Extended Forecast Discussion NWS Weather Prediction Center College Park MD 224 PM EDT Sat Jul 31 2021 Valid 12Z Tue Aug 3 2021 - 12Z Sat Aug 7 2021 ...Heavy rainfall threat for parts of the South and the Southeast... 18Z Update: Model and ensembles remain in very good agreement regarding the synoptic scale features across the continental U.S. next week. The pattern will initially be a rather amplified one, and then transitioning to a more quasi-zonal pattern by next Saturday and this is where some more timing differences emerge. The GFS becomes slightly quicker with the upper low across the Pacific Northwest and also a bit more progressive with the trough exiting the Northeast. The CMC becomes stronger than the consensus with a shortwave trough crossing the northern Plains, but not considered an outlier solution. In terms of the heavy rainfall potential across the Southeast U.S., the strong model signal for 3 to 6 inch totals with locally higher amounts remains near the coast, although the forecast has trended down some across interior portions of the Carolinas compared to last night. Flooding could make weather headlines across portions of eastern North Carolina and South Carolina, with the highest probability of this for Wednesday and Thursday. The previous forecast discussion is appended below for reference. /Hamrick ...Overview... Troughing will persist next week over the East, in between flattening ridging out of western/central Canada and over the Atlantic. This favors bouts of rainfall along a slowly-moving/stalled front across the Carolinas back through the Southeast into the southern Plains. Multi-day rainfall could exceed several inches over parts of the Carolinas. The upper high over the Southwest will meander in place, shifting the moisture axis primarily to the Rockies and Front Range. ...Guidance/Predictability Evaluation... Latest models and ensembles remain in good agreement on the large scale pattern through the period. Recent deterministic runs continue to waver on how quickly (or slowly) to bring height falls into the Pac NW later in the week with ECMWF/Canadian/ECENS mean a little bit slower than the GFS and many GEFS members. The lack of a clear upstream kicker over Alaska would favor the slower solutions, but the upper low could unwind in pieces which would send in some lower heights into WA/OR. Favored a blend of the deterministic guidance days 3-5, trending toward the 12Z ECMWF and the 18Z GEFS/12Z ECENS means by days 6 and 7. ...Weather Highlights/Hazards... Focus for impactful weather will lie over the North/South Carolina coast in the vicinity of the frontal boundary. With ample moisture off the Atlantic, daily rounds of showers and storms will be likely through the period. Guidance indicates several inches of rainfall over a fairly large area which seems reasonable given the setup. Less rainfall is expected farther west along the boundary along the Gulf Coast back into Texas. Over the Northwest, approaching system will eventually bring in some light rain to coastal Washington by Friday into the northern Rockies (ID/MT) as the front moves ashore. Temperatures will be within a few degrees of normal over much of the West with areas along the Canadian border likely to see temperatures 5 to 10 degrees above normal from eastern Montana to the western Great Lakes. 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 for early August on some days over Texas/New Mexico and over the Carolinas. Low temperatures will be closer to normal in the Southeast. 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