Extended Forecast Discussion NWS Weather Prediction Center College Park MD 201 AM 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... Through the 12Z/18Z cycle, models and ensembles showed good clustering to start, but then drifted apart one-by-one from Friday onward. The UKMET was first to depart from otherwise good agreement in the West (stronger than most other guidance) and East (slower than most). The GFS became farther south in the PacNW with the incoming upper low late Fri into Sat vs the ECMWF/Canadian which were farther north in southern BC (which has had good continuity). Favored the ECMWF/Canadian as the pattern unfolded into quasi-zonal flow but increased ensemble weighting from both the GEFS and ECMWF ensemble mean to account for increased uncertainty in an inherently more unpredictable pattern. ...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. 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