Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 PM 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 the middle of next week... ...Overview and Guidance Evaluation/Preferences... Expect the heaviest concentration of rainfall during the period to be along parts of the western/central Gulf Coast into midweek. There is still a lot of uncertainty regarding how a wave emerging into the southwestern Gulf of Mexico may ultimately evolve and track farther north, tempering confidence in the details of how much moisture will reach the Gulf Coast region and where. Continue to monitor National Hurricane Center outlooks for the latest information on this feature. Meanwhile progressive flow across the northern tier of the U.S. will push along multiple frontal systems with varying amounts of accompanying rainfall. By the time the fronts reach the East, mean ridging or at least above normal heights aloft will make it difficult for the fronts to reach any farther south than the Lower Great Lakes/Ohio Valley and northern Mid-Atlantic--yielding above average temperatures over a decent portion of the eastern U.S. Around the periphery of strongest Atlantic upper ridging, a wave/system may track near the Bahamas and East Coast. In contrast to the eastern warmth, a deepening upper trough forecast to reach the Northwest by next Saturday should bring an early taste of a cool-season pattern with below normal temperatures and wetter conditions that may include some high elevation snow. For the large-scale pattern and associated features, a composite of 00Z models/06Z GFS provided a reasonable starting point for about the first half of the period while a steady trend toward about half models and half means (06Z GEFS/00Z ECens) accounted for the moderate increase of detail spread that develops later in the period. The primary change in continuity results from the latest consensus being stronger/faster with the upper trough that reaches the northern Rockies/Plains by Thursday, ultimately leading to greater southeastward progression of the leading cold front during the latter half of the week. Ensemble means supported the majority scenario while the old 12Z/10 ECMWF was quite slow even versus its mean. The 12Z GFS has backed off from this trend somewhat but the new ECMWF is consistent. Issues with this trough ultimately extend into the East late in the period and could affect the progression of the surface wave/energy aloft that may track near the Mid-Atlantic coast. A favored intermediate approach would lead to somewhat higher heights aloft/slower progression than in the 00Z ECMWF. Guidance seems to be navigating toward an intermediate solution for the Northwest upper trough by next Saturday, as the 00Z ECMWF lowered heights versus its prior run while the latest GFS runs have backed away some from the relatively deeper 00Z run. This trough may contain multiple pieces of energy so details may remain ambiguous for a while. Confidence remains low for the details of possible tropical development in the western Gulf of Mexico late this weekend and the resulting heavy rainfall threat along the Texas/Louisiana coasts through at least the middle of next week. Most of the guidance agrees upon a general heavy rainfall threat associated with moist flow from the Gulf and surface troughing or low pressure. The degree and track of any well-defined surface system will be crucial in determining the magnitude and location/duration of heaviest rainfall though, and unfortunately surface details could depend on exactly how convection organizes well into the short-range time frame. Current preferences (including from today's 17Z coordination with the National Hurricane Center) lean more to recent GFS/ECMWF runs in general with greater definition/northward track versus the CMC/UKMET that have been much weaker and southwestward. Consult the National Hurricane Center's Tropical Weather Outlook for latest details on the development potential for this system. ...Weather/Hazard Highlights... Expect multiple rounds of heavy rainfall near the Texas coast and into parts of Louisiana through the early/middle portions 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. Rausch/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