Extended Forecast Discussion NWS Weather Prediction Center College Park MD 238 AM EDT Tue Sep 24 2024 Valid 12Z Fri Sep 27 2024 - 12Z Tue Oct 01 2024 ...Potential Tropical Cyclone Nine to bring impactful heavy rain, high wind, and coastal flooding threat to the Central to Eastern Gulf Coast/Southeast ahead/with landfall as Hurricane Thursday... ...Guidance/Predictability Assessment... The main forecast story remains the threat and impacts associated with impending Potential Tropical Tropical Cyclone Nine. All eyes will be on the pattern over the eastern U.S. ino later this week. Recent guidance has come into better agreement in depicting a split flow pattern with an upper-trough lifting northward over the Northeast as a cut-off upper-low drops southward over the Middle Mississippi Valley/Ozarks Region/ArkLaTex. This upper-low then lingers in the general vicinity into the weekend as upper-ridging moves in to the north over the Great Lakes/southeastern Canada. At the same time, the official National Hurricane Center (NHC) forecast track for Potential Cyclone Nine brings the system northward across the Florida Panhandle Region as a Hurricane and into the Southeast on Thursday. The GFS/ECMWF and the UKMET have trended over the past few runs to consistently depict Potential Tropical Cyclone Nine remaining a separate entity for a longer period of time after moving inland, moving northward into the Appalachians/Upper Ohio Valley through the day Friday. The system is then eventually absorbed into the upper-low by early Saturday over the Middle Mississippi Valley. The CMC remains an outlier with the upper-low absorbing the system faster. Specifics on the exact track of the system, the upper-low to the west, and the timing in which the energy is absorbed will be important to the overall QPF footprint, which will likely stretch from the Southeast, robustly north into favored terrain of the southern/central Appalachians and back westward through the Ohio Valley into the Ozarks Region. The position/strength of the upper-low not surprisingly becomes less defined by the latter part of the period late this weekend/early next week. Differences increase with the overall pattern elsewhere by the end of the period as well with the very uncertain potential for more amplified northern-stream shortwave energy over the northern Rockies/High Plains suggested by the deterministic guidance, particularly recent GFS runs. However, the 00 UTC GFS has strongly trended toward the rest of guidance. The WPC forecast includes a blend of the GFS/ECMWF/UKMET for Friday and Saturday given good overall agreement with respect to the track of Potential Tropical Cyclone Nine and the handling of the cutoff low to the west. Swictched to the GEFS/ECENS means for the latter part of the period as uncertainty the overall pattern grows. ...Weather/Hazards Highlights... The NHC and the Weather Prediction Center among others continue to monitor Potential Tropical Cyclone Nine. The official NHC track brings the system northward into the Gulf of Mexico to threaten maritime interests and the central to eastern Gulf Coast/Southeast mid-late week, with the system set to move inland Thursday. Confidence is growing in a large system that will bring significant rainfall/winds/surf/coastal flooding for the central to eastern Gulf Coast/Florida, with impactful weather possible through the Southeast, and wrapping back into the Mid-South late week into the weekend. The Day 4/Friday ERO period features Nine proximity and moisture feed into the southern Appalachians as well as over the Ozark Region under the influence of the slow moving closed-low, with both regions covered with Slight Risks Outlook threat areas. A similar area depicted for the Day 5/Saturday ERO shows a scaling down ERO version as Nine weakens and dissipates. However, ample lingering deep tropical moisture will remain to fuel the elongated Marginal threat area. More certain local focus closer to occurance would likely lead to Slight Risk issuance upgrades, especially given earlier period heavy rainfall will create saturated soils. Elsewhere, the Washington coastal ranges and Cascades will be an exception with enhanced rain late week with Pacific system passages, with some additional modest potential downstream later into the northern Rockies. Warmer than average temperatures are expected to translate from the Northern Plains east into the Midwest later this week into the weekend following upper-level ridging, with anomalies decreasing by early next week. Cooler than average temperatures are expected centered around the Ozarks Region under the influence of the upper- low aloft, also returning closer to average by early next week as the upper-low dissipates. Schichtel 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 (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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