Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Sun Sep 22 2024 Valid 12Z Wed Sep 25 2024 - 12Z Sun Sep 29 2024 ...Tropical System Development threat from the Caribbean may impact the Gulf of Mexico and Central to Eastern Gulf Coast/Southeast by late week... ...Guidance/Predictability Assessment... It remains the case that there is quite alot of uncertainty surrounding the upper level pattern over the CONUS mid to late week. While unresolved, this also has huge implications for the eventual track of a possible tropical system through the Gulf of Mexico. This may help to pull any sort of tropical development in the Gulf northward a little faster than previous thoughts and latest guidance and latest guidance from NHC reflected this. Very little confidence in any of the specifics associated with the overall medium range flow pattern over the CONUS and, of course, Gulf of Mexico tropical activity as models continue to flip-flop. There also remains uncertainty late week as a cut off low tries to form over or off the Northeast Coast, and timing inconsistencies Pacific energies move into and onward from the West/Northwest. The WPC medium range product suite was mainly derived from a blend of the 18 UTC GEFS mean, 12 UTC ECMWF ensemble mean and especially the 12 UTC ECMWF for much of this forecast period. These solutions were reasonably clustered/compatible, and also importantly best fit latest NHC guidance on a possible formation and northward track of a tropical system into the Gulf of Mexico late week. However, recent model runs continue to suffer from much less than normal cycle to cycle continuity, both with interacting flow evolution over the lower 48 and with any tropical development. ...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. WPC Excessive Rainfall Outlook (ERO) Marginal Risk areas shift from the Mid-Atlantic through New England Day4/Wednesday and Day/5 Thursday. Meanwhile, NHC and WPC will continue to monitor possible tropical development in the Caribbean over the next few days that may 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. 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. 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