Extended Forecast Discussion NWS Weather Prediction Center College Park MD 319 PM 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 for the interior Mid-Atlantic into New England late this week... ...Excessive Heat lingers over portions of Texas and the central Gulf Coast Friday into this weekend... ...Overview... Tropical Storm Debby will lift through the East Coast at the start of the medium range period, eventually completing extratropical transition into New England and merging with the baroclinic zone to the north. The influx of tropical moisture will set the stage for heavy rainfall that may be exacerbated by the in situ frontal boundary from the Mid-Atlantic northward into the Northeast, mainly along the I-81 corridor to northern parts of the I-87/91 corridors. Upstream over the West and central states, 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. Troughing into the Northwest will linger through the period. ...Guidance/Predictability Assessment... For Debby, the 15Z NHC forecast favored the quicker solutions once Debby starts to get picked up by troughing moving through the Great Lakes. This was nearer to the 00Z ECMWF and somewhat the 06Z GFS, with a bit quicker trend noticed in some of the 12Z guidance. Upstream, a consensus of the ECMWF/GFS deterministic and ensemble guidance sufficed as differences were generally with the noise at the ~D5 time range, but mostly showed good agreement overall. Largest changes were with the QPF from the NBM, which was still mostly based on older/slower guidance. Quickened the pace of the rainfall up the East Coast but amounts were generally on par. ...Weather/Hazards Highlights... For Friday, in coordination with the offices from the Mid-Atlantic northward, raised the ERO level to Moderate from roughly the Mason-Dixon line northward and northeastward into northern New England. Much of VA has seen well below normal rainfall and widespread excessive rainfall may not be realized, but will continue to see future QPF trends in amounts/rates. Farther north, FFG values are lower and the areas are more susceptible to flooding, partly contingent upon short term rainfall. By Saturday, rainfall will be on its way out of the Northeast and maintained the Slight Risk for parts of the Northeast. These focus for the rainfall impacts is through the interior, though everyone will see rainfall as the large moisture plume lifts northward/northeastward irrespective of the tropical classification. 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 Friday with a Slight Risk maintained 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 (highs only in the 60s/70s east of the Rockies into the Northern/Central Plains) into the weekend before trending toward normal. Fracasso/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