Extended Forecast Discussion NWS Weather Prediction Center College Park MD 253 PM EDT Sat Sep 21 2024 Valid 12Z Tue Sep 24 2024 - 12Z Sat Sep 28 2024 ...Eyes to the Caribbean for next week... ...Guidance/Predictability Assessment... There continues to be a lot of uncertainty surrounding the upper level pattern over the CONUS mid to late next week, which has huge implications for the eventual track of a possible tropical system through the Gulf of Mexico. Latest guidance runs seem to be trending towards more split flow over the CONUS, as energy or an upper low splits from a leading trough racing through the Midwest/Northeast next Wednesday-Friday and meanders over the Central Plains-Middle Mississippi Valley. This may help to pull any sort of tropical development in the Gulf northward a little faster than previous thoughts and latest guidance from NHC reflects 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 and will probably continue to do so. Also a lot of uncertainty late period as a cut off low tries to form over or off the Northeast Coast, and timing inconsistencies as another shortwave moves into the West. Through the 00z/06z guidance, the GFS was the biggest outlier and was not used in today's updated guidance blend. The new 12z run (available after forecast generation time) is more in line with consensus. The WPC forecast for today leaned more heavily on the ECMWF and Canadian, with some smaller contributions from the UKMET and the means early period. By the second half of the period, transitioned to as much as 70 percent ensemble means in the blend to help smooth out unresolvable details at this point. ...Weather/Hazards Highlights... Energy digging southward over the Rockies/High Plains in still uncertain separating flow may offer a period favorable for enhanced rainfall over the southern High Plains under upper diffluent flow and with frontal proximity. A WPC Marginal Risk area remains in place on the WPC Day 4/Tuesday Excessive Rainfall Outlook (ERO). Meanwhile, upper trough energy and wavy frontal systems will be kicked slowly into the Midwest/East early next week along with an associated threat of showers/thunderstorms with some local runoff issues given motion. WPC (ERO) Marginal Risk areas are depicted to shift eastward Day4/Tuesday and Day/5 Wednesday across the Upper Ohio Valley and northern Mid-Atlantic. Upper trough reinforcement may support additional rainfall potential to monitor over the East- Central U.S. mid-later next week in amplified but uncertain flow. The eastward progression of this remains contingent on far upstream development of a progressive upper trough from the northeast Pacific early next week. Meanwhile NHC and WPC will continue to monitor possible tropical development in the Caribbean that may slowly lift northward into the Gulf of Mexico to threaten the Gulf Coast late next week. The ultimate position and southward breath of the upper trough over the lower 48 will determine extent and speed of northward lifting. Given some of the latest guidance trends, there is potential that heavy rainfall may begin impacting parts of the central to eastern Gulf Coast by late next week. The timing and exact location of impacts is still very uncertain at this time. 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