Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Thu Aug 1 2024 Valid 12Z Sun Aug 04 2024 - 12Z Thu Aug 08 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 sustain heat across the Southwest while bringing a heatwave into the central Plains together with ongoing heat and humid 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... Latest guidance continues to show overall good agreement on the large scale pattern evolution across mainland U.S. through next week. However, there remains 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. 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 this weekend but with a wide variety of solutions to the track of the tropical system. Following an overwhelming westward shift in the guidance from last night, all models have settled with tropical cyclone development over the eastern Gulf of Mexico this weekend as the ECMWF abandons its long-held idea of tropical cyclone development east of Florida. Thereafter, the 00Z and 06Z GFS showed the possibility of turning the cyclone back west toward the Southeast U.S. However, the 12Z guidance including the ECMWF, UKMet, Icon, and 06Z EC-AI models now generally support an east-northeasterly track near/off the coast of the Southeast U.S., which is the track proposed by the National Hurricane Center through Day 7. The CMC offers an alternative scenario by recurving the cyclone and turning it extratropical across the Southeast U.S., which is considered to be an outlier at this point. In any case, 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 used a blend of the operational models and their ensemble means for the first half of the period, but excluding the CMC and westward-moving GFS solutions for the tropical system while leaning more towards the ensemble means days 6 and 7 to account for northern stream uncertainties. ...Weather/Hazards Highlights... The main focus for heavy rainfall threats during the period will likely be associated with whatever becomes of a tropical disturbance likely to affect parts of Florida and the Southeast. Guidance this cycle showed a drastic shift towards heavy rainfall potential for much of the northern to central parts of the state. The Days 4 and 5 Excessive Rainfall Outlooks covering Sunday-Monday show slight risk upgrades across this region, with a broader marginal covering much of the state to account for the lingering uncertainty. 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 possible extension into parts of the Southeast dependent on track of this system. Elsewhere, a front exiting the Northeast on Sunday may bring a period of locally heavy rainfall, which continues to be covered in the Day 4 ERO. To the West, 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 eastward, but did include a small slight risk across parts of central Minnesota and Wisconsin on the Day 5 ERO given ample moisture, instability, and some soil sensitivity. Some rain will continue east into the Ohio Valley and Northeast middle of next week. Monsoonal moisture on the western side of the ridge should continue a wet and unsettled pattern stretching from the Four Corners region northward into the Rockies/high Plains most of the period. Marginal risks on the ERO continue across parts of the Southwest/Four Corners Sunday and Monday. 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. Above normal temperatures across the Central U.S. should slowly sink south and eastward with time. This equates to moderate to major HeatRisk across the south-central to Southeast U.S.., 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. Kong/Santorelli 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