Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Tue Aug 6 2024 Valid 12Z Fri Aug 09 2024 - 12Z Tue Aug 13 2024 ...Heavy rain and flash flooding threat associated with Debby expected to spread up the East Coast toward New England late this week... ...Excessive Heat lingers over portions of Texas and the central Gulf Coast Friday into this weekend... ...Overview... The latest National Hurricane Center forecast track for Debby takes it up the Carolinas Friday and then along the northeastern Seaboard this weekend where there is the threat for additional heavy rainfall. 00Z model guidance continues to depict frontal interaction of tropical moisture, with heavy rain, associated with Debby over the Mid-Atlantic and Northeast. Upper ridging centered over Texas will maintain monsoonal flow over the Southwest through this weekend with heat concerns persisting from Texas to the central Gulf Coast. ...Guidance/Predictability Assessment... The focus of heavy rainfall associated with Debby as it begins to eject north from the Carolina Coast on Friday and the associated moisture interacting with a stalled frontal boundary over the northeastern Seaboard is the major forecast concern for the medium- range period. The ECMWF favors a farther inland track of the heavy rain, with the 00Z run focusing on the eastern slopes of the central Appalachians and the eastern Great Lakes on Friday before shifting over New England on Saturday. The CMC has been closer to the coast and the GFS remains a significant outlier, maintaining remnants of Debby over GA into Friday a few states south of the 00Z consensus. The 00Z UKMET did not arrive in time for evaluation for this discussion. Elsewhere, model solutions agree rather well even into Day 7. The WPC medium-range forecasts were derived based on 50% from the 12Z ECMWF, 20% from the 12Z CMC and the remaining from the 12Z UKMET/18Z GFS and/or the 12Z ECENS and 18Z GEFS means. The QPF is generally a blend of the 01Z NBM and the 12Z ECMWF. The 00Z ECMWF continues the inland track of heavy QPF on Friday over the east. Data from the 18Z GFS was minimized across the Southeast on Day 4 due to the aforementioned slow progression of Debby. ...Weather/Hazards Highlights... Given variability of guidance on the interaction of tropical moisture from Debby and a frontal zone near the Northeastern Seaboard, a broad Slight Risk was maintained for the Day 4 ERO (though northern Maine was removed). A targeted Moderate Risk is likely to be needed eventually, but not until greater model consensus on where that targeted area should be drawn. A Slight Risk was introduced for the Day 5 ERO in New England given timing of heavy QPF moving through Saturday in recent ECMWF solutions. Monsoonal moisture on the western side of the ridge will keep a wet and unsettled pattern in place from the Four Corners region northward into the Rockies/High Plains through this weekend . Marginal EROs are in effect for parts of the Southwest/Four Corners (especially the sensitive slot canyons in southern Utah) through the northern High Plains Thursday with a Slight Risk maintained for the new Day 4 over northern NM/southern CO where higher QPF is noted. Above seasonal normal temperatures across the southern tier are forecast to continue Friday and this weekend, mainly over Texas to the central Gulf Coast. Below normal temperatures over central U.S. persists behind a cold front Friday into the weekend before trending toward normal. 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