Extended Forecast Discussion NWS Weather Prediction Center College Park MD 318 PM EDT Fri Aug 2 2024 Valid 12Z Mon Aug 05 2024 - 12Z Fri Aug 09 2024 ...Potential Tropical Cyclone Four will threaten parts of Florida into the coastal Southeast into next week... ...Hazardous heat persists across the Southwest early next week as heat intensifies over the south-central U.S. as the week progresses... ...Overview... The National Hurricane Center has designated Potential Tropical Cyclone Four, which by the start of the medium range period Monday is forecast to have crossed Florida and be making its way into the far western Atlantic near the Southeast Coast. This tropical system will spread heavy rain and flash flooding as well as gusty winds lingering in Florida and pushing into the coastal Southeast for the first part of the week, with uncertainty in the timing and how far inland the impacts get. Farther west, upper ridging centered near the Four Corners will maintain heat concerns across the West through parts of next week. The ridging should get suppressed by shortwaves in the Northwest but expand east to focus potentially hazardous heat in the south-central U.S. mid- to late next week. ...Guidance/Predictability Assessment... The first model diagnostics concern is with the track, timing, and strength of what is currently Potential Tropical Cyclone Four. Even by the start of the period early Monday, considerable differences were evident in the model guidance. At least in general, recent models are more agreeable in showing a track from the far northeastern Gulf of Mexico crossing northern Florida/possibly southeastern Georgia into the far western Atlantic early next week. GFS runs have been on the faster side and strongest with the cyclone taking this track. However, the 00Z CMC was even faster in tracking the low northeast, but the newer 12Z CMC was slower but also more inland. UKMET runs have also been slow and inland. The ECMWF seemed to be the most reasonable middle ground in position early in the period, but initially on the weaker side. By Tuesday-Wednesday the GFS and ECMWF are not too different in position, and the AI/machine learning models tended to be in between. This type of middle ground solution matched pretty well with the NHC forecast. How close to land/inland that the storm tracks will be the primary driver of QPF, along with the speed of the system. Then as the week progresses, the possible ejection of the storm northeastward is uncertain as well, depending on if an upper trough steers it northeast. The 00Z ECMWF was generally alone in taking the low northwestward toward the Mid-Atlantic, and now the new 12Z run retrogrades it into the Tennessee Valley. The EC solutions appear less likely considering the full suite of guidance, but could occur if the system completely misses the northern stream flow. Ensemble members also also show ample spread. Further forecast adjustments will be needed and see NHC for the official track. Elsewhere, upper ridging centered near the Four Corners weakening a bit with time is agreeable in the guidance, while the northern stream shortwaves rounding this ridge and making their way into the northeastern U.S. show more variability. Frontal and QPF positioning in the Midwest to Northeast varies because of this. A blend of models/ensembles seemed reasonable though. The WPC forecast favored the 00Z ECMWF early on given its initially favorable track with Potential TC Four, while excluding the slow 00Z UKMET and the fast 00Z CMC. This also worked for other areas. However, ramped up the proportion of ensemble means to more than half by Day 7 as model spread increased. ...Weather/Hazards Highlights... Heavy rain is likely along the path of Potential TC Four, but with plenty of uncertainty with how much falls onshore versus offshore depending on the system's exact track. A Slight Risk lingers into Monday across northern Florida with any feeder bands behind the storm, and is also in place northward into southeastern Georgia and eastern South Carolina for Monday/Day 4. A Slight Risk was introduced for Day 5/Tuesday for much of the coastal Carolinas as the system slowly moves northeast. These risk areas attempt to follow the current forecast track of Potential TC 4, i.e. offshore but near enough to land to spread heavy rain amounts that may cause flash flooding inland. Expect further adjustments as the forecast evolves. Farther north, models show a potential mesoscale convective system riding the top of the ridge into the Upper Midwest/Upper Great Lakes on Monday. For this activity, a Marginal Risk on the Day 4 ERO stretches from the northern High Plains to the Lower Great Lakes region with an embedded Slight Risk across parts of southeast Minnesota and southern Wisconsin given ample moisture, instability, and some soil sensitivity. While rainfall amounts are more uncertain farther east, some guidance showing 1-2 inches of rain across northern New England in an unstable environment front seemed to warrant a Marginal Risk there. Then by Day 5/Tuesday, moisture streams could combine in the Lower Great Lakes to Northeast to focus rain near the front in what might be considered a predecessor rain event (PRE). Lack of model consistency in placement of rain amounts precluded any embedded Slight Risk within the larger Marginal at this time. 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 for Monday and Tuesday, with some expansion of the risk area west on Tuesday with the meandering moisture plume and to cover the sensitive slot canyons in southern Utah. The West can expect above normal temperatures into next week that gradually reduce in magnitude. The Southwest should see the longest-lasting heat concerns, with daytime highs exceeding 110F likely, but this is only a few degrees above average. Meanwhile, hazardous heat will be likely for the Central/Southern Plains and Middle/Lower Mississippi Valley early in the week. After a cold front passes southward through the central U.S. Tuesday-Wednesday, excessive heat will then focus across the Southern Plains, Lower Mississippi Valley, and Southeast. Experimental HeatRisk values are generally major for much of this region to even extreme for the central Gulf Coast area. The north-central U.S. should remain near to below normal through the period as weak shortwave energies move through. Tate 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