Extended Forecast Discussion NWS Weather Prediction Center College Park MD 327 PM EDT Mon Aug 02 2021 Valid 12Z Thu Aug 05 2021 - 12Z Mon Aug 09 2021 ...Lingering heavy rainfall threat for the coastal Carolinas Thursday... ...Overview... Upper pattern will undergo a reshuffling of the anomaly centers from the Pacific across CONUS into the western Atlantic later this week into next week. Initial trough/ridge/trough pattern over the PacNW/Rockies/Great Lakes and Midwest will shift toward broad troughing from the northern Rockies to the Upper Midwest with weaker ridging into the East. This translates to a generally drier pattern with warming temperatures out of the Plains into the East. The Northwest will see cooler and unsettled weather overall. ...Guidance/Predictability Evaluation... For the 00z/06z cycles, the models and ensembles show good clustering the first half of the period, with the typical detail and timing uncertainties arising thereafter. The GFS has shifted north with it's placement of the upper low into the PacNW Friday into Saturday, and is now more in line with the ECMWF and ensemble means. The CMC is now the model farthest south with the low, and the ECMWF by days 6 and 7 takes it a little north into Central Canada, whereas the GFS hangs close to the US/Canadian border. For days 3-5, a majority deterministic blend worked well as a starting point for this cycle of the WPC forecast. After that, opted to lean more on the ensemble means, while still maintaining at least 50% of the deterministic ECMWF/GFS. This approach also worked well for the Eastern U.S. and maintains good continuity with the previous WPC forecast. ...Weather Highlights/Hazards... Stalled frontal boundary over the Carolinas and along the East Coast will slowly dissipate this coming weekend, after likely being a focus for several days of widespread rain and locally heavy amounts that could cause flooding issues. Upper trough responsible for its orientation will weaken in concert, allowing the focused rainfall to abate. With increased flow across the U.S./Canadian border, rainfall will expand out of the PacNW/northern Rockies through the Upper Midwest and Great Lakes as a system traverses the northern tier. Rainfall will focus near and north of an area of low pressure as well as south of a northern stream boundary. Rainfall in the Southwest will be more isolated this weekend but could increase a bit next week as the upper high shifts away and weak troughing settles into California and Nevada. Temperatures will trend cooler as the Pacific front moves ashore the Pacific Northwest Friday--about 5-15 degrees cooler than normal. Ahead of the front, temperatures will be 5 to 10 degrees above normal over the Plains Thu/Fri then into the Corn Belt/Great Lakes this weekend and to the East Coast by next Monday. Much of the East/Southeast will see near to below normal max temperatures along and behind the stationary front--perhaps as much as 10 degrees below normal on some days over Texas/New Mexico and over the Carolinas Thu-Fri before a warm-up this weekend back toward more typical values. In the Southwest, excessive heat may continue into Thursday but should moderate Friday and into the weekend. As the upper high shifts out of the Southwest and into the Deep South, heat and humidity may combine to bring heat indices to near 100-110F late period across portions of the Lower Mississippi Valley. Santorelli/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 Hazards: - Heavy rain across portions of the Southeast, the Mid-Atlantic, and the Northeast, Thu, Aug 5. - Heavy rain across portions of the Southwest, Sat-Sun, Aug 7-Aug 8. - Excessive heat across portions of the Central Great Basin, California, and the Southwest, Thu, Aug 5. - Excessive heat across portions of the Middle Mississippi Valley, the Lower Mississippi Valley, the Southern Plains, the Ohio Valley, and the Tennessee Valley, Sun-Mon, Aug 8-Aug 9. - Much above normal temperatures across portions of the Central Plains, the Middle Mississippi Valley, the Upper Mississippi Valley, and the Northern Plains, Fri-Mon, Aug 6-Aug 9. - Much above normal temperatures across portions of the Northeast, the Central Appalachians, the Middle Mississippi Valley, the Mid-Atlantic, the Upper Mississippi Valley, the Great Lakes, and the Ohio Valley, Sun-Mon, Aug 8-Aug 9. - Heavy rain across portions of mainland Alaska, Thu-Fri, Aug 5-Aug 6. 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