Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Sat Aug 26 2023 Valid 12Z Tue Aug 29 2023 - 12Z Sat Sep 02 2023 ...A tropical system is forecast to impact portions of the southeastern U.S. with heavy rain likely from Florida Panhandle northeastward through midweek next week... ...Overview... A tropical disturbance currently over the northwestern Caribbean Sea is forecast to develop into a tropical cyclone per the National Hurricane Center as the system tracks northward through the eastern Gulf of Mexico and into Florida around midweek, likely with a turn northeastward into the western Atlantic thereafter with heavy rain spreading into coastal Georgia and the Carolinas. However, there continues to be a fair amount of spread in model guidance with this system's track, so notable changes to the forecast could occur. Elsewhere, an upper low could spread locally heavy rainfall into the Northwest Tuesday-Wednesday, while the next surge of monsoonal moisture appears to move up across the Desert Southwest into the Four Corners states late next week. Cooler weather is likely across much of the country at least compared to the past week, but temperatures are forecast to rebound to above normal in the central U.S. especially by the latter part of next week. ...Guidance Evaluation/Predictability Assessment... Model guidance is generally in good agreement with the synoptic-scale flow especially early in the period, featuring an upper trough digging into the Great Lakes Tuesday-Wednesday before lifting and retreating into the Canadian Maritimes late week, as well as an upper low moving eastward across the Pacific Northwest into the northern Plains through the medium-range period. Model guidance was within reason for these features, with a multi-model deterministic blend working fine. By next weekend, the deterministic guidance begins to diverge near the West Coast where an upper trough could develop into a closed low. The ensemble means appear much more agreeable with this feature. Thus, a large portion of today's Day 7 WPC forecasts was based on a composite blend of the 06Z GEFS, 00Z EC mean, and the 00Z CMC mean. The largest uncertainty continues to be related to what comes out of the tropical system that is forecast to develop over the eastern Gulf of Mexico early next week. Since yesterday, the GFS has been offering an alternative scenario, taking this system on a more northerly track into the interior southeastern U.S. verses a track toward the northeast across northern Florida and just off the Carolina coasts as depicted by the ECMWF. However, recent ECMWF runs, as well as runs from the CMC and ICON, have shown a westward trend in the forecast track of this system over the eastern Gulf of Mexico, which is in closer agreement with the GFS scenario. Thereafter, the ECMWF continues to take the system on a more northeasterly course toward the Carolina coasts verses the slower and more inland track depicted by the GFS. It appears that models continue to have notable disagreements on how to juggle between an upper low currently near the central Gulf Coast, the lifting East Coast trough, and their mutual interaction with the tropical system. The WPC medium-range forecast package for Days 3 - 5 was based on a composite blend of 40% from the 00Z ECMWF/EC mean, 40% from the 06Z GFS/GEFS, 20% from the 00Z CMC/CMC mean. This blend yielded a forecast track for the tropical system slightly more to the west across the eastern Gulf of Mexico than in previous forecasts, followed by a similar forecast track near the Carolina coasts thereafter. Forecasts for the remainder of the country are similar to WPC continuity. ...Weather/Hazards Highlights... The tropical cyclone that is likely to form is forecast to move through the eastern Gulf of Mexico and into Florida Tuesday-Wednesday, with the most likely track of the system northeastward across Florida and into the western Atlantic by Thursday near the Georgia/Carolina coast. Slight Risks of excessive rainfall are in place for these areas for the Days 4-5 EROs as a starting point. The forward speed of the system hopefully will put some limit on the flash flood potential, but regardless the heavy tropical rainfall with high rain rates could cause flash flooding even if the system is moving quickly. In addition, tropical downpours from the outskirt of the system may pose some flash flooding threat. These are certainly subject to change as forecasts for the tropical system are refined. A cold front located west of the tropical low could help elongate the footprint of the heavy rainfall. Farther north into the Mid-Atlantic and Northeast, some rain and storms are possible Tuesday-Wednesday with a couple of frontal systems, before a surface high sneaks in for a drying trend. Fronts across the Gulf Coast will provide a focus for moisture to pool, and scattered thunderstorms are likely there through much of the week. In the Northwest, the strong upper low coming through on Tuesday-Wednesday will provide good forcing 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, while rain totals are likely to exceed at least low (1 year) average recurrence intervals. Marginal Risks are in place in the EROs with this activity. After a relative lull in the monsoon around midweek, increasing convection appears likely by late week into Arizona and the surrounding states. Finally some relief of the heat is in store for the Gulf Coast region by Tuesday as considerably lower heat indices are forecast as a front or two come through. The Desert Southwest could see another hot day on Tuesday though, with highs in the 110s, which should abate some by Wednesday and beyond. Multiple fronts crossing the East will keep temperatures somewhat cooler than average for much of next week. The same will be true for the Northwest with the upper low overhead. The central U.S. will likely start off somewhat cooler than average but with a warming trend in store through the week. Temperatures are forecast to rise into the 90s into the north-central Plains and Midwest, around 10-15 degrees above average. 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 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