Extended Forecast Discussion NWS Weather Prediction Center College Park MD 205 AM EDT Tue Aug 03 2021 Valid 12Z Fri Aug 06 2021 - 12Z Tue Aug 10 2021 ...Overview... Troughing will continue through the medium range in the West (especially the Northwest) as ridging builds into the East. A drier pattern overall is expected with rainfall focused mostly along the Canadian border but also along the Gulf Coast and into the Southwest, though with less coverage/intensity. Temperatures will warm up in the East well into the 80s and 90s while the Northwest stays below normal. ...Guidance/Predictability Evaluation... Through the 12Z/18Z cycle, the models/ensembles that paired best included the 12Z GFS/ECMWF/Canadian and 12Z GEFS mean/ECMWF ensemble mean while the 12Z UKMET and 18Z GFS/GEFS were not preferred. The UKMET has had a string of runs with questionable details and timing of systems that has not seemed to pan out, so it was discounted again from day 3 onward. The 18Z GFS/GEFS seemed to have an odd run where it likely over-amplified a shortwave out of northern Canada across Hudson Bay late Fri into Sat which led to a much slower system through the Northeast compared to the consensus. Its 12Z run was much more aligned with the ECMWF/Canadian and ensembles. By next week, the deterministic consensus was decidedly quicker with the trough progression along ~50N compared to the ensembles, but this could be a function of the lower amplitude flow and lack of details in the broader ensemble means. Utilized a much larger deterministic component to day 7 despite the usual increase in uncertainty given a preference for a quicker pace as evidenced by a multi-day trend from the Northeast Pacific eastward this weekend into next week. ...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 around the start of the medium range. 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 dipping into the Great Lakes. Some rainfall could be heavy on Saturday over the Upper Midwest. Rainfall in the Southwest will be more isolated but could affect terrain locations and perhaps some lower valley locations as well. Temperatures will be about 5-15 degrees cooler than normal in the Pacific Northwest behind the lead cold front Fri-Sun before a slight moderation. Ahead of the front, temperatures will be 5 to 10 degrees above normal over the Plains Fri-Sun and to the East Coast Sun-Tue (especially the Ohio Valley/Mid-Atlantic/Northeast). As upper ridging builds into the Deep South next week, heat and humidity may combine to bring heat indices to near 100-110F across portions of the Lower Mississippi Valley and around 100F in the Mid-Atlantic. 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