Extended Forecast Discussion NWS Weather Prediction Center College Park MD 424 PM EDT Fri Aug 25 2023 Valid 12Z Mon Aug 28 2023 - 12Z Fri Sep 01 2023 ...Threat of a tropical system increasing for the eastern Gulf of Mexico region midweek with heavy rain most likely across Florida... ...Heat and humidity will be confined to the Gulf Coast region with some return of the heat late week over the central Plains... ...Overview... A gradually amplifying upper trough will likely overspread much of the eastern half of the country as next week progresses. The ongoing extreme heat and humidity over the mid-section of the country will be dispelled by a large dome of Canadian cool air. As the heat and humidity persists across the Gulf Coast region, a tropical disturbance over the northwestern Caribbean is forecast to drift into the eastern Gulf of Mexico next week and possibly track across Florida midweek. Meanwhile, rain will likely return to the Pacific Northwest midweek next week as an upper-level low moves onshore, while the next surge of monsoonal moisture appears to move up across the Desert Southwest into the Four Corners states late next week. ...Guidance Evaluation/Predictability Assessment... Model guidance is generally in good agreement with the synoptic-scale flow, featuring amplifying troughing across the East for the first half of next week. Meanwhile, an upper low over the eastern Pacific on Monday is forecast to advance eastward through the Pacific Northwest and into north-central U.S. through the end of next week, with ridging persisting over the southern and southwestern U.S. Model differences appear relatively minor even out to Day 7 for much of the mainland U.S.. The largest uncertainty continues to be related to what comes out of the tropical system over the eastern Gulf of Mexico and how it eventually interacts with the eastern U.S. amplified trough. The majority of the guidance takes this system northeastward across the eastern Gulf early next week, followed by Florida midweek, and off the Carolina coasts thereafter as the East Coast upper trough retreats into eastern Canada. Models generally have shown a larger uncertainty on the forward speed of this tropical system than its direction of motion due to a larger uncertainty with the timing of interaction with the downstream trough. The 12Z GFS takes the system on a more northerly course, pushing it into the interior Southeast which would help separate the tropical system from the East Coast upper trough. This alternative scenario would result in a slower-moving tropical system to impact the Southeast through late next week. ...Weather/Hazards Highlights... As the workweek begins, a stalling wavy front with moisture pooling along it will likely produce a heavy rainfall threat from the central Gulf Coast through the Southeast and into the Mid-Atlantic, remaining into Tuesday and potentially spreading into the Northeast Tuesday as well depending on frontal positions. A primary focus is likely to be across the Carolinas where a Slight Risk has been maintained for next Monday. Despite dry conditions overall in the past few weeks there, a wet day on Day 3/Sunday will likely prime the area to experience some flash flooding issues. Also by Monday and especially Tuesday, the possible tropical system in the eastern Gulf could begin bringing heavy tropical rains to parts of Florida, for a Marginal Risk in southern Florida on Monday and a Slight Risk from the Florida Big Bend southward along the West Coast as the system is assumed to be approaching. Of course, this may be upgraded to a higher category depending on the eventual speed of motion of the tropical system. In addition, tropical downpours from the outskirt of the system may pose some flash flooding threat. If the system continues to track across northern Florida and emerge into the western Atlantic, heavy rainfall could also brush coastal Carolinas by Wednesday. If the system continues to move off the East Coast, thunderstorms could still linger across Florida into late week with a frontal boundary trailing and lingering behind the system. Above normal moisture is in place across portions of the central/southern Rockies to the Front Range on Monday, where scattered to widespread showers and storms may be slow-moving and could lead to localized flooding. A Marginal Risk was maintained in this area for Monday. Much of Texas could get some welcome rain as well near a frontal system. Some convection may remain into Tuesday with lesser amounts overall. By Wednesday, monsoonal convection could focus farther west likely into the Mogollon Rim, with increasing coverage once again by Thursday for the Four Corners states. In the Northwest, the strong upper low coming through for the first half of the week will provide support for rainfall there. There does look to be some instability in place, so localized rain amounts/rates may be high enough to cause a few instances of flash flooding. Another day of hot temperatures and high humidity is forecast for the Gulf Coast region on Monday. Additional record high maximum and minimum temperatures are possible as heat indices reach well above 110F. Finally some relief is in store by Tuesday and Wednesday with near to just above normal highs expected, but considerably lower heat indices as a front or two come through. Meanwhile heat is forecast to build again across the lower elevations of the Desert Southwest with some 110s likely on Monday and Tuesday, but this should also abate some by Wednesday and beyond. Multiple fronts crossing the East will keep temperatures somewhat cooler than average for much of next week. The central U.S. will likely start off somewhat cooler than average but with a warming trend in store through the week, high temperatures are forecast to reach into the mid to possibly upper 90s over the central Plains ahead of a cold front by late next week. Kong/Tate 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 Hazards: - Heavy rain across portions of the Southeast, the southern Mid-Atlantic, and the Southern Appalachians, Mon, Aug 28. - Heavy rain across portions of the Southeast and the southern Mid-Atlantic, Tue-Wed, Aug 29-Aug 30. - Flooding possible across portions of the Great Lakes. - Flooding likely across portions of the Great Lakes. - Hazardous heat across portions of the Southeast, the Southern Plains, the Lower Mississippi Valley, California, and the Southwest, Mon-Tue, Aug 28-Aug 29. WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw