Extended Forecast Discussion NWS Weather Prediction Center College Park MD 307 AM EDT Sun Aug 4 2024 Valid 12Z Wed Aug 07 2024 - 12Z Sun Aug 11 2024 ...Heavy rain and flash flooding increasingly likely to continue for areas of the Southeast in the middle to late parts of the week from Debby... ...Excessive Heat/Major HeatRisk for the southern Plains to central Gulf Coast Wednesday through Friday... ...Overview... The National Hurricane Center forecast for Debby remains slow along the Southeast Coast into the Carolinas Wed/Thur, increasing the threat of very heavy rainfall over portions of at least the coastal Southeast along with gusty winds. Uncertainty remains with the track forecast and the associated heavy rains, so please stay alert to forecast updates. Upper ridging centered near the Four Corners through at least Thursday night will maintain monsoonal flow over the Southwest with heat concerns shifting south ahead of the ridge, over the southern Plains to the central Gulf Coast (areas west of Debby). ...Guidance/Predictability Assessment... Very strong ridging over Northwestern Canada persists through at least Thursday with broad ridging over the Four Corners region, favoring elongated (E-W) troughing across much of southern Canada later in the week. With additional upper ridging over the central Atlantic, this leaves troughing over the East Coast, allowing Debby to drift northward later in the week. The latest NHC track favors the 12Z ECMWF for the Days 4/5 track that is over the Carolinas, though there is considerable spread among global guidance, so expect changes to the forecast track which will have significant implications for the rather heavy QPF given the slow motion. The 12Z ECMWF was favored for QPF Days 4/5 given its similarity to the track forecast, but the 00Z ECWMF has come in more progressive to the north after Wednesday while the GFS retains a much farther inland track over GA. Elsewhere, upper ridging centered near the Four Corners suppressing south a bit, then drifting east to the southern Plains with time is agreeable in the guidance. Troughing over Canada will still support an eastward- moving cold front that will eventually meet up with another frontal boundary hung up near the East Coast atop Debby. ...Weather/Hazards Highlights... Slow motion to Debby raises the threat for very heavy rain in the Southeast. A Moderate Risk for excessive rainfall is noted for the coast of South Carolina into the GA coast Wednesday and Thursday around the track for the center of the system which is forecast to be just off the Southeast Coast for Wednesday before turning north Thursday. The Day 4 area is quite similar to the previous Day 5 with expansion of the Moderate Risk around Savannah. Farther north, the approaching front will likely stall and linger to the north of Debby, favoring a PRE setup with additional rainfall stretched northeastward across the Mid-Atlantic by later in the week. The new Day 5 ERO features a Slight up through south-central VA for such frontal convergence precip. Expect further adjustments as the forecast evolves, especially given the broad track solutions among the 00Z global guidance. 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) Wednesday and Thursday with a Slight Risk raised for Thursday/Day 5 over northern NM/southern CO where higher QPF is noted.. Above seasonal normal temperatures across the Southwest and southern Plains on Wednesday are forecast to trend toward normal through Friday as the ridge suppresses south under troughing along the Canadian border. Hazardous heat is likely for the Southern Plains and Lower Mississippi Valley through midweek, then suppress south to the central Gulf Coast. Experimental HeatRisk values are generally major for much of this region with extreme for the central Gulf Coast area. Much of the central U.S. will cool off in the wake of a cold front midweek. Jackson 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