Extended Forecast Discussion NWS Weather Prediction Center College Park MD 240 PM EDT Sun Sep 22 2024 Valid 12Z Wed Sep 25 2024 - 12Z Sun Sep 29 2024 ...Tropical System Development threat for the Gulf of Mexico and Central to Eastern Gulf Coast/Southeast by late week... ...Guidance/Predictability Assessment... Guidance seems to be finally trending towards a more consistent flow pattern across the CONUS during the medium range period, which includes the eventual track of a possible tropical system through the Gulf of Mexico and towards the central-eastern Gulf Coast region late this week. Models have trended towards more splitting of flow and a cutoff upper low lingering over the middle Mississippi Valley much of the period. This should help to pull a Gulf tropical system northward and faster than previous guidance. Latest QPF/ERO forecasts from WPC and NHC thoughts reflected this. Still, very little confidence in any of the specifics associated with especially the tropical and until an actual system forms, models will likely continue to flip-flop. Should also note that the ensemble means are a little slower than their deterministic counterparts, which highlights the uncertainties in individual ensemble members. Elsewhere across the CONUS, there is some timing and amplitude uncertainty with another shortwave into the Western U.S. late weekend and reinforced troughing/a closed low near the Northeast. The WPC medium range suite used a blend of the deterministic models Days 3-4, with more weighting towards the ECMWF which was closest to NHC thoughts on potential Gulf tropical development. After this, increased ensemble mean contributions to half of the blend by day 6-7 given increasing uncertainty. Overall faster than WPC continuity with the Gulf Coast system and faster to bring QPF into the coast and the Southeast. ...Weather/Hazards Highlights... Upper trough energy exiting the Great Lakes/Ohio Valley midweek may be further enhanced by uncertain reinforcing energy into later week over the Northeast. Wavy frontal systems will also focus deepening moisture to fuel a threat of showers/thunderstorms with runoff issues given slowed motion and possible closed low. The day 4 (Wednesday) WPC Excessive Rainfall Outlook (ERO) expanded the marginal risk slightly across the Mid-Atlantic/central Appalachians given newest guidance. On Day 5 (Thursday) ERO, and after collaboration with the impacted WFOs, was able to remove the marginal risk across New England as soils are dry and rainfall should be mostly beneficial. Meanwhile, NHC and WPC will continue to monitor possible tropical development in the Caribbean over the next few days that looks to lift into the Gulf of Mexico to threaten maritime interests and the central to eastern Gulf Coast/Southeast mid-late week. Given planned development, ultimate timing and track remains quite uncertain in guidance. Even so, the vast bulk of recent guidance offers a growing signal for impactful weather, including a threat for heavy rainfall/winds/surf for the central to eastern Gulf Coast, with impactful weather possible through the Southeast/East late week/weekend. With the overall faster trend in the latest guidance, the Day 4 ERO marginal risk was expanded northward to include the FL Panhandle. On Day 5/Thursday, also expanded the marginal and slight risks northward well into the Southeast states. If the current forecast hold, it is likely at least a moderate risk will be needed closer to the coast for this time period, but with ongoing uncertainty, it seems prudent to hold off for now. Regardless, latest models and ensembles continue to support a heavy rainfall/storm surge/flooding threat across the central to eastern Gulf Coast region. Out West, most areas should stay dry. The Washington coastal ranges/Cascades will be an exception with growing daily enhanced rain potential with Pacific system energy and moisture approaches. Santorelli/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