Extended Forecast Discussion NWS Weather Prediction Center College Park MD 301 PM EDT Sun Aug 01 2021 Valid 12Z Wed Aug 04 2021 - 12Z Sun Aug 08 2021 ...Heavy rainfall threat for parts of the South and the Southeast Wednesday... ...Overview... During the extended period, the eastern U.S. will remain in a trough as ridging persists over the Atlantic Ocean and along the Rockies. This favors a quasi-stationary boundary astride the East Coast with embedded areas of low pressure that will promote bouts of rainfall across especially the Carolinas, but also up the coast to New England. Accumulating rainfall during the course of the extended period will likely be several inches for North and South Carolina. On the West Coast, a Pacific system will gradually move inland over Washington and Oregon by the end of the week. ...Guidance/Predictability Evaluation... The latest guidance and their ensemble means continue to have a good handle on the overall pattern, with typical model spread noted by the end of the extended period. Maintained continuity by utilizing a combination of the ECWMF, GFS, CMC and UKMET initially then increased weighting of the ECWMF ensemble and GEFS mean day 5 through 7. Models continue to suggest increased coverage of showers across northern Idaho with a stronger system moving into portions of northern California/southern Oregon ...Weather Highlights/Hazards... With a frontal boundary stalled nearby, the coastal areas of the Carolinas will have daily showers rainfall resulting in several inches of rainfall. Chances for rainfall will be slow to decrease across this area as there will be ample moisture streaming from the Atlantic into the region. Along this boundary, particularly across the central and western Gulf, the coverage of convection will be more scattered but may be more enhanced over New Mexico and southeastern Arizona as mid-level energy meanders over the area. Over the Northwest, lead-in and Pacific system will bring in some light rain to the northern Rockies and coastal Washington, respectively, as the upper trough and surface front into WA/OR move ashore. Temperatures will be within a few degrees of normal over much of the West but will begin to cool as the Pacific front moves ashore the Pacific Northwest. Areas along the Canadian border from eastern Montana to the western Great Lakes will likely see temperatures 5 to 10 degrees above normal under upper ridging. Much of the East/Southeast will see near to below normal max temperatures along and behind the cold front--perhaps as much as 10 degrees below normal on some days over Texas/New Mexico and over the Carolinas. Low temperatures will be closer to normal in the Southeast owing to cloud cover but perhaps a few degrees below normal in the mid-Mississippi Valley where skies will be clearer. Campbell 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