Extended Forecast Discussion NWS Weather Prediction Center College Park MD 236 PM EDT Wed Aug 4 2021 Valid 12Z Sat Aug 7 2021 - 12Z Wed Aug 11 2021 18Z Update: With the quasi-zonal flow pattern becoming established over the north-central U.S. and southern Canada, more noteworthy model differences are becoming apparent compared to the recent amplified pattern. The trough entering the Pacific Northwest this weekend is now in better agreement with the arrival of the 12Z ECMWF, compared to the much slower and amplified 00Z ECMWF. By the middle of the week, the 12Z CMC becomes out of phase by having a ridge over the Midwest states, whereas the model consensus favors a building trough over this region. More of the ensemble means were incorporated for the middle of next week to account for the increasing spread in the deterministic guidance. Temperatures for the most part should be near to above average for the majority of the nation, with the only exception being across the Pacific Northwest early in the forecast period with the upper level trough moving through. The previous forecast discussion is appended below for reference. /Hamrick ...Overview... Troughing will persist in the West and along the Canadian border next week as upper ridging builds into the East and South. This favors a drier pattern than in recent weeks with rainfall focused mostly over the Upper Midwest/Great Lakes as systems converge in the center of the continent and lift northeastward. Mid-summer heat will return with a vengeance by the middle of next week in much of the East. ...Guidance/Predictability Evaluation... Through the 12Z/18Z cycle, the deterministic models largely paired well with the ensembles except that the GFS runs were largely quicker than the consensus. Though there has been a several day trend of a quicker pace, and the quasi-zonal pattern may support it, did not favor this quicker solution as many GEFS members were as slow as the ECMWF (on the slowest end) while the UKMET/Canadian fell in the middle. Opted to rely more on the ensembles means by the end of the period in order to compensate for uncertainty in timing by next Tue/Wed in the west-to-east flow. Favored a bit higher heights over the Southeast per the GFS/GEFS just over the 5940m threshold. ...Weather Highlights/Hazards... Rainfall will expand across the Upper Midwest and Great Lakes this weekend ahead of a shortwave trough south of a main low over BC/Alberta. Some rainfall could be locally heavy over MN into WI/MI. As the system lifts into southern Ontario/Quebec, rainfall coverage and intensity may wane a bit before perhaps picking up again next Tue/Wed along the tail-end of the boundary again over the Corn Belt into the Great Lakes. In the Southwest, some monsoon rains may flare up over southeastern AZ as some moisture rides up the west side of mainland Mexico. Temperatures will be about 5-15 degrees cooler than normal in the Pacific Northwest behind the lead cold front this weekend with a slow moderation next week. Ahead of this lead cold front, temperatures will be 5 to 12 degrees above normal over the Plains this weekend and expanding to the East Coast next week. A ridge over the Southeast sets up by Sunday and looks to persist well into next week bringing an extended heat wave over the Lower Mississippi Valley eastward to the Mid-Atlantic. Temperatures will climb into the 90s with heat indices into the low 100s as dew points rise into the 60s and low 70s. 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