Extended Forecast Discussion NWS Weather Prediction Center College Park MD 255 PM EDT Sun Aug 15 2021 Valid 12Z Wed Aug 18 2021 - 12Z Sun Aug 22 2021 ...Tropical Storm Grace to track into the Gulf of Mexico... ...Emerging North-Central U.S. Heavy Rain Threat... ...Guidance/Predictability Evaluation... Models and ensembles do not offer a strong and consistent guidance signal for Grace with emergence from the Caribbean after midweek, but recent guidance trends have tended to shift the track more westward across the Gulf of Mexico later week into next weekend. There is potential for system intensification to monitor for maritime interests along with a possible Florida/Gulf Coast threat. Elsewhere, The WPC medium range product suite was primarily derived from a blend of the 00 UTC ECMWF/Canadian and ECMWF ensemble mean that offered more mid-latitude flow amplitude and slower progression than the 00/06 UTC GFS/GEFS mean. This seemed reasonable considering anchoring upper ridge amplitude in most guidance set over the eastern Pacific. The 12 UTC GFS/GEFS and 12 UTC UKMET have generally trended (GFS/GEFS slower and UKMET faster) toward the WPC blend and the 12 UTC ECMWF remains overall in line. WPC continuity is well maintained. ...Pattern Overview and Weather Highlights/Hazards... Residual tropical moisture from a dissipated Fred will fuel lingering but increasingly scattered showers/thunderstorms and localized downpours spread over the east/northeastern U.S. past midweek. T.S. Grace may also with some uncertainly track from the Caribbean into the Gulf of Mexico midweek into next weekend. Please refer to the latest advisories from the National Hurricane Center for updated information about Grace. Elsewhere, monsoonal moisture activity lifting out from the Desert Southwest/Great Basin mid-late week will offer a localized flash flood risk. Moisture should stream northward and northeastward ahead of the incoming Northwest to north-central U.S. upper trough, with an emerging potential for moderate to heavy rainfall/convection from the northern and central Rockies/Plains to the mid-upper MS Valley mid-late week and then the east-central U.S. with gradually diminishing support into next weekend. Expect organized and enhanced rainfall will focus along/ahead of a developing frontal system aided by cyclogenesis as height falls/instability/inflow enhance with amplified upper trough progression. Taylor/Schichtel 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, winter weather outlook probabilities and heat indices are at: 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml