Extended Forecast Discussion NWS Weather Prediction Center College Park MD 318 AM EDT Sun Aug 27 2023 Valid 12Z Wed Aug 30 2023 - 12Z Sun Sep 03 2023 ...What is currently Tropical Depression Ten is forecast to impact portions of Florida northeastward into Georgia and the Carolinas Wednesday-Thursday with heavy rain and high winds... ...Overview... Tropical Depression Ten is forecast to move northward through the eastern Gulf of Mexico during the short range period and strengthen per the National Hurricane Center's forecast, reaching near the Florida Big Bend by early Wednesday potentially as a hurricane. The system is likely to take a northeasterly track after that, spreading heavy rainfall from Florida into Georgia and the Carolinas. However, there continues to be a fair amount of spread in model guidance with this system's exact track, so notable changes to the forecast could occur. Elsewhere, the Northwest could see lingering heavy rain into Wednesday, while the next surge of monsoonal moisture appears to move up across the Desert Southwest into the Four Corners states around Thursday-Friday. Initially cooler weather is likely across much of the country at least compared to last week, but temperatures are forecast to rebound to above normal in the central U.S. especially by late week into next weekend. ...Guidance Evaluation/Predictability Assessment... Model guidance is generally in good agreement with the synoptic-scale flow in the midlatitudes especially early in the period. The midweek pattern features an upper trough atop the northeastern U.S. before lifting into the Canadian Maritimes late week, as well as an upper low moving eastward from the Northwest across the north-central U.S./south-central Canada border. The latter feature shows a bit more spread, with the 12Z ECMWF a bit slower than other 12/18Z guidance, while on the whole the newer 00Z guidance ends up weaker and more progressive with this feature by Thursday-Friday. Additional energy looks to dig through the eastern Pacific late in the week and appears likely to form a closed low near or over the West Coast. There is some spread with the position of this low meandering late in the week, but the only real outlier seemed to be the 12Z CMC that ejects the low into the Northwest and phases it with the northern stream by next Sunday. The 00Z CMC was more in line with the guidance consensus in keeping the feature held back west. The largest uncertainty continues to be related to the track and timing of the tropical cyclone (currently T.D. Ten), with spread in solutions evident in the ensemble members and deterministic models. In general over the past couple of days, the GFS runs have been on the western side of the envelope after landfall, CMC runs have been fast, and the ECMWF has been generally favored as it has been persistent--though with a gradual westward trend over the past few runs for better odds for a system onshore near the western Atlantic coast rather than offshore. There was some convergence of models in the 12/18Z cycle, especially as the 18Z GFS hopped to the east to be in better alignment with the ECMWF. The 00Z GFS did shift back west a bit though, while the 00Z ECMWF continues a slight trend west and more onshore. Model spread is also evident in the timing of the system. Most 12/18Z model guidance was slower than the National Hurricane Center's track from the 03Z advisory, but their advisory was very similar to their previous one. So the timing is another aspect that will continue to be monitored. It appears that these differences are related in part to disagreements on how to juggle between an upper low/trough over the Gulf and Gulf Coast states and the lifting East Coast trough, and their mutual interaction with the tropical system. The WPC forecast was based on a mostly deterministic model blend early in the forecast period. Gradually increased the proportion of the ensemble means as the period progressed to around half by days 6-7, and phased out the CMC given its differences across the West. The forecast for the tropical cyclone followed the NHC advisory. ...Weather/Hazards Highlights... Current Tropical Depression Ten (which is likely to strengthen) is forecast to move into Florida around the Big Bend area by Wednesday, with the forecast track of the system northeastward across the Atlantic coastlines of Georgia and the Carolinas. Slight Risks of excessive rainfall are in place for these areas for the Days 4-5 EROs on Wednesday and Thursday. The forward speed of the system and how far inland it tracks will determine the flash flood potential over the affected land areas. But regardless, the heavy tropical rainfall with high rain rates could cause flash flooding even if the system is moving quickly. A cold front located west of the tropical cyclone could help elongate the footprint of the heavy rainfall. Wednesday (Day 4 in terms of ERO timing) may see the heaviest rain totals from the tropical cyclone. But with continued spread in model guidance as to the exact track, plus the currently forecast heaviest totals over eastern Georgia and southern South Carolina not really overlapping heavy rain forecasts in the short range, held off on any Moderate Risk at this point. But the ERO risk level on Wednesday is likely to be more enhanced than some Slight Risks. The positions of the risk areas are also subject to change as forecasts for the tropical system are refined. In addition, tropical downpours and squalls from the outer rainbands of the system may pose some flash flooding threat farther south into Florida for example. Additionally, fronts across the Gulf Coast will provide a focus for moisture to pool, and scattered thunderstorms are likely there through much of the week into next weekend. In the Northwest, heavy rain could continue into Wednesday across parts of the northern Rockies as the strong upper low overhead provides good forcing for rainfall while some instability may linger. Light rain is also possible into the Pacific Northwest with additional height falls through the latter half of the week. Farther south, after a relative lull in the monsoon around midweek, increasing convection appears likely by late week into Arizona and the surrounding states. Plenty of instability in place and above average moisture levels at least into the Desert Southwest seems sufficient to have a Marginal Risk in the ERO for Day 5/Thursday with this activity. Most of the excessive heat concerns look to abate by Wednesday. The western Gulf Coast may see highs a few degrees above average, but heat indices will be considerably lower after a front or two come through. The Desert Southwest could see highs in the low 110s for one more day on Wednesday before a moderating trend later in the week. Meanwhile the Northwest and the East should generally be lower than average in terms of temperatures Wednesday-Friday, warming a bit to around normal in the East by next weekend, while below average highs drift south a bit from the Northwest into California and the central Great Basin. However, the central U.S. is likely to see heat building gradually through late this week and into next weekend. Highs well into the 90s will expand into the north-central Plains and Midwest, while highs exceeding 100 are likely for the southern half of the Plains, around 10-15 degrees above average. 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