Extended Forecast Discussion NWS Weather Prediction Center College Park MD 243 AM EDT Thu Aug 1 2024 Valid 12Z Sun Aug 04 2024 - 12Z Thu Aug 08 2024 ...Hazardous heat threats to focus across the Southwest and back into the South-central U.S. next week... ...Overview... Upper ridging over the Western U.S. this weekend will gradually get suppressed with time and expand eastward back into parts of the south-central U.S. next week. This will promote heat threats initially across the Southwest but also into the central Plains and Southeast. 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 the Southeast. ...Guidance/Predictability Assessment... Latest guidance continues to show good agreement on the large scale pattern, but lots of uncertainty in the details. There remains growing 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, as highlighted in the latest NHC outlook. There remains a lot of uncertainty in the exact track, and resulting rainfall footprint, but there has been an overwhelming westward shift in the guidance compared to 24 hours ago. Most models now suggest something will lift through the far eastern Gulf of Mexico Sunday-Monday. What happens with it next week is extremely uncertain and dependent on possible interactions with northern stream troughing over the East. Majority of guidance shows some sort of slowing/meandering of this system near the northeast Gulf or over Florida, but the new 00z CMC came in much more progressive taking it north through the Southeast. The GFS also continues to be a much stronger outlier with the UKMET the farthest west into the Central Gulf coast region. Either way, 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 for the first half of the period (more heavily weighted towards the ECMWF and CMC). Leaned more towards the ensemble means days 6 and 7 to account for northern stream uncertainties and also the possible tropical disturbance. ...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. 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