Extended Forecast Discussion NWS Weather Prediction Center College Park MD 228 AM EDT Fri Aug 2 2024 Valid 12Z Mon Aug 05 2024 - 12Z Fri Aug 09 2024 ...Hazardous heat persists across the Southwest as heat intensifies over the south-central U.S. next week... ...Watching tropical threats from Florida to the southeast U.S. coast next week... ...Overview... Upper ridging over the western U.S. this weekend will be up against upper troughing digging down central Canada toward the northern Plains, causing the ridge to get suppressed with time and extend east toward the south-central U.S. next week. This pattern will be favorable in maintaining the heat across the Southwest while spreading the heatwave across the South. Northern stream flow and energy digging to the lee of the upper ridge will reinforce troughing over the Upper Midwest to the East that lasts through the middle of next week. The National Hurricane Center is also monitoring the potential for tropical development that could affect the eastern Gulf Coast and then the southeast U.S. coast. ...Guidance/Predictability Assessment... There is generally good agreement with the large scale pattern and how it evolves over the next week. However, noticeable uncertainty in the timing and amplitude of energies through the northern tier to carve out a Midwest/eastern U.S. mean upper trough, especially later in the period persist in the last few run model cycles. Also question on the extent and strength of the upper ridge from the Southwest into the Central Plains/Southeast. Models and ensembles continue to favor possible tropical development near/south of Florida over the weekend but with a wide variety of solutions to the track of the tropical system. Consensus favors tropical cyclone development to track east-northeast near/off the coast of the Southeast U.S., which reflects the National Hurricane Center through Day 7. Although the 12Z GFS decidedly turns the cyclone toward the east- northeast, there is still quite a number of the 12Z GEFS members maintaining a west- northwest track across the northeastern Gulf of Mexico. This system will need to be watched for possible heavy rainfall impacts to parts of the Gulf Coast/Florida and the Southeast. The WPC medium range forecast suite maintained continuity by utilizing a multi-model blend of the operational models and their ensemble means. The CMC and westward- moving GFS solutions for the tropical system were reduced/omitted while leaning more towards the ensemble means days 6 and 7 to account for northern stream uncertainties. ...Weather/Hazards Highlights... The National Hurricane Center is monitoring a tropical disturbance that is expected to increase the threat for heavy rainfall for parts of Florida and the Southeast in the coming days. The Day 4 Excessive Rainfall Outlook covering Monday maintains a Slight Risk across this region, with a broader marginal covering much of the state to account for the lingering uncertainty. The area of heavy rainfall is expected to shift up the coast by Tuesday, therefore a D5 Marginal Risk was raised for portions of the Carolina Coastline. It is likely these areas will need to be modified as we get closer in time, but for now, this seemed the most plausible solution covering the better model track consensus. Heavy rainfall across Florida may linger into next week with further modifications dependent on track of this system. Models show a potential mesoscale system riding the top of the ridge into the Upper Midwest early next week. A Marginal risk on the Day 4 ERO stretches from the Northern High Plains to the Lower Great Lakes region with a small Slight Risk across parts of southeast Minnesota and southern Wisconsin given ample moisture, instability, and some soil sensitivity. Some rain will continue east into the Ohio Valley and Northeast middle of next week; a Marginal Risk area was raised for Day 5 for the Lower Great Lakes to the Northeast. Monsoonal moisture on the western side of the ridge should keep a wet and unsettled pattern in place from the Four Corners region northward into the Rockies/high Plains through much of the extended period. Marginal Risks for excessive rain are in effect for parts of the Southwest/Four Corners through Tuesday morning. Much of the West Coast should be modestly above normal much of the period, but the greatest threat for hazardous heat will be across parts of southern California and the Desert Southwest this weekend into early next week, with daytime highs exceeding 110F likely. For the Central U.S. the above normal temperatures are expected to shift toward the South-central and Southeast regions. This will likely result in developing a moderate to major HeatRisk across the South/Southeast, with some localized extreme across parts of the central Gulf Coast states. Northern tier should remain near to below normal through the period as weak shortwave energies move through. Campbell/Kong 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