Extended Forecast Discussion NWS Weather Prediction Center College Park MD 249 PM EDT Sun Jul 26 2020 Valid 12Z Wed Jul 29 2020 - 12Z Sun Aug 02 2020 ...Excessive rainfall/runoff threat for the Mid-South and vicinity mid-late week... ...Tropical system threat for the Caribbean then possibly the Bahamas/Southeast U.S. in a week... ...Guidance/Predictability Assessment... The 06 UTC GFS/GEFS mean and 00 UTC ECMWF/ECMWF ensemble mean offer a better clustered solution for days 3-7, bolstering forecast confidence. A composite blend was mainly used to derive the WPC medium range product suite along with guidance from the 13 UTC National Blend of Models, NHC guidance, and WPC continuity. Smaller scale differences of the 12 UTC GFS/ECMWF still show inherent difficulties with convective focus in summer patterns. ...Weather/Hazard Highlights... A northern stream upper low over eastern Canada will maintain troughing across the Northeast as a very wavy trailing frontal boundary settles from the Mid-Atlantic to the Mid-South and Plains. This slow moving front will become the focus for strong to severe thunderstorms and a threat for locally heavy rainfall through much of the period. There is a mid-late week threat for more organized moderate to heavy rainfall/runoff issues from the mid-MS Valley to the TN/lower OH Valleys under digging southern stream troughing. The region may be in the favorable right entrance region of the upper jet, have pooled precipitable water values upwards of 2.25" and enhanced convergence/instability with frontal wave approaches. Activity tending to work downstream into the Appalachians and southern Mid-Atlantic may gradually reform northward through the OH Valley with gradual flow amplification, but 12 UTC guidance still shows uncertain wave developments. NHC is also monitoring a risk for tropical system development over the Caribbean later this week that may threaten the Southeast U.S./Bahamas in a week, but there is guidance uncertainty at these longer time frames to monitor. Meanwhile, the subtropical high will settle over the Southwest, with building upper ridging also supporting moderately anomalous summer heat across the Intermountain West/Rockies/High Plains. This occurs as amplifying upstream troughing establishes just off the West Coast. Trough energy/height falls gradually work inland over the Northwest to drive an unseasonable series of showery fronts, with moderate rainfall amounts most likely later week over favored terrain of the Pacific Northwest. Schichtel Additional 3-7 Day Hazards information can be found on the WPC medium range hazards 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