weaker and south. WPC prefers a blend towards the GFS/ECMWF which is also more consistent with the previous forecast. Consult the National Hurricane Center's Tropical Weather Outlook for the latest details on the development potential with this system. Meanwhile, progressive flow across the northern tier of the CONUS will push along multiple frontal systems. There's good enough agreement in the day 3-5 time frame for a general model compromise, but details in timing and amplitude especially with the next trough entering the Western U.S. late period emerge by day 6/Saturday. The ECMWF and CMC get noticeably faster with the trough axis which reaches the central U.S. by Sunday. The 18z GFS is much slower keeping the axis over the west coast. Prefer a blend towards the better agreeable ensemble means which kind of splits the difference between the two camps. The last system to note is an area of low pressure expected to form east of the Bahamas and track towards the north, off the East Coast. The GFS is the strongest with possible tropical development of this system bringing the low very near/over the Mid-Atlantic coast. The ECMWF and CMC are farther off shore. A shortwave moving over the Great Lakes should help to eventually push the system east shunting any progression farther into the Northeast. These differences obviously have huge implications for any kind of rainfall across an already over saturated coastal Mid-Atlantic/Northeast. ...Weather/Hazard Highlights... Expect multiple rounds of heavy rainfall near the Texas coast and into parts of Louisiana through the middle of next week. Details will be very sensitive to the evolution/track of a tropical disturbance now emerging into the southwestern Gulf. A well-defined/northward system could bring up to 10-15 inch localized totals but potential also exists for a weaker and farther southwest feature that would lead to rainfall that is less extreme/focused. Even in this latter case the general pattern could still favor areas of sufficiently heavy rainfall to be hazardous. The forecast requires close monitoring given the potential for 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. Mostly lighter rainfall will accompany frontal passages through the central and northeastern parts of the CONUS. Some potential exists for isolated/localized pockets of heavier amounts, especially from the Midwest into far interior New England with a leading front Tuesday-Wednesday. 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. Some locations along the East Coast may see rainfall with a wave that could track close to the Mid-Atlantic by late this week. The most persistent warm anomalies should be over portions of the Great Lakes/Ohio Valley and Mid-Atlantic/Northeast with some readings 10-15F or so above normal over multiple days. In general morning lows should be somewhat more above normal than daytime highs. Surrounding areas may see more brief episodes of warmth depending on frontal progression. The possible system near the Mid-Atlantic late in the week would decrease high temperatures somewhat. More moderate warm anomalies (plus 5-10F) should extend back through the southern half of the West while clouds and tropical moisture will keep western-central Gulf Coast highs somewhat below normal. By the end of the week highs in the Northwest should decrease to as much as 10-15F below normal as the upper trough reaches the area. Downstream the amplifying pattern could increase coverage of highs 10-15F above normal over the Plains for next Saturday and into the upper midwest on Sunday. Santorelli/Rausch 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