Extended Forecast Discussion NWS Weather Prediction Center College Park MD 253 AM EDT Thu Aug 05 2021 Valid 12Z Sun Aug 08 2021 - 12Z Thu Aug 12 2021 ...Overview... Troughing in the West will move into the central CONUS around the middle of next week as heights build over much of the area in another push of summertime heat. Rainfall will largely focus along the Canadian border (Northern Rockies Sunday, then the Upper Midwest/Great Lakes into the Ohio and Tennessee Valleys) in an overall drier pattern than in recent weeks. Uncertain flow evolution out of the Pacific may squeeze some ridging into the Pacific Northwest late in the period. ...Guidance/Predictability Evaluation... Through the 12Z/18Z cycle, the deterministic models largely paired well with the ensembles, but this represented a change out of the Pacific toward more ridging into the Pacific Northwest around midweek next week rather than persistent, albeit weak, troughing. Though this may be a relatively small shift eastward, this also nudged the downstream trough eastward to the Great Lakes rather than the Plains or Upper Midwest. The GFS/ECMWF were stronger with this troughing than earlier runs and the ensemble means, but since this change was manifested in just the recent 12Z cycle, opted to rely mostly on the ensemble means in case an adjustment back emerges. This was a good midpoint between continuity and the more aggressive GFS/ECMWF. Otherwise, still expect heights build over much of the CONUS midweek next week and beyond in line with the CPC day 6-10 forecast. ...Weather Highlights/Hazards... Rainfall will expand across the Corn Belt and Great Lakes Sunday as a shortwave lifts toward Wisconsin. Some rainfall could be locally heavy as this system has trended more defined in recent runs. As the system lifts into southern Ontario/Quebec, rainfall coverage and intensity may wane a bit before perhaps picking up again next Tue-Thu along the tail-end of the lead boundary and in advance of another Canadian front, in addition to afternoon convection over the Tennessee and Ohio Valleys. Temperatures will be about 5-15 degrees cooler than normal in the Pacific Northwest and northern Rockies behind the lead cold front this weekend into Monday with a moderation thereafter. Ahead of this cold front, temperatures will be 5 to 12 degrees above normal over the Plains Sun-Mon and expanding to the East Coast Mon-Thu. A ridge over the Southeast Monday will maintain temperatures into the 90s with heat indices into the low 100s as dew points rise into the 60s and low 70s. Temperatures into the 80s and 90s will reach into the Northeast/ Mid-Atlantic ahead of the Great Lakes trough which is about 5-10 degrees above normal. 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