Extended Forecast Discussion NWS Weather Prediction Center College Park MD 513 PM EDT Mon Aug 5 2024 Valid 12Z Thu Aug 08 2024 - 12Z Mon Aug 12 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 continues for portions of Texas and central Gulf Coast Thursday into this weekend... ...Overview... The National Hurricane Center forecast track for Debby takes it up the Eastern Seaboard Thursday into this weekend where there is the threat for heavy rainfall along with gusty winds. Latest model guidance has trended toward greater frontal interaction of tropical moisture associated with Debby leading to increasing heavy rain potentials up toward New England. Upper ridging centered near the Four Corners will maintain monsoonal flow over the Southwest through this weekend with heat concerns shifting south ahead of the ridge, over Texas to the central Gulf Coast (areas west of Debby's cloud cover). ...Guidance/Predictability Assessment... With Debby coming onshore and forecast to meander near/over the Southeast U.S., heavy rainfall will obviously be the major focus during the medium-range period. A key question regarding how far north the heavy rainfall will surge up the East Coast will be how much interaction of an upper trough dipping into the northern Plains will cause the tropical moisture associated with Debby to lift northward. Recent model and ensemble consensus have continued to indicate a noticeably quicker motion of the northern Plains upper trough toward the eastern U.S. This setup would result in a more robust interaction between the upper trough and the circulation of Debby during the medium-range period, leading to a quicker northward expansion of the heavy rain up the East Coast late this week toward/into New England by the weekend. This setup bears resemblance to cold season East Coast cyclogenesis out of a triple-point low, except that Debby will act as the triple- point low with tropical moisture being lifted and overrun a coastal front as well as another frontal boundary farther inland. Elsewhere, model solutions agree rather well even into Day 7. The WPC medium-range forecasts were derived based on 40% from the 06Z GFS/GEFS, 40% from the 00Z EC/EC mean, and 20% from the 00Z CMC/CMC mean, while leaning toward the consensus of the ensemble means. The QPF from the 06Z GFS was minimized across Georgia on Day 4 due to GFS's track of Debby being too far to the west. ...Weather/Hazards Highlights... Given the trend of a more robust interaction of the northern stream trough with Debby, Day 4 Moderate Risk for excessive rainfall has been expanded northward into southern Virginia, and westward toward the foothills of the Appalachians, with a corresponding expansion of the Slight Risk. With the northward trend of the heavy rainfall continues into Day 5, the Slight Risk ERO has been expanded from the Mid-Atlantic through New England. A Moderate Risk of ERO has been held off in order to accommodate local offices' IDSS constraints as well as inherent forecast uncertainty. However, a Moderate Risk within the Slight Risk area appears warranted in future ERO updates. 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 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) through the northern High Plains Thursday with a Slight Risk maintained for Day 4 over northern NM/southern CO where higher QPF is noted. A Slight Risk has been introduced on Day 5 from the footfills of the central Rockies to the adjacent High Plains given increasing model signals for higher QPFs. Above seasonal normal temperatures across the Southwest and southern Plains on Wednesday are forecast to trend toward normal into the weekend as the ridge suppresses south under troughing along the Canadian border. Hazardous heat is likely for Texas and Lower Mississippi Valley/central Gulf Coast late this week into the weekend. 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. Kong/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