Extended Forecast Discussion NWS Weather Prediction Center College Park MD 249 AM 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... Troughing will persist through the week over the East, in between flattening ridging out of the northern Rockies and over the Atlantic. 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. Multi-day rainfall could exceed several inches over parts of the coastal Carolinas. A system over the Pacific will slowly work its way into Washington/Oregon later in the week. ...Guidance/Predictability Evaluation... Latest models and ensembles remain in good agreement on the large scale pattern through the period. Favored a blend of the deterministic guidance (GFS/ECMWF/Canadian/UKMET) days 3-5, trending toward the 18Z GEFS/12Z ECENS means by day 7 as the models diverge on timing of the systems over the central CONUS. Trends have been for a bit stronger lead-in system into northern CA/southern OR by early Thu (touching off more showers for northern ID) and a tad quicker with the upper low into British Columbia. With support from the deterministic models in each area, blended/middle ground solution was prudent. ...Weather Highlights/Hazards... Areas of the North/South Carolina coast in the vicinity of the frontal boundary will see the highest chance for multi-inch rainfall totals during the first few days of the period. With ample moisture off the Atlantic, daily rounds of showers and storms will be likely with a slow trend in decreasing coverage/intensity by next weekend as the front washes out. Less rainfall is expected farther west along the boundary along the Gulf Coast back into Texas, but some could be enhanced over New Mexico into 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 trending cooler in the Pacific Northwest as the front moves ashore. 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. Fracasso 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