Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Sat Aug 3 2024 Valid 12Z Tue Aug 06 2024 - 12Z Sat Aug 10 2024 ...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 continues to forecast Tropical Cyclone Four to track across Florida into the western Atlantic Ocean with a notable slow down near the Southeast/Carolina 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. There continues to be uncertainty in regards to 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... There continues to be a fair amount of spread among the global and ensemble guidance in regards to the track, timing and strength of Tropical Cyclone Four. The last couple of runs did reveal a slowing trend, especially for days 3-5, and favored a track that crossed the state of Florida (possibly southeast Georgia) and track north/northeast offshore the Carolina Coast early next week. The GFS has been persistently faster/deeper with the track while the CMC and UKMET have been slower and further inland. As previously noted, the ECWMF has been a reasonable middle ground approach that reflects a slightly slower track but not was far west as the CMC and UKMET. This type of middle ground solution continued to compliment the NHC forecast. The WPC QPF will be dependent on how close to land/inland that the storm tracks and the speed of the system. The possible ejection of the storm northeastward is uncertain as well, depending on if an upper trough steers it northeast. There has been a trend to suggest that the storm retrogrades toward the Appalachians/Tennessee Valley 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 leaned toward the 00Z ECMWF more so than the GFS early on given its initially favorable track with Tropical Cyclone Four. The UKMET and CMC were not utilized for this forecast. The proportion of ensemble means were increased to more than half by Day 7 as model spread increased. ...Weather/Hazards Highlights... Heavy rain is expected to accompany the tropical cyclone along its path. Given the degree of uncertainty how much occurs onshore versus offshore will be dependent on the exact track the storms takes, especially with the notable slowing trend as it progresses northeast. A Slight Risk for Excessive rainfall was maintained for the Day 4 period for eastern portions of Georgia and the Carolinas. In addition, a Slight Risk was raised for the Day 5 to account for the continued threat for heavy rain along the Carolina Coast and into southeast Virginia. The multiday accumulation of 8 to 12 inches with locally higher amounts is the current thought. These risk areas attempt to follow the current forecast track of tropical cyclone, 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. Models continue to show a potential mesoscale convective system riding the top of the ridge into the Upper Midwest/Upper Great Lakes on Monday. By 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 (especially the sensitive slot canyons in southern Utah) Tuesday and Wednesday. Above seasonal normal temperatures across the West is anticipated to gradually lower through the extended period. The Southwest should see the longest-lasting heat concerns, with daytime highs exceeding 110F likely, but this is only a few degrees above average. Hazardous heat will be likely for the Central/Southern Plains and Middle/Lower Mississippi Valley early in the week. Much of the central U.S. will cool off in the wake of a cold front midweek; however, 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. Campbell/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